Top Banner

of 57

UV Radiation Irradiation Dosimetry

Apr 03, 2018

Download

Documents

feketega
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
  • 7/29/2019 UV Radiation Irradiation Dosimetry

    1/57

    3

    ContentsConcerning the Nature of Optical Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Characterization of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Spectral Composition of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Quantitative Features of Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Distinctive Features of UV Radiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Inuences That Can Change the Radiation of a Lamp . . . . . . . . . . . . . . . . . . . . . . . 40Daylight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Dosimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

    Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

    Core Messages

    Basic principles on the behavior of light and radiation

    Important properties of light and UV radiation with respect to photobiology

    Denitions and explanations of quantities used in dosimetry Technical properties of radiators and lamps

    Basic principles of spectroradiometric instruments and presentation of spectrallyresolved data

    Werner Jordan, Dr. rer. nat. ()

    OSRAM GmbH, Central Laboratory for Light Measurements, (I OSR QM CL-M),

    Hellabrunnerstrae 1, 81543 Munich, [email protected]

    Jean Krutmann et al. (Eds.),Dermatological Phototherapy and Photodiagnostic Methods

    DOI: 10.1007/978-3-540-36693-5, Springer 2009

    1UV Radiation, Irradiation, and DosimetryL. Endres*, R. Breit

    Revised by W. Jordan, W. Halbritter

    *This article is dedicated with regard, respect, and gratefulness to

    Mr. Ludwig Endres, who passed away in January 2008.

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    2/57

    Concerning the Nature of Optical Radiation

    The existence of invisible rays in sunlight was not known until the beginning of the nine-teenth century. First, in 1800, by studying the rainbow-colored solar spectrum adjoining tothe red side, Friedrich Wilhelm Herschel was able to detect invisible rays generating heatwhen they impinged on absorbent surfaces. Shortly thereafter, in 1802, Johann WilhelmRitter likewise discovered radiation at the other end of the visible spectrum, beyond the vi-olet, that was capable of initiating intense chemical effects.

    By reason of the detection processes and the geometric positions within the spectrum, itwas accordingly an obvious idea to designate these two newly discovered radiation rangesas infraredand ultravioletradiation respectively. (With regard to wavelengths, it wouldhave been correct to refer to ultra red and infra violet.)

    But there were not yet any clear concepts concerning the nature of those kinds of radi-ation, their propagation, or in particular the manner in which light is able to generate ef-fects [16, 20]. Certainly there were various theories, the best known of these being the em-anation theory of Isaac Newton, dating back to the year 1669, and the undulatory theory ofChristiaan Huygens, dating back to the year 1677. Newton, in his theory, postulated thatlight consists of small particles that, when absorbed in material, were capable of generat-ing the known effects. In contrast, Huygens took the view that light was a wave that, justlike a water wave, required a medium for its propagation. He named this medium the op-tical ether, which was omnipresent in his opinion but was not detectable with the means

    available to him.Each one of these theories was able to offer conclusive explanations for specied phe-

    nomenaNewtons for the radiation effects, Huygenss for the interference phenomenabut neither was capable of offering an all-inclusive solution.

    It is therefore understandable that these contradictory matters led to many discussionsand attempts to set up a theory of light that was valid for all types of phenomena. How-ever, for almost two centuries, there were no further noteworthy ndings in this matter.

    A decisive advance took place in 1871, when James Maxwell propounded an electro-magnetic theory of light that inspired Heinrich Hertz to the experiments that led to the dis-

    covery of electrical oscillations (1888). These results furnished the proof that any electro-magnetic radiation, including the complete optical radiation (light and UV and IR radia-tion), propagates in the manner of waves and does not need any medium for this purpose.He found that all electromagnetic waves, independent of wavelength or frequency, prop-agate in a vacuum with the velocity of light, which was already known at that time. How-ever, attempts to explain the generation and absorption of these waves continued to be un-satisfactory.

    The processes implemented in this connection were not established until the beginningof the past century: In 1900, Max Planck published the radiation laws in which light is

    considered not as a steady process, but as a discontinuous sequence of small energy statesthat cannot be further divided. In 1902, in the course of investigations of the photoelectriceffect, Philipp Lenard discovered particular properties of light that led him to formulate anoptical quantum hypothesis. And in 1905, Albert Einstein was able to show that the exper-imental results of Lenard may be fully explained by the quantum theory of Planck.

    4 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    3/57

    Thus, starting at this time, two theories standing side by side were necessary to providea complete description of the behavior of electromagnetic waves. With regard to all ques-tions concerned with the creation, absorption, and effect of radiation, whether it be the vi-sual process, the perception of heat, or the reactions to ultraviolet radiation, it was neces-sary to apply the laws of quantum theory, while the processes involved in the propagationof electromagnetic radiation and its behavior in optical systems could be described only

    by the wave theory.Not until the second half of our century did new ndings of quantum physics provide

    a connection between these two theories, which, in a mathematical presentation, form ini-tial principles appertaining to a generally valid theory of radiation [16]. Nevertheless, asso frequently occurs in modern physics, even when applying this model, all attempts forexplanation go beyond the conceptual power of non-specialists for whomalthough it isomnipresentthe manifestation of light continues even nowadays to be a mysterious pro-

    cess.

    Characterization of Radiation

    In spite of the very complicated interrelationships that underlie the various manifestationsof electromagnetic waves, for the purpose of many technical applications, their behaviorcan be described with sufcient accuracy by just a few formulae [12, 16, 20]. These con-

    cern, on the one hand, features such as wavelength, frequency, photon energy, and spectralcomposition of a mixed radiation, while in the second main group, statements are madethat are concerned with the quantitative recording of the transferred radiation intensity andits spatial distribution.

    Features of a Wave

    Wavelength and Frequency

    Electromagnetic radiation propagates in an undulatory form within a wide range of differ-ing shapes. But mathematics (Fourier analysis) proves to us that any possible shape can be

    built up from a number of sinusoidal curves [16]. So, for understanding the points treatedin this chapter, it is sufcient to consider only the properties of a simple sinusoidal wave.

    Figure 1.1 shows how a sinusoidal wave can be created. A rotating arrow describes acircle around a central point. By transferring several positions of the arrowhead withina full turn to an axis divided into rotation angles, a graph with a sinusoidal shape can beformed. In this way, the length of the arrow corresponds to the maximum amplitude of the

    wave. The distance between the points at the beginning and the end of a full revolution iscalled one period or cycle of the wave.

    If the arrow is rotating with constant velocity, one can transfer the points of the arrow-head to a time axis in the same manner. The higher the speed, the more cycles there will bewithin a given time section (Fig. 1.2a). By knowing at least the propagation velocity of a

    1 UV Radiation, Irradiation, and Dosimetr y 5

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    4/57

    wave, one can transform the ordinate axis to length segmentation (Fig. 1.2b). With the ex-amples of Fig. 1.2, we are now able to deduce the basic characterizing quantities of a si-nusoidal wave:

    Frequency means cycles per second (cps); in Fig. 1.2a, frequencies of 2, 4, and 8 cps

    have been plotted.Wavelength denotes the distance between the beginning and the end of a period. In Fig.1.2b, wavelengths of 0.5, 0.25, and 0.125 m have been plotted.

    Fig. 1.1 Construction of a sinusoidal wave

    Fig. 1.2a,b Frequency and wavelength of a sinusoidal wave a on a time scale and b on a distancescale

    6 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    5/57

    Propagation velocity is dened by the distance that covers an optional point (P) in aspecied time period; the dimension is m/s. (In Fig. 1.2, the propagation velocity of allthree waves is 1 m/s.)

    These quantities are associated with each other:

    frequency(/s) =propagation velocity v(m/s)

    wavelenght (m) =

    v

    (1.1)

    wavelength (m) =propagation ve ocity v m/s

    frequency(/s) =

    v

    (1.2)

    propagation velocity v(m/s) = wavelength (m) frequency(/s)

    v= (1.3)

    These equations are valid for all types of electromagnetic waves, independent of fre-quency, wavelength, and intensity [11, 12, 16, 20, 22].

    Optical radiation has shorter wavelengths and higher frequencies, as in the examplesshown above. In order to avoid inconveniently large or small numbers, derived units arefrequently used for the numerical characterization of electromagnetic waves. It is con-ventional to denote the wavelengths of light, UV, and IR radiation in one of the follow -

    ing units:1 angstrom () = 1 10 10m1 nanometer (nm) = 1 10 9m1 micrometer ( ) = 1 106m

    In characterizing frequencies, it is common practice to use the following units (cps meanscycles per second):

    1 kilohertz (kHz) = 1 10 3 cps1 megahertz (MHz) = 1 10 6 cps

    1 gigahertz (GHz) = 1 10 9 cps

    Propagation Velocity

    Any electromagnetic radiation energy propagates in the vacuum with the velocity oflight.

    velocity of light c= 299,792.458 km/s (exact denition) (1.4)

    The emphasis is on the word vacuum. In all other cases, if radiation enters a medium,the propagation velocity is reduced in accordance with the refractive index n of the perti-nent substance. (For examples of velocities in different media, see Table 1.1.)

    1 UV Radiation, Irradiation, and Dosimetr y 7

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    6/57

    Normally, this fact is without any inuence for common measurement practice. Thepropagation velocity in earths atmosphere differs so little from that in vacuum that thedifference is negligible in the vast majority of cases of laboratory work.

    Equation 1.3, which states that the propagation velocity equals the product of frequencyand wavelength, is also valid for radiation passing through a medium. At a reduced veloc-ity, therefore, either the frequency or the wavelength must vary. Physics tells us [16] thatthe frequency remains constant in all media, and that the wavelength becomes shorter in

    proportion to the reduced velocity. Hence, it should be more evident to characterize a radi-ation by its frequency. Nevertheless, within the optical spectral range, the characterization

    of radiation by stating the wavelength (in a vacuum) has become established, while withinthe range of the longer waves, for instance radio waves, stating the frequency of radiationis in most cases customary.

    Table 1.1 Velocity o light

    Medium Refractive index Symbol Velocity Percent ofc

    Vacuum n = 1 c = 299,792 km/s

    Air n = 1.0003 cair = 299,690 km/s 99.9%

    Water n = 1.33 cwater = 225,410 km/s 75.2%

    Quartz glass n = 1.46 cquartzglass = 205,337 km/s 68.5%

    The different velocities given here refer to visible radiation with a wavelength of 500 nm.

    Fig. 1.3 Beam model of optical radiation

    8 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    7/57

    Energy of an Electromagnetic Wave

    A single beam of optical radiation can be imaginedvery simpliedas an arrow inspace (mathematically, as a vector; see Fig. 1.3, gray straight line) with electric (E, redline) and magnetic (B, green line) sinusoidal wave components [16].Based on electrodynamics theory, this vector (the so-called Poynting vectorS) representsthe ux of energy density of an electromagnetic wave in space. In terms of physics nota -tion, it is written as a vector equation:

    S = c E B ( = electric constant) (1.5)

    The dimension ofS(J/m s) represents the energy density of a beam passing through an ar-bitrary surface within a time interval and results in the well-known term of irradianceEe

    (W/m) in laboratory practice when incident on a detector surface (for common units, seealso Table 1.3):

    S = cEB = cE

    (W/m) (1.6)

    Note: All the other models we use to explain the behavior of radiation (Gaussian optics, straight-lined propagation of radiation) and many other macroscopic-based calculations are mainlyfounded on the wave nature of optical radiation.

    Photon Energy

    Electromagnetic waves transport energy. In the particle view of radiation (quantum me-chanics), the porters of the energy are photons. According to the Einstein relation, the en-ergyEof a photon has a xed relationship to its frequency [16, 20]:

    Ephoton =h (h= 6.626 1034 J s, Planck constant) (1.7)

    Related to wavelength in a vacuum, equation (1.7) may also be written as:

    Ephoton = h =hc

    (1.8)

    Another common unit for the energy of radiation is the so-called electron volt (eV); 1 eVcorresponds to the kinetic energy acquired by an electron passing through a potential dif-ference of 1 V in a vacuum. Since 1 watt second (W s) = 1 joule (J) = 0.624 1019 electronvolt (eV), it is also possible to write:

    Ephoton (eV) =, ( enm )

    number of wavelength(1.9)

    As can be easily seen from this relationship, photon energy (energy of electromagnetic ra-diation) increases as wavelength decreases. Therefore, UV radiation has a greater energycontent than visible light (Ephoton, 250 nm = 4.96 eV >Ephoton, 550 nm = 2.25 eV).

    1 UV Radiation, Irradiation, and Dosimetr y 9

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    8/57

    Classification of Electromagnetic Waves

    The wavelengths of the electromagnetic spectrum cover the enormous range of roughly1016m to 107 m. Accordingly, the whole spectral range is subdivided into various divisions

    based on the nature of the generation of the radiation [11]. Table 1.2 shows only a coarseclassication on this basis.

    Thus, within the regions of overlap, X-ray radiation can be generated using both X-raytubes and the processes of generation of ultraviolet radiation, or electric waves can be gen-erated using oscillator circuits and by thermal processes.

    According to international standardization [11], infrared, visible, and ultraviolet radia-tion form the range of optical radiation. These wavelength ranges are classied even morenely with regard to their chemical and physical effects.

    Infrared Radiation

    Long-Wavelength InfraredIR-C (3 1 mm) This spectral range denotes a low-en-ergy radiation with little biological signicance.

    Medium-Wavelength InfraredIR-B (1,400 nm3 ) This is the main emission rangeof heated glasses (lamp bulbs). IR-B is not perceived as heat by humans, since it is alreadyabsorbed in the outermost layer of the skin. Staying under intense IR-B radiation is per-

    ceived as an unpleasant feeling, since no regulation of the body temperature takes place.

    Short-Wavelength InfraredIR-A (7801,400 nm) This represents the main emissionrange of the thermal radiation of the sun. The radiation penetrates deeply into the skin and,within broad limits, is perceived as pleasant. The range 780 nm1,400 nm is also referredto as the therapeutic thermal octave.

    Table 1.2 Coarse overview o the electromagnetic spectrum

    Designation Wavelength Frequency Generation

    Electric waves 107103m 1011011Hz Oscillator circuits

    Infrared radiation 1038 107m 10114 1014 Hz Thermal radiators

    Visible radiation 8 1074 107m 4 10148 1014 Hz Thermal excitation,electron transition

    Ultraviolet radiation 4 1071 107m 8 10143 1015 Hz Electron transition

    X-ray radiation 5 1081 1013m 6 10153 1021 Hz Electron transition(inner atomic orbits)

    Nuclear radiation 1 10131 1016m 3 10213 1024 Hz Nuclear reaction

    Hertz (Hz) is an ofcial SI unit and denotes a periodic oscillation per second (1 Hz = s1)

    10 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    9/57

    Visible RadiationLight

    The main distinctive feature of visible radiation is the visual stimulus and the color im-pression generated by the individual wavelength ranges in the human eye. The sensitivityof the eye to different colors differs in magnitude. The eye is most sensitive to shades ofgreen and least sensitive to violet and red colors. Since the eyes sensitivity has quite es-sential signicance with regard to the economy of light sources, a standard eye responsein regard to brightness sensitivity was determined on the basis of experimental investiga-tions. This sensitivity progression is incorporated into international standardization as thespectral luminosity factor, or V () curve [6].

    In the psychological area as well, the wavelengthand thus the colorcan be rele-vant. Bluish colors are intended to increase activity, while reddish color shades are in-tended to have a calming and relaxing effect.

    Although color impressions merge continuously into each other as the wavelength in-creases, it is nevertheless possible to stipulate approximate limits for the individual colorranges:1

    Ultraviolet Radiation

    Long-Wavelength UV RadiationUV-A1 (340380 nm) This is a denite componentof all natural and articial, unltered light and UV sources and is not absorbed by un-

    stained types of glass. This part of UV radiation has the lowest energy properties, and con-cerning its chemical effectiveness, it can be combined with the short-wavelength visibleradiation (up to 440 nm) because of its photobiological effects, especially in safety consid-eration with regard to the eye (blue light hazard) [7].

    1 In international literature, the boundaries are frequently dened differently: UV-A1 340 nm400 nm; UV-A2 320 nm340 nm; UV-B 280 nm320 nm.

    Table 1.3 Color description and the corresponding wavelength range

    Common color description Wavelength rangeViolet 380420 nm

    Blue 421495 nm

    Green 496566 nm

    Yellow 567589 nm

    Orange 590627 nm

    Red 628780 nm

    1 UV Radiation, Irradiation, and Dosimetr y 11

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    10/57

    Long-Wavelength UV RadiationUV-A2 (315340 nm) This denotes the transitionalrange between UV-A and UV-B, in which effects of both spectral ranges can be found.

    Medium-Wavelength UV RadiationUV-B (280315 nm) This was dened in accor-dance with the erythema action curve for human skin on the basis of the fundamental in-vestigations made by Karl Wilhelm Hauer and Wilhelm Vahle.

    Short-Wavelength UV RadiationUV-C (100280 nm) This represents the short-est wavelength and thus the highest energy part of UV radiation. In physical denition,UV extends to 15 nm and thus directly adjoins X-ray radiation. But the short-wavelength

    boundary of UV radiation of the optical range was stipulated as 100 nm in order to avoidconicts with radiation protection regulations. Between 100 and 200 nm air absorbs UVradiation. Thus, radiation of this wavelength range (also designated as vacuum-UV) can-

    not occur in air [16, 20, 22].

    Spectral Composition of Radiation

    Only in exceptional cases, e.g., in the case of lasers, does optical radiation consist of radi-ation that can be described by one single wavelength. Normally, a mixture of many wave-lengths of different intensities is emitted. The spectral composition of radiation determines

    its effects; therefore, to evaluate the efcacy of radiation, it is necessarily to know the por-tions of the power emitted in the various wavelength domains. The representation of awavelength-related composition of a mixture of radiation is designated as spectrum, or, iflisted in a numerical manner, as spectral power distribution (SPD).2

    Spectroscopic Instruments

    Investigations and analysis of such mixtures of optical radiation are performed with spec-

    trometers or spectrographs [14]. A principle construction is shown in Figs. 1.4 and 1.6.Functionally, one can subdivide these instruments into the following components:

    Entrance Area The radiation to be analyzed is collected at the aperture of the spectrom -eter, generally shaped as a slit.

    2 Spectral power distributions can be analyzed with spectroscopic equipment (physical methods)only. The eye can perceive only one impression at one place of the retina. Of course, this im-

    pression primarily depends on the spectral distribution of the light, but it allows no conclusionto the structure of the spectrum. Different spectral power distributions can cause identical im-

    pressions called metameric color stimuli.

    12 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    11/57

    Imaging Optics Concave mirrors transform the divergent incident beam coming from the

    entrance slit to a parallel incidence. After passing the dispersing elements, they retrans-form them into a convergent bundle, focused in the exit plane of the spectrometer.

    Dispersing Elements (Either a Prism or Gratings) Prisms refract the radiation, depend-ing on the wavelength, into different angles. This basic principle may be represented in asimple manner. If a prism is held in sunlight and a piece of paper is placed behind the exitsurface, one can see the well-known colors of the rainbow appearing on the paper (see Fig.1.5). Accordingly, the rainbow is the spectrum of sunlight, but the eye cannot separate thespectral composition of sunlight. The visual process only states the impression white.

    Gratings reect the radiation into different directions (optical diffraction). Generally,the shorter the wavelength, the greater the direction is changed. Blue is deected moregreatly than red, and ultraviolet radiation is deected more greatly than visible.

    Exit Area The exit area is dened by a virtual plane on which the spectral dispersed ra-diation is focused.

    Fig. 1.4 Principle of spectral dispersion

    Fig. 1.5 Spectral refraction seen with a prism

    1 UV Radiation, Irradiation, and Dosimetr y 13

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    12/57

    To record the intensities of radiation in this area, three main procedures of mounting de -

    tectors are in use.

    Photographic Detector After exposing and developing a photographic material, one cannd a very ne structured picture of the spectrum. The blackening of the photographiclayer is typically for the spectral intensity at this point. A quantitative interpretation takes

    pains, because the blackening of the material is not linear with respect to intensity.

    Narrow (Exit) Slit with a Single Detector Turning the dispersing element in tempo-ral sequences, the whole spectrum will move through the slit and can be recorded step by

    step, but a lot of time is necessary to carry out these sequential measurements. Addition-ally, rotating prisms or gratings necessitate high precision of the mechanical constructionand stability, which can be very costly.

    Pixel Detectors Arranged as an array, pixel detectors are closely packed on a disk andplaced directly in the image plane of the spectrum so that each pixel position is assignedto a denite wavelength range. The response (exposure) time with such an arrangement iswithin a tenth of a second. By evaluating the distribution with electronic equipment, it is

    possible to perform measurements on spectral power distributions in a very short time.

    The width of the exit slit and respectively the dimensions of the detector are the limitingfactors of spectral resolution. The narrower the slit, or the more packed the pixel detec -tor, the smaller is the selected range and the higher is the spectral resolution that can beachieved. It is primarily limited by the signal intensities picked up, which become pro -gressively weaker the smaller the wavelength area has been sized. In practice, therefore,it is recommended to stay with a resolution that is necessary for the respective application

    Fig. 1.6 Principle of the optical path in a prism monochromator

    14 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    13/57

    or task to be done. In most cases, it is sufcient to record a value every 5 nm in the visiblerange, while in the ultraviolet range, the spectrum must be recorded at least in 1 nm stepsto guarantee a sufciently precise allocation of the spectral radiation intensities.

    Representation of Spectral Power Distributions

    Spectral power distributions can be represented in the form of a table or as a diagram. Theintensity of the spectral dispersed radiation is plotted against wavelength (or frequency).The unit of spectral radiation conforms to the unit of the radiation entering the spectralequipment supplemented with the unit of wavelength, which for the UV region usually isthe nanometer [12]. For example, measuring irradiance, the unit is W/m; the correspond-ing spectral unit is then W/(m nm), according to the wavelength to be characterized.3

    In Fig. 1.7, three graphic possibilities are shown of how to plot spectral power distribu-tions. For this purpose and for comparison, a radiation source is selected that emits a radi-ation power of 100 W in the wavelength range from 295 to 405 nm and that has been mea-sured in steps of 10 nm.

    3 In some diagrams, the spectral unit is W/m. This is based on mathematics: one can combine mand nanometer (109m), resulting in 109m.

    Fig. 1.7ac Presenting spectral power distributions: a lines, b continuous, and c staircase

    1 UV Radiation, Irradiation, and Dosimetr y 15

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    14/57

    In the rst example (Fig. 1.7a), the intensities measured are plotted as lines drawn in thecenter of the wavelength range (the line at 300 nm encloses the range from 295 to 305 nm).The ordinate unit is W, supplemented with the index in a range of 10 nm.

    If one can be sure the measured spectrum is continuous, one can connect the top ofthe lines to form a curve (Fig. 1.7b). The characterization of the ordinate now is spec-tral power with the unit W/nm. Note that the numerical values are 10 times lower than inFig. 1.7a.

    If we intend to plot the spectrum by hand, it is more favorable to draw the distributionin the shape of a staircase (Fig. 1.7c).

    In Fig. 1.7b, one can seen a so-called continuous spectrum, which is characterized bythe fact that radiation is emitted in the whole wavelength range. If we are analyzing a con-tinuum, the shape of the distribution stays nearly the same, independent from the spectralsolution chosen. But especially if we are dealing with low pressure discharge lamps, we

    will nd spectral power distributions with emission located in a few denite wavelengthregions while the neighborhood is free of any emission. Figure 1.8 shows a line spectrum,which was recorded with different spectral resolutions.

    You can see that the lower the spectral resolution, the broader the shape of the line con-tour becomes. Through evaluating the line intensity, you will nd the same result, inde-

    pendent of resolution, but the deceptive shift from parts of the line intensity to adjoiningwavelength can lead to errors if one calculates the efciency according to actinic effectsof radiation. For this reason, it is recommended never to specify a spectrum within smallerwavelength steps than the spectral resolution of the spectral instrument.

    In practice, there are continuous spectra, line spectra, and spectra exhibiting both ofthese characteristics [1, 3, 8] (see Fig. 1.9).

    Fig. 1.8 Records of the linespectrum (mercury line at253.7 nm) with spectralresolution of 10 nm, 1 nm,and 0.1 nm

    16 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    15/57

    The spectral distributions of sunlight and of incandescent lamps are typical representa-tives of spectral radiation distributions that have no gaps, i.e., irrespective of how narrowthe slit is, radiation intensity can be detected at any wavelength setting. Such spectra aredesignated as continuous (Fig. 1.9a). This behavior is typical for sources generating theiremission through high temperature excitation.

    If, however, as in the case of sodium and mercury lamps, gases or vapors are excited toemit, then a spectral power distribution is found in which at preferred wavelengths, radi-ation of very high intensity occurs, while in the neighboring areas, virtually no emissioncan be detected. Accordingly, such spectra are designated as line spectra (Fig. 1.9b).

    From the majority of articial radiation sources emerges an SPD that represents a mix-ture of continuous and line distribution. The reason for this is that modern light manufac-turers have been making increasing use of several physical processes for radiation genera-tion. For example, in the case of uorescent lamps, the fundamental discharge of the mer-

    cury gives a line spectrum that is distributed over the UV and visible range. The visiblelines contribute to light directly, while the ultraviolet lines excite a mixture of luminescentmaterial to uorescence applied to the internal surface of the bulb. The SPD of the phos -

    phors is approximately continuous and superposes the line spectrum. This type of distribu-tion is designated as a mixed spectrum (Fig. 1.9c).

    Fig 1.9ac Continuous spectrum (a), line spectrum (b), and mixed spectrum (c)

    1 UV Radiation, Irradiation, and Dosimetr y 17

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    16/57

    The real quality of a spectrum is of great interest for physicists and chemists. From this,fundamental information about the structure and/or the elements of a radiator can be ob-tained. In the case of continuous radiating sources, the type of distribution gives an indi-cation of the temperature of the radiator (see Thermal Radiators); in the case of line andmixed spectra, the position of the lines and the form of the continuum give indications ofthe components that are excited for emission in a source (spectral analysis).

    If the primary interest is with the effectiveness of specic radiation, the presentation ofan SPD like a staircase, showing the intensity within denite wavelength ranges, is suf-cient and can be well handled.

    Spectral radiation distributions are of great importance for all investigations that areconcerned with arithmetic determination of irradiation effects [19]. To this end, there isa need not only for the emission spectrum of the radiation source (absolute values for thewavelength intervals chosen) but also for an action spectrum that denes the spectral sen-

    sitivity within the individual wavelength ranges. Action spectra are known from many ef-fects and have in some cases also been published in national and international standards.Examples of frequently used action spectra are, within the visible range, the luminos-ity factor of the eye and the spectral value curves for color computation or, in the photo-

    biological sector, the activity spectra of erythema, pigmentation, or destruction of bacte-ria [12].

    Actinic effective radiation is calculated by multiplying the intensity value of eachwavelength interval (SPD) with the associated value of the action curve, andfor practi-cal purposesthese arithmetic products are summed rather than integrated. The result is

    one number that is typical for the efciency of the radiation mixture under investigationand for this actinic effect [12]:

    SPDeffective actinic radiation =

    SPD s, action curve (1.10)

    Actinic Radiometers

    Radiometric measurements concerning actinic radiation are costly and time consuming. Inparticular, eld measurements or measurements for controlling working places are not yetpracticable with monochromator systems. Therefore, in the last 10 years, handheld actinicradiometers, like illuminance meters, have been developed. Such a radiometer normallyconsists of a detector head and a combined signal processing and display unit. The detec-tor head is adjusted to the respective action spectrum by use of appropriate optical lter-ing. But this adaptation to an action spectrum is also the main source of error in measure-ments with such a system, because the spectral sensitivity is often very bad compared tothe ideal action curve. Therefore, only when spectral distributions of the radiation mea-

    sured are similar to the source used for calibration is there a good agreement with spec -trally resolved radiation analysis. Measurements done concerning different spectral powerdistributions reveal that systematic errors up to a factor of two or three can be stated, con-trary to the simple use of actinic radiometers. In order to rank measurements done withthese systems and to help the user with some guidance, a standardization attempt is madeconcerning the properties and the features of actinic radiometers [13].

    18 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    17/57

    Quantitative Features of Radiation

    In addition to spectral properties of radiation in application engineering, quantities alsoare needed that enable us to make statements concerning the intensity and to describe thegeometric conditions under which the radiation is present. From the vast amount of quan-tities that have in the past been proposed for these purposes, nowadays seven quantitieshave become established [12]. These denitions are also internationally standardized andare sufcient to solve all tasks occurring in practice (see Table 1.4).

    Light is the best-known example of evaluated actinic radiation. In this case, the radia -tion is evaluated by applying the spectral luminous efciency of the human eye. By rea-son of their importance, in Table 1.4, luminous units have been added, too, which are cor-related to the radiant units.

    Statements concerning these quantities are also found in the majority of product spec-ications of radiation sources and irradiation systems, so that the interpretation of thesequantities permits an evidentially cogent comparison.

    These quantities are constructed in such a simple manner that, in the mode of writingthem related to the particular application, only the four basic types of mathematics are re-quired to be understood.

    The unit watt is the generally valid unit for any type of power. It applies not only to ra-diation but likewise also to the electrical power take-up of a system or to the power of acar engine.

    For this reason, in the case of all physical radiation measurements, it is absolutely nec-essary to make more detailed statements concerning the nature of the measured power. Inthe case of unweighted radiation, the wavelength range must be stated, e.g., radiation of300400 nm (or UV-B radiation), and in the case of evaluation by reference to an actioncurve, it is necessary to state the nature of the action and the evaluation done, e.g., radiant

    power of erythema-effective radiation.

    Table 1.4 Comparison o the most important radiant and luminous quantities and units [2]

    Radiance Luminosity

    Quantity Unit Quantity Unit

    Radiant ux e Watt (W) Luminous ux v lumen (lm)

    Radiant efciency e W/W Luminous efcacy v lm/W

    Radiant energy Qe Joule (J) = W s Quantity of light Qv lm s

    Radiant intensityIe W/sr Luminous intensityIv cd = lm/sr

    RadianceLe W/m sr LuminanceLv cd/mIrradianceEe W/m IlluminanceEv lx = lm/m

    Radiant exposureHe J/m Light exposureHv lx s

    Generally, the index e denotes energetic whereas the index v denotes visual quantities. Thisdistinction should be made for clearness.

    1 UV Radiation, Irradiation, and Dosimetr y 19

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    18/57

    Some Comments on Radiant Quantities and Units

    Radiant Power e This denotes the total radiation emitted by a source. Whether theemission of radiation takes place uniformly into the entirety of space or only in preferreddirections is of no importance in this connection.

    Radiant Efciency e This denotes the ratio of the radiant ux of the radiation emittedto the electrical power consumed by the source. It must be specied whether or not the

    power dissipated by an auxiliary, such as ballast, is included in the power consumed bythe source.

    Radiant Energy Qe This denotes the radiant power that is emitted during a specied timeinterval.

    Radiant Intensity Ie This denotes the radiant power that is emitted in a specied direc-tion. The unit of radiant intensity is W/sr. The sr unit is for the solid angle and is calledsteradian (Fig. 1.10). A plane angle can be dened using degrees or by length that an an-gle cuts from a circle. If this length equals the radius of the circle, then this angle has avalue of 1 radian. This principle has been adopted in three-dimensional geometry to denea solid angle. If a cone or a pyramid cuts out an area equal to the square of the radius froma sphere, you can say that this solid angle has an amount of 1 steradian (sr). The shape ofthis areaA is of no importance. As the whole surface of a sphere is 4r, it can be also de-

    ned by 4 sr.In dealing with lamps, the amount of energy radiated in various directions about the

    lamp may be signicant. The distribution of energy around a lamp is rarely symmetrical,but it can be characterized by the spatial intensity distribution of a lamp (see Fig. 1.11).

    Fig. 1.10 Radian and steradian

    20 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    19/57

    Also, reectors or other optical devices are often used to give marked directional effects.Accordingly, the radiation in a given direction may be more signicant the total output of

    a lamp. For this reason, the energy distribution is expressed in terms of the amount radi-ated per unit solid angle.

    Radiance LeNeither with the denition of the radiant ux nor the radiant intensity havestatements been made about the size of the source. The source is here regarded as a math-ematical point without any dimensions.

    With the denition of radianceLe, information about the properties of the radiating part ofa source is included. Radiance describes the radiant intensity emitted from a denite area.

    Inphotometry, radiance corresponds to luminance or brightness, which is responsible forthe glare of a light source. (Fig. 1.12 illustrates the correlation of intensityI, radiation eld

    f, and radianceL.)

    Fig. 1.11 Example: Spatialdistribution of a lamp

    Fig. 1.12 Correlation of intensityI, radiation fieldf, and radianceL

    1 UV Radiation, Irradiation, and Dosimetr y 21

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    20/57

    Irradiance Ee This characterizes the radiant power that is incident on a surface. In thisinterrelation, it is insignicant whether the radiation originates from one or more radia-tors and from which directions it is incident. Accordingly, the irradiance is a quantity thatrelates only to a receiving surface, in contrast to the source-related quantities previouslydiscussed. It is dened by incident radiant power per receiver surfaceA with the unitW/m2.

    Only under certain circumstances does a correlation exist between the radiant intensity ofa single lamp and the irradiance found on a plane. If the distance from source to plane is10 times greater than the size of the source, it is possible to calculate the irradiance on the

    plane with the radiant intensity Imultiplied with the cosine of the incidence angle di-vided by the square of the distance r(Fig. 1.13).

    E=cos

    r

    Radiant Exposure He This characterizes the radiant power that is incident on a surface

    (irradiance) multiplied by time. In photochemistry, phototherapy, and photobiology, theradiant exposure is also called dose.

    Distinctive Features of UV Radiators

    In the case of all known natural and technical light sources, UV, visible, and IR radia-tion always occur together, even though the radiation intensities in the individual ranges

    may exhibit great differences, depending upon the type of lamp [14, 21]. Nevertheless,the classicationwhich is customary in technical languageinto UV radiators, lightsources, and IR radiators does make sense, for it gives initial indications as to the spec -tral center of the radiation or reveals the main eld of application. The transitions betweenthese types of radiators are uent, so that it is not possible to specify numerical or otherlimiting values.

    Fig. 1.13 Definition ofthe quantity irradi-anceE

    22 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    21/57

    A further classication criterion, which is independent of the eld of application, isthe physical structure and design of the radiators, which also determines the nature ofthe radiation generated. If, in terms of this criterion, the design construction of the radia-tors is considered in detail, then both in the external dimensions and in the internal struc-ture, a multiplicity of illustrative examples are found that may be classied into three maingroups (Fig. 1.14).The rst group comprises thermal radiators, in which solid and in a few exceptional casesalso gaseous substances are heated up so far that they generate useful radiation. They emita temperature-dependent, material-specic, and in most cases, a continuous spectrum. Themost important representative of a thermal radiator is sunlight. One can regard the surfaceof the sun as a thermal radiator with a temperature of approximately 6,000C.

    The most widespread type of an articial thermal radiator is the incandescent lamp, inwhich a tungsten wire is heated up to about 2,800C, and its radiation emission is primar-

    ily determined by the temperature generated in the coil [1, 3]. The family of thermal ra-diators also includes candles, in which glowing carbon particles represent the radiationsource, and also the heated charcoal plugs of arc lamps.

    The second group comprises discharge lamps, in which gases or vapors are excitedby electric current or by electric elds to radiate [1, 3]. The spectrum is in most cases aline or band spectrum and typical of the excited substances. The higher the inside pres-sure of such a lamp, the more the spectrum approaches to a continuous shape. This groupincludes the majority of UV radiators, because it is possible, by choosing suitable sub-stances, to set the centre of the emission spectrum within desired spectral ranges. Accord-

    ing to the radiation produced, one distinguishes between gaseous, metal vapor, and metalhalide discharge lamps. Components of gas-lled lamps are, for example, carbon dioxide,helium, krypton, neon, nitrogen, and xenon. Metal vapors can be generated by iron, mer-cury, or sodium, and metal halide vapors are based on rare earth elements, or cobalt, in-dium, nickel, and tin. In newer developments, lamps can be found including both gas andvapor (xenon and sodium) using these components as emitting components.

    Fig. 1.14 Possibilities for generation of radiation

    1 UV Radiation, Irradiation, and Dosimetr y 23

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    22/57

    The third group includes lamps that generate the majority of the emitted radiation byuorescence. In this case, as a prerequisite, a gas discharge is employed to generate highenergy (short wavelength) UV radiation, which, when incident on a uorescent layer, isthen converted into longer wavelength radiation, e.g., visible or UV radiation. The shapeof the spectrum may be line or band type.

    On the basis of the primary radiation generation, it would also be possible to includethese uorescent lamps among gas discharge lamps, but, by reason of the special technicaland economic importance of these types, the manufacturers have formed a specic groupfrom these lamps.

    Amongst the generation mechanism, further distinctive features for characterizing alamp are possible and in use:

    Electrical operating conditionsGeometric dimensions

    Average lifeRadiation outputOptical, ultraviolet, and infrared propertiesRadiance and magnitude of the radiation eldBurning positionColor temperature 4

    Radiators are offered in such a multiplicity of forms and designs that it is frequently dif-cult to nd the correct type or to assess its suitability. The search for radiators suitable

    for specic applications within the ultraviolet radiation range is becoming easier, since insome cases the manufacturers have combined these types of lamps in separate lists, or atleast in separate sections of their product specications [4].

    Radiators may also be selected in terms of economic aspects such as operating costsand service life for the respective practical purpose. For the case that even further datashould be of interest, it is recommended to request the missing information from distribu-tion engineers of the pertinent manufacturers, who, by virtue of their experience, have ex-tensive additional information available concerning lamp properties and operating condi-tions.5

    The following paragraphs contain descriptions of the main groups of UV radiatorsavailable on the market, together with their typical properties. Where trade names of indi-

    4 The typical spectral distribution of a Planckian radiator, depending on its temperature, alsocauses a typical color. But color is an attribute of the visible region and gives no evidence aboutthe radiation properties in the UV region.

    5 In general, it can be stated that modern lamps, which are suitable for general lighting, are notuseful for optical applications like UV irradiation. For general lighting purposes, the aim is toreduce the medium- and short-wavelength UV components to a minimum to exclude any possi-

    bility of danger to health due to UV effects, and in some cases, also to prevent radiation damageto materials, principally to plastic materials and colored pigments. In order to achieve this, theinterior of lamps are directly altered, but in most cases, lamp bulbs with properties of reducedUV transmissivity (keyword UV stop) can be used. By this means, it is possible in particularto reduce the short-wavelength ranges, in some cases by orders of magnitude.

    24 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    23/57

    vidual manufacturers are mentioned in this connection, this is intended merely to facilitatethe search for suitable types but does not mean that other manufacturers not mentioned donot also have equivalent and compatible types.

    In a similar way as can be found in product lists, the classication is made on the basisof the method of radiation generation. Therefore, in the present compilation, reference ismade to the technical lamp data only in the form of statements on range [1, 3, 4, 8].

    Thermal Radiators

    Anybody whose temperature is above absolute zero (Kelvin = 273C) emits electromag-netic radiation. The higher the temperature, the more intense the radiation becomes andalso the larger the short-wavelength radiation component becomes. In the case of an ideal

    black body with an entirely black surface that thus reects nothing, the intensity and thespectral distribution are dependent exclusively upon its temperature. Accordingly, theemission of these black bodies or cavity radiators may be unambiguously described bytwo laws.

    The Stefan-Boltzmann law (Eq. 1.11) states that the total electromagnetic radiation of abody varies with the fourth power of its temperature. In this case, total radiation signiesthe wavelength range between zero and innity.

    Total radiationL= T4 where: = .

    cm

    K

    (1.11)

    On this basis, a doubling of the temperature generates an increase in radiation by 16-fold,and a triplication of temperature gives rise to an 81-fold greater radiant power.

    Table 1.5 shows, for a number of temperatures, the total radiation of a black body sur-face, computed in accordance with the Stefan-Boltzmann law.

    Table 1.5 Total radiation o a black body and the appropriate wavelength o maximum emission

    (see Eq. 1.11)

    Temperature T

    K

    Total radiationLe

    Wcm2

    Wavelength at maxi-

    mum spectral emission

    nm

    1,000 5.7 2,890

    1,500 29 1,930

    2,000 91 1,450

    2,500 224 1,150

    3,000 459 960

    3,400 765 850

    4,000 1,465 725

    5,000 3,580 580

    6,000 7,420 480

    1 UV Radiation, Irradiation, and Dosimetr y 25

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    24/57

    The progression of the spectral intensities of a cavity radiatorST is computed according toPlancks law. This relatively complicated equation (Eq. 1.12) shows again that the spectraldistribution is determined by the temperature only:

    ST = c

    (ecT )(1.12)

    where c1 = 3.72 1012 W/cm2 and c2 = 1.438 WK.

    If the spectral distributions of the cavity radiator (Planckian radiator) are computed in ac-cordance with the Planck formula, then the following interrelationships emerge: The max-imum of the spectral radiation distribution is found at shorter wavelengths as the tempera-ture increases; in this case, to a rst approximation, a simple, linearly proportional interre-lationship between the temperature and the wavelength of the radiation maximum exists:

    max =/T(= 2.898 106 nm K) (1.13)

    This interrelationship had already been discovered by Wilhelm Wien before Planck statedhis formula. Accordingly, Eq. 1.13 is designated as Wien displacement law (for numeri-cal values, see Table 1.5).

    With rising temperatures, the short-wavelength ranges increase more strongly, the lon-ger wavelength ranges less strongly than the total radiation. Table 1.6 shows the increaseof radiation bands with temperature for a number of wavelength ranges. It demonstrates

    that only upwards from 3,000 K is there any expectation of UV intensities that can be uti-lized for practical purposes.

    At present, incandescent lamps with tungsten laments reach their technical limita-tion at 3,400 K. In the crater of a carbon arc, 4,000 K can be generated, and, in the visibleand UV-A range, the surface of the sun can be regarded as a thermal radiator of approxi-mately 6,000 K.

    Table 1.6 Cavity radiator (black body)percentage o radiation or dierent spectral ranges

    Tempera-

    ture (K)

    Wavelength range (nm)

    220280 280315 315400 380780 7801,400 1,4003.000

    1,000 1015 1014 109 0.001 0.78 26.2

    1,500 109 108 105 0.17 8.1 47.0

    2,000 106 105 0.002 1.70 21.0 48.8

    2,500 104 0.001 0.033 5.94 32.0 42.1

    3,000 0.004 0.015 0.21 12.7 38.1 33.8

    3,400 0.019 0.058 0.59 19.0 39.7 27.8

    4,000 0.11 0.25 1.72 28.5 38.6 20.5

    5,000 0.74 1.14 5.12 40.6 32.3 12.5

    6,000 2.35 2.73 9.35 46.6 25.3 7.8

    26 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    25/57

    Conventional Incandescent Lamps

    Conventional incandescent lamps are thermal radiators with a coil-shaped tungsten wire.The coil, which incandesces through the passage of an electric current, is mounted in asealed glass bulb with a vacuum or in an inert gas atmosphere. Tungsten is used as coilmaterial because out of all metals, it has the highest melting point (3,653 K).

    The spectral distribution of tungsten is based, like a cavity radiator, also on its temper-ature and follows the Planckian law. However, there are two pieces of missing informa-tion that cause problems when trying to calculate its power distribution in the ultraviolet

    region. One is the spectral inuence of the bulb, and the other is the spectral properties ofthe tungsten surface.6

    The emissivity of Tungsten reaches about 0.400.45 (depending on temperature andwavelength) compared to a cavity radiator; therefore, the radiance emitted by a tungstencoil is only about half that of a cavity (black body) radiator operated with the same tem-

    perature.Conventional incandescent lamps are intended to combine good light-output levels,

    i.e., high coil temperatures and a long service life. However, the two requirements are mu-tually contradictory (see Fig. 1.15), so, by way of a compromise, lamps in general illumi-

    6 There is a law of Kirchhoff that states that absorptivity = emissivity . That means that only atotal black surface (absorptivity = 1) is able to produce full radiation power. The other extremesare ideal white surfaces (absorptivity = 0), which cannot radiate, whatever temperature theyhave.

    Fig. 1.15 Average life of incandescentlamps as functions of temperature, oper-ating voltage, and efficacy

    1 UV Radiation, Irradiation, and Dosimetr y 27

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    26/57

    nation are designed for temperatures below 3,000 K, giving rise in these circumstances toservice lifetimes of about 1,000 h. However, as Table 1.6 shows, at these temperatures, theoutput of UV radiation is so low that this type of lamp is unsuitable in practice as a UV ra-diation source.For photographic and other technical applications, lamps are offered having color temper-atures of up to 3,400 K and UV-transmitting hard glass bulbs. However, their service life isonly a few hours, thus they cannot be used for the purposes of UV therapy.

    For scientic applications, e.g., for the calibration of spectral instruments, there arehighly stable lamps with a tungsten strip in place of the coil and a fused-on quartz windowthat transmits into the UV-C range. These lamps, which are available under the name WI17/G (Trademark OSRAM), do not have a very great intensity but are suitable for radiancecalibrations down to 250 nm.

    Halogen Incandescent Lamps

    At high temperatures, tungsten vaporizes and deposits on the internal bulb wall. This re-sulting blackening of the bulb can be reduced by a high pressure of a lling gas; but lim-its are set for any possible increase by reason of the restricted mechanical strength of thepear-shaped bulbs of standard incandescent lamps.

    In order to minimize this bulb blackening, it is possible to add a small quantity of hal -ogen (uorine, bromine, iodine) to the lling gas. At lower temperatures, occurring in the

    vicinity of the bulb wall, the halogens form together with tungsten gaseous halides, whichare then decomposed again at high temperatures in the vicinity of the coil. In this process,the tungsten is deposited on the coils, and the halogens are available again for repetitionsof this process (halogen circuit process).

    Two further advantages are achieved by this technique: the bulbs can be made to besubstantially smaller, and in that small, heavy bulb, the pressure of the lling gas may bedrastically increased; this, in turn, leads to a marked reduction in the vaporization of tung-sten. Accordingly, for the same coil temperature, halogen incandescent lamps last longerthan standard incandescent lamps. Conversely, where a shorter lifetime is still acceptable,

    the coil temperature and thus the UV emission can be increased. Depending upon the in-tended area of application, these lamps are supplied with envelopes of quartz glass, hardglass, or doped glass bulbs.7

    7 In spite of the advantages of the halogen incandescent lamp, even this type dies in accor-dance with the classical aging mechanisms: incandescent coils are hottest at the centre and va-

    porize most tungsten there. However, high vaporization leads to a reduction in cross-section of

    the tungsten wire, which in turn leads to an increase in temperature, until the coil nally burnsthrough at this position in these circumstances. Unfortunately, even the halogen circuit cannot

    prevent this process, for the vaporized tungsten is not actually deposited at its starting point, butpreferentially at the cold coil ends and the current leads, where it is not of benet either to thegeneration of radiation or to the regeneration of the coil.

    28 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    27/57

    The small dimensions of halogen lamps make them well suited for incorporation in re-ectors. They were built in low-voltage designs up to 24 V, already mounted and ready foruse, with reector surfaces of differing spectral reection factors, both with parabolic andelliptical contours, where paraboloids are used to perform irradiation tasks, and ellipsoidsfor injection into light guides.

    In the medical sector, the UV-A radiation of halogen incandescent lamps is currentlyused in dental engineering to harden plastic materials. This ultraviolet special design re-ects mainly the range 400500 nm and is transmissive for visible and infrared radiation.As a result, both glare and thermal loading are reduced within the irradiation eld.

    The most important technical properties of halogen incandescent lamps are summa-rized in Table 1.7.

    Discharge Lamps

    Discharge lamps generate radiation when current ows through gases or metal vapors. Toconvert the initially non-conductive gas to a conductive state, every discharge lamp needsto be ignited. This takes place by means of high voltages, which are briey applied andwhich are generated by specially designed starters or ignition systems. The higher the va-

    por pressure in an arc tube, the longer is the warm-up time. This warm-up time can take upto 15 minutes; exact information can be found in technical specications.

    In contrast to incandescent lamps, in which the tungsten resistance increases as the cur-

    rent increases, and which accordingly become self-stabilized at any lamp voltage, dis-charge lamps have a so-called negative currentresistance characteristic. That is to saythat the greater the lamp current, the smaller the resistance of the discharge path becomes.This, in turn, results in a further increase of lamp current. Accordingly, without externalcurrent control, discharge lamps are destroyed through overheating, even after the brief-est period of operation. To limit the current, all discharge lamps must be operated with acontrol gear.

    In their simplest form, ballast systems are resistors that are designed in such a waythat a predetermined current ow becomes established within the lamp. If this is achieved

    Table 1.7 Important technical properties o halogen incandescent lampsProperty Value

    Operating voltage 6240 V

    Power level, low voltage types 5150 W

    Power level, high voltage types 2520,000 W

    Color temperature 2,6003,400 K

    Average lifetime 254,000 h

    Low voltage operation Using conventional or electronic transformers

    High voltage operation Directly on the line voltage

    1 UV Radiation, Irradiation, and Dosimetr y 29

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    28/57

    by ohm resistors, these consume in the course of their operation a lot of electrical power,which may even extend the lamp power. Besides the poor economy of such an operationmode, the large amount of heat generated is also a disturbing effect. This type of currentlimitation is used only in the case of lamps with wattages lower than 5 W or for specialapplications. For example, ULTRA VITALUXreector lamps are radiators for cosmeticand therapeutic applications and are intended to generate ultraviolet and short-wavelengthinfrared radiation simultaneously. The source for UV radiation is a mercury high pressurearc tube; the infrared radiation is generated by a tungsten coil, which additionally per-forms the function of lamp ballast resistance (Fig. 1.16).

    In traditional systems, however, inductive coils or capacitive condensers are used asballasts, because in the case of AC impedances, the losses can be reduced down to 1530%of lamp power.

    For about 15 years, electronic adapter systems have also been commercially available,

    reducing the power losses further down to 310%. In addition, these systems are substan-tially lighter than the heavy, iron-jacketed chokes, they ignite the lamp more rapidly, andthe lamps give a nearly steady light without ickering since the lamp operates not at linefrequency, but at frequencies of a few kilohertz or at rectangular current characteristics.

    Discharge lamps are supplied in many sizes and designs. The principal classicationcriterion is the nature of radiation generation and thus the dominant spectral output of thelamps. The following types are commercially available:

    Low pressure mercury lamps (without phosphors)High pressure mercury lamps

    Low pressure sodium lampsHigh pressure sodium lampsMetal halide lampsHigh pressure xenon lampsHigh pressure krypton lamps

    Fig. 1.16 Spectral distribution of an ULTRA-VITALUX UV/IR radiator (black radiation of amercury tube; gray radiation of an incandescent coil)

    30 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    29/57

    With the exception of sodium lamps, which have low UV emissions and therefore will notbe considered, the following sections describe all discharge lamps with UV transmittingbulbs that can also be used as ultraviolet radiation sources.

    Low Pressure Mercury Lamps (Without Phosphors)

    Low pressure mercury vapor lamps are tubular radiators. They are available as bar-shapedlamps, as tubes arranged in pairs, and as compact uorescent lamps (hairpin lamps). Theygive a pure line spectrum, with line widths of only fractions of a nanometer. A typical fea-ture of these types of lamp is the high radiation output in the UV-C range; up to 30% of theelectrical power taken up is emitted by one line at 254 nm. Lines of less intensity appear

    between 240 and 200 nm, followed by the mercury resonance line at 185 nm, the intensity

    of which amounts to 1020% of the radiation of the 254 nm line and which is responsiblefor the generation of ozone, since it is absorbed by the oxygen of the air.

    The lamps are therefore supplied in an ozone-free (OFR) design (bulbs that transmitonly up from 220 nm) and in ozone-generating (OZ) versions (bulbs that also transmit the185 nm line). The main area of application for the OZ types is the sterilization of water.Water also transmits radiation below 200 nm, so that microorganisms dispersed in watercan be inactivated by high energy radiation of the short-wavelength UV-C lines.

    The radiance of these lamps is low. Thus, they are less suitable for use in reectors.However, they generate only a small amount of heat and accordingly can be used at short

    distances for the generation of high irradiance.Further areas of application besides the sterilization of water are the sterilization of air,

    deodorization of air, and the sterilization of the surfaces of pharmaceutical products.The technical properties of low pressure mercury vapor radiators are summarized in

    Table 1.8.

    Table 1.8 Properties o low pressure mercury vapor radiators

    Property Value

    Wattage, compact lamps 524 W

    Wattage, L-lamps 4115 W

    Lamp length 7120 cm

    Radiant power of the 254-nm line 135 W

    Supply voltage 125240 V

    Operation Using magnetic ballasts or electronic adapter systems. A starter is necessary for ignition.

    Brand designations TUV radiators, HNS radiators

    1 UV Radiation, Irradiation, and Dosimetr y 31

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    30/57

    High Pressure Mercury Lamps

    These are compact lamps, in most cases having an ellipsoidal bulb, containing a so-calledburner lled with mercury. Just like all discharge lamps, this type also has to be operatedwith current-limiting adapter systems because of the negative currentvoltage curve. Afterignition, which takes place automatically by means of a built-in ignition probe, a few min-utes are required before the mercury has fully vaporized and the vapor pressure of some

    bar has been built up. This high mercury pressure has two effects: the discharge is con-stricted in the middle of the discharge vessel, so that a high radiation concentration (i.e., ahigh radiance) occurs there. The center of the spectral emission is shifted, compared withthe low pressure discharge, from the 254 nm line to the UV-A line at 365 nm and the visi-

    ble lines at 405 nm, 436 nm, 546 nm, and 578 nm. Thus, by reasons of its simple construc-tion and, compared to the incandescent lamp, its high light output, the high pressure mer-

    cury lamp was one of the rst discharge lamps to be used for illumination tasks. This lampis also economical, has a long service life of many thousands of hours, and is accordinglystill used even in the therapeutic treatment of psoriasis nowadays. Lamps are available onthe market under the brand names HOK or HQA at the power levels 1251,000 W.

    A special design of a high pressure mercury lamp is the already-mentioned ULTRA VI-TALUX radiator, in which the burner is integrated into a mushroom-shaped reector. Witha total power take-up of 300 W, it can be screwed into any normal lamp mounting (E27)and, by reason of this simple operation, is even nowadays still a widespread system forhome therapy and for cosmetic applications.

    High pressure mercury lamps are manufactured at the power level of 125 W, also witha so-called black glass bulb, which absorbs the visible lines and transmits only the UV-Aline at 365 nm. By reason of uorescence effects which thus are made noticeable, the lampis also used in diagnostics.

    Mercury high pressure lamps for illuminating purposes are unsuitable for UV applica-tions because their outer bulb, in some cases additionally covered with phosphors to con-vert UV into light, allows emitting UV radiation only in an insignicant quantity. Table1.9 details the technical attributes of high pressure mercury lamps.

    Table 1.9 Properties o high pressure mercury lamps

    Property Value

    Wattage 1251,000 W

    Lamp lengths 130390 mm

    Efcacy UV-A, -B, -C Each 5% (max)

    Supply voltage 125230 VOperation Using chokes. At a line voltage lower than 220 V, a com-

    bination of transformer and magnetic ballast is required.

    Brand designations HOK, HQA, HQS, HQV radiators

    32 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    31/57

    Metal Halide Lamps

    These are likewise high pressure mercury lamps, but with metal halides added. These ra-diators emit not only the mercury spectrum but also the typical spectrum of the metals in -troduced, in most cases consisting of many lines. The admixing of various halides pro-vides the lamp developer with the possibility of generating almost any desired spectrum.By means of iron, nickel, and cobalt halide additives, the spectral range between the UV-Aand -B lines is lled in to such an extent that an almost continuous spectrum is emitted be-tween 280 and 450 nm. A higher radiation output in the ultraviolet range is also associatedwith this improvement in the spectral progression. Accordingly, this lamp type has dis-

    placed the pure mercury lamps to a large extent.These types of lamp are supplied mostly only as bare arc tubes (discharge vessels)

    without an outer bulb. The small dimensions, the short discharge paths between 1 and 3

    cm, and thus the association with high radiance make these lamps particularly suitable forbuilt-in reectors. Accordingly, their application resides principally in relatively large ir-radiation systems, which allow high irradiances, even at relatively large distances fromthe source.

    The arc tubes of the halide UV radiators are produced from two different types of glass:rstly, as a UV-AB radiator with a short-wavelength transmission limit at 280 nm and in aUV-ABC design with a special quartz glass, which also transmits UV-C as far as 250 nm,

    but holds back the ozone-generating radiation, which starts with wavelengths < 230 nm.The spectra of this type of lamp are shown in Fig. 1.17.

    If these lamps are used in therapy or for cosmetic applications, it is absolutely neces -sary to give consideration and information to appropriate ltering because of the high ra-diation intensity of these lamps, to avoid undesired radiation effects. Information on thesubject should be requested from the manufacturer of the system or radiator. Table 1.10shows the typical properties of this type of lamp.

    Fig. 1.17 Spectra of an UL-

    TRAMED metal halideUV radiator in UV-A, -B,and -C design. Thick linesshow the UV-A and -Bversion

    1 UV Radiation, Irradiation, and Dosimetr y 33

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    32/57

    Short Arc Mercury Lamps (Maximum Pressure Lamps)

    These lamps have such small arc tubes that they behave almost as point radiation sources.

    Their radiance is comparable to that of the sun in some UV ranges. This property makesthem also suitable as a radiation source in uorescence microscopy and in uorescenceendoscopy. They are also successfully employed as irradiation sources of the monochro-mator, allowing the generation of high irradiances with sufcient high spectral purity for

    photobiological investigations.The lamps require a few minutes to start while the mercury vaporizes. The spectra of

    these lamps show, indeed, radiation maxima in the spectral region of the mercury lines.The line contour, however, at the pressures of some hundred bar prevailing in the lamp, is

    broadened to such an extent that it is possible to refer to an almost continuum-like spec-

    trum. The tails of the lines overlap, so that within the entire range, useful radiation is pres-ent at every wavelength, in contrast to low and high pressure lamps, showing low-radia-tion regions between the dominant lines (see Fig. 1.18).With some low power lamp types, a typical arc length is only fractions of a millimeter (thesmallest eld of illumination is only 0.25 0.25 mm2 in magnitude), but an arc length of4 mm is not exceeded, even in the case of the larger types.

    These lamps have ideal characteristics for injecting high radiation intensities into lightguides with small diameters. Built in ellipsoidal mirrors, the total eld of luminance of thelamp may be imaged on the entrance surface of a light guide on the scale 1:1 with only

    slightly attenuated intensity and may be transmitted with low loss if aperture angles of thereectors used are suitably designed. Such combinations are already supplied in preas-sembled mountings.

    No special UV types are offered, but all types have sufcient energy in UV-AB for UVapplications. Typical properties of the lamps are outlined in Table 1.11.

    Table 1.10 Typical properties o metal halide lamps

    Property Value

    Power level 1502,000 W

    Lamp length 50200 mm

    Supply voltage 125400 V

    Area of the elds of illumination 0.55 cm2

    Operation Possible only if using special control gears.

    Ignition Two versions are offered: cold ignition (the lamp can beignited only when cooled down) and hot ignition (after

    being switched off, the lamp can be ignited again immedi-ately). Since in this case high frequency ignition voltages of

    several thousands of volts are used, appropriate screen-ing for protection against leakage pulses is required.

    Brand designations ULTRAMED, ULTRATECH, HMI-S, MSR, HPA, and HPI lamps

    34 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    33/57

    Xenon Short Arc Lamps

    These radiation sources provide high radiance with a daylight-like spectrum in the visiblerange. In industrial applications, they are mainly used for lm projection and for solar sim-ulation. In the scientic sector, they are used in photochemistry, for analytical measure-

    ments, and also as monochromator irradiation sources for photobiological investigations.Just like the spectrum of the sun, the lamp emission proceeds almost equally energet-

    ically within the entire visible range; only within the range between 450 and 500 nm arethere a few band-like peaks. This reveals that the color temperatures of the xenon radiationand of the sun are almost identical (5,800 K) (Fig. 1.19).

    Fig. 1.18 UV mercury line at 366 nm oflow- (black), high- (gray), and maximum-(light gray) pressure lamps

    Table 1.11 Typical properties o short arc mercury lamps

    Property Value

    Power level 508,000 W

    Lamp length 545 cm

    Area of the elds of radiance 0.064 mm2

    Electrical supply Direct or alternating current, depending upon the type

    Operation Using special systems

    Brand designations CS lamps, HBO lamps

    1 UV Radiation, Irradiation, and Dosimetr y 35

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    34/57

    In the ultraviolet range as well, the xenon discharge shows a purely continuous spectrumwith an intensity related to the visible range, even greater than in the solar spectrum. Theintensity declines slowly towards the shorter wavelengths and is limited only by the trans-

    mission of the bulb material used. Using appropriate transitive quartzes as bulb material,radiation may still be detected down to 170 nm.

    The discharge vessel contains only the rare gas xenon. Accordingly, just like incandes-cent lamps, xenon lamps give full radiant power within a few seconds after ignition.

    Further important characteristic properties of these lamps are a high constancy of spec-tral radiation distribution and only a slight decline of radiant power over the entire life ofthe lamp. These properties and the high radiation intensity make xenon short arc lampswell-proven reference radiation sources for the entire ultraviolet range. However, partlyfor reasons of cost but also due to the lifetime (maximum of about 3,000 h), they are used

    in irradiation therapy only in special cases.In a similar constructive design, there are krypton lamps, which exhibit a marked ra-

    diation center at 220 nm. Table 1.12 highlights some of the important properties of xenonshort arc lamps.

    Fig. 1.19 Spectral power distribution of a xenon short arc lamp and global(sun + sky) solar radiation. Both distributions are referred to by the samephotometric value

    36 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    35/57

    Fluorescent Lamps

    The primary radiation source in uorescent lamps is a low-pressure mercury discharge.The line spectrum of this discharge exhibits two strong lines in the UV-C range, at 185 nmand 254 nm, in which approximately 50% of the energy emitted as radiation is contained.The remainder of the energy is distributed among the UV-A and -B range (~20%), the vis-

    ible (~20%), and some weak lines in the IR.Adhered to the internal surface of the bulb wall are uorescent substances in a thick-

    ness of 2030m, converting the short-wavelength UV lines into longer-wavelength UVor visible radiation. Almost every UV quantum is converted into a photon with longerwavelength. The energy content of the photons converted is, however, in accordance withthe Einstein relation, lower than that of the exciting radiation. Generalizing, one can say amaximum of 50% of the radiation generated in the mercury discharge is available as uo-rescence radiation. Nevertheless, uorescent lamps are one of the most economical typesof lamp.

    Fluorescent lamps are mainly used in general lighting applications. The UV radiationof these so-called white lamps starts with low intensity at about 300 nm and is governed byspectral bulb properties such that the currently stipulated limits for protection from radia-tion dangerous to health will not be exceeded. By using appropriate bulb material, it is ad-ditionally ensured that the exciting UVC lines will not penetrate to the outside.

    For UV applications and for phototherapy, lamps are supplied with various spectra (seeFig. 1.20).

    Lamps whose radiation centers lie within the UV-A range are used in photochemo-therapy, in phototherapy, and, in large unit numbers, in solaria and sun beds. In the case

    of these lamps, the emission maximum is between 350 and 370 nm; some types also havetails extending into the UV-B range, but all still have residual radiation in the violet and

    blue and are therefore clearly distinguishable, even visually by their bluish light color,from white-colored uorescent lamps.

    Table 1.12 Typical properties o the xenon short arc lampProperty Value

    Power level 7512,000 W

    Lamp length 848 cm

    Area of the elds of illumination 0.2540 mm2

    Electrical supply Direct current

    Operation Using special systems

    Brand designations XBO lamps, CSX lamps

    1 UV Radiation, Irradiation, and Dosimetr y 37

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    36/57

    Fluorescent lamps for UV-A applications also exist with semilateral reector layers. Bythis means, the lamps can be mounted closely side by side in irradiation systems, and the

    irradiation intensity can be increased up to twofold as compared with systems having ex-ternal reectors.

    Only an indication of the suitability of the individual types for specied practical appli-cations may be taken from the graphical representations of the radiation spectra. As a re -sult of the differing progressions of action curves, the center of an action response can be

    Fig. 1.20 Spectra of UV-B,-AB, -A, and -A1 fluorescentlamps

    38 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    37/57

    displaced markedly, so that only a spectral evaluation can give clear information on the ac-tive photobiological output.

    In terms of the lamp shape, a distinction is drawn between line-shaped, ring-shapedand U-shaped lamps. During recent years, compact lamps have been developed from the

    U-shaped uorescent lamps, and these are approximately of the same size as incandes-cent lamps; by reason of their high light output and longer life spans, they are increasinglygaining acceptance in areas that were previously reserved for incandescent lamps. Thesecompact uorescent lamps are also manufactured for special UV applications, e.g., fortreating psoriasis of the scalp. Table 1.13 outlines a number of the characteristics of uo -rescent lamps.

    Solid-State UV Radiation Sources

    Due to the development of light-emitting diodes (LED) for use in lighting applications,UV-emitting solid state devices (here named UVED) also have been produced since2001. These UVEDs can be fabricated from AlInGaN materials using advanced semi-conductor epitaxial growth methods [4]. Their spectral emission characteristic is simi-lar to visible LED and exhibits a bell-shaped emission curve over about a 20-nm spec-tral spread. Emission maxima reported lie at 247 nm, 254 nm, 264 nm, 280 nm, 304 nm,338 nm, and 365 nm. In 2004, for the rst time, a UVED with emissions at 280 nm and aradiant power of more than 1 mW were realized. Due to conventionally used UV sources,

    this means much less UV radiant power, but typical advantages of this technology, likesmallness, robustness, and fast modulation times (nanosecond response time) and the abil-ity of making surface contact irradiations due to very small heat production, make theseradiation sources really interesting for some special medical and technical applications inthe future.

    Table 1.13 Typical properties o fuorescent lamps

    Property Value

    Power level, compact lamp 7120 W

    Power level, tube 40100 W

    Lamp length, compact lamp 1424 cm

    Lamp length, tube 60180 cm

    Supply voltage 125230 V

    Operation Using magnetic ballasts and starters or usingelectronic adapter systems. All systems must

    be approved for the respective type of lamp.

    Brand designations Light colors 78 and 79, EVERSUN SU-

    PER TL/10, 12, TL/09 CLEO

    1 UV Radiation, Irradiation, and Dosimetr y 39

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    38/57

    Influences That Can Change the Radiation of a Lamp

    Lamp specications include lamp data for operating under normal conditions. But, de-pending on time, temperature, or mechanical and electrical adjustments, variations of thelamp emission can result. Sometimes, the bulb material will change without consequencesin the visible range, but it is possible by this constructive measure that the UV radiationwill be changed.

    Some of these inuences can be negated, for example, by prolongation of the irradia -tion time or by better cooling. With others effects, nothing can be done. The following sec-tions, therefore, shall provide some remarks about what kind of effects these inuencescan bear [1, 3, 8].

    Bulb Material and Its Influence on UV Emission

    Untreated bulb glass has a spectral transmission curve in the form of a cut-on lter, i.e.,it does not transmit short-wavelength radiation. It starts transmitting radiation in a wave-length range typical of the material and stays transmissive up to the short-wavelength IR.Figure 1.21 shows, in this respect, a few typical examples.

    Depending upon the composition of the glass, the cut-on may lie within the vacuumUV below 180 nm, but also in the UV-B range, at 315 nm. In the UV-A range, up from

    340 nm, untreated bulb glasses and broad glasses are always transmissive.It is possible to characterize a cut-off lter by three wavelengths that correspond to

    transmission values of 1%, 50%, and 90%. In Table 1.14, these values are presented forfrequently employed types of bulb glass.Principally, bulb material is selected for its process capability and its thermal and chemicalresistance. Only when possibilities for further selection criteria still remain, can the spec-tral transmissivity also be included in the deliberations. In many cases, however, the tech-nological and optical requirements cannot be satised together, so it is necessary to treatthe glass in some special manner.

    Fig. 1.21 Typical progression of the spectral transmission of bulb glasses in UV

    40 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    39/57

    When assessing the spectral transmission values of bulb glasses, it is necessary to giveconsideration to the temperature dependence of this quantity. The customary list onlyspecies the values measured at room temperature, whereas the properties at operating

    temperatures are of importance to the user. All bulb glasses have a tendency to shift theirtransmission cut-on towards the long-wavelength direction as the temperature rises.

    The magnitude of this displacement depends upon material properties and can be be-tween 0.03 and 0.15 nm per 1C temperature rise. At temperatures of 800C, which occurin the case of tungsten halogen incandescent lamps and high pressure burners, displace-ments of up to 100 nm are therefore possible.8

    This cut-on displacement is also signicant with regard to the question of whether alamp emits UV-C or UV-B. According to brochure details in the case of the majority of

    bulb glasses, this would have to be the case. In fact, however, as is shown by the spectra

    of the lamps, the UV components are substantially smaller than would be expected on thebasis of these curves.

    8 The magnitude of the effects of the phenomenon of cut-on displacement in the event of a tem-perature rise may even be demonstrated without measuring equipment in the case of xenonlamps with quartz glassUltrasil bulbs. Xenon lamps have only a very short warm-up period,so that the full spectrum is emitted already when the lamp is switched on. According to Table1.14, Ultrasil transmits to below 200 nm at room temperature, and, since xenon also radiateswithin this range, ozone-generating radiation must emerge after ignition. This fact is rapidlyrecognized by the typical odor of ozone, which, however, disappears again after a brief pe -riod. (Ozone is an odorless gas. The pungent odor originates from oxides of nitrogen which areformed together with the ozone.) Since, however, the upper limit for the generation of ozone byradiation is approximately 240 nm, the transmissivity must have been shifted beyond this range

    by heating of the bulb material.

    Table 1.14 Spectral transmission o some bulb glasses

    Type of glass Wavelength (nm) for the spectral transmission of

    1% 50% 90%

    Broad glass d= 2.5 mm 295 330 360

    Broad glass d= 5 mm 310 340 380

    Soft glass/soda glass 290 330 350

    Hard glass/silicate glass 260 290 340

    Vycor glass 210 230 250

    Quartz glass 175 190 240

    Suprasil quartz 165 175 220

    1 UV Radiation, Irradiation, and Dosimetr y 41

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    40/57

    Alteration of Bulb Transmissivity by Thin Layers

    By using distinctive techniques likeAlteration of the glass wall thickness,Staining within the mass, andColoring the surface,

    the transmission of the bulb material can be altered. These processes are mainly used inthe visible range. By reason of the temperature rise associated therewith, these techniquescan be used only for lamps that are not under excessively high loading.

    By coating with thin, interfering layers, it is possible to alter the transmission proper-ties of the bulbs in a variety of ways. As the effect is not achieved by absorption, but byselective reection, it does not bring about any direct rise in temperature of the bulbs and

    can therefore be used in the case of lamps under high loading, by reason of the high ther-mal stability of these layers as well. Cut-on replacements are possible, as well as the selec-tion of specied spectral ranges.

    Besides the possibility of altering the color of light, there are at the present time twofurther areas of application of coating techniques. In the rst, a layer reecting only inthe infrared is applied to incandescent lamp bulbs (Infrared Reective Coating; IRC lay-ers), by which the thermal radiation is retained within the bulb. This improves the radia-tion output of the lamps, since less electrical energy needs to be supplied for the same ra-diant power. The energy saving achieved thereby amounts to approximately 30% at the

    present time.In the second area of application, the layer reects approximately the range of one

    wavelength octave and is transmissive for the adjacent ranges. Applied to reectors, onlyradiation of the desired spectral range is then present in the main beam path, while the re-maining radiation is transmitted or absorbed within the glass body. The preferred radiationmay lie within the UV-A range (see Fig. 1.22), within the visible, or within the near IR.

    Fig. 1.22 Alterationof the lamp spec-trum by spectralselectively actingreflectors

    42 L. Endres, R. Breit

  • 7/29/2019 UV Radiation Irradiation Dosimetry

    41/57

    Alteration of the Bulb Transmissivity by Doping

    A further possibility for reducing the excessively high UV transmissivity of a bulb is theso-called doping of the glass. Doping can take p