The Light Chapter-1

7/30/2019 The Light Chapter-1

1/25913LIGHTING ENGINEERING 2002

Chapter 1.

THE LIGHT

1.1. General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.2. Wave characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.3. Frequency spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.4. Dual nature of light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18


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1.1. General remarks

It is well known that there are several types of energy: mechanical, thermal, electrostatic and electromagnetic.

If m echanical energy is applied to a body at rest, it tends to set into motion, thus, transforming the energy applied into kinetic energy.

This energy is taken along and is also transmitted to other bodies, in case it collides with them.

Heat is a form of energy which diffuses through convection, conduction or radiation.

When a switch is "turned on", the metallic filament of an incandescent lamp is connected by means of a potential difference. Thus,

electric charge flows through the filament in a similar way pressure difference in a hosepipe makes water flow through it. Electron flow

constitutes the electric current. Current is usually associated to charge movement in bridge conductors, but electric current emerges

from any charge flow. When electric current diffuses through conductors and reaches a receptor, this receptor is transformed into

another type of energy.

If the body or the emitting source irradiates energy, propagation takes place by means of radiation in the form of waves* which are

those physical disturbances which diffuse in a certain medium or in the vacuum.

Mechanical waves diffuse this kind of energy through an elastic material medium. They are longitudinal sound waves because particle

vibration coincides with their propagation direction. Two examples of this phenomenon are vibrations of spring and sounds. In a spring,

vibrations propagate in only one direction. In the case of sound, vibrations propagate in three different dimensions.

Electromagnetic waves propagate the energy produced through oscillations of electric and magnetic fields and do no t need a

propagation material medium. For example, the light.

O ut of the di fferent ways waves propagate, there are several regimes. From the point of view of lighting engineering, the periodical

regime is the one which interests us. It may be defined as regular time interval repetitions and expressed graphically as several wave

forms.

Thus, wave form represents oscillations as phenomena in which physical quantity is a periodical function of an independent variable ( time) ,

whose average value is null. That is to say, we are talking about simple or fundamental harmonic functions, like the sine or the cosine, of a

single, one-dimensional and transversal variable ( propagated perpendicularly to the direction in which particles vibrate) . In short, there is a

wide range of physical, electric and electromagnetic phenomena, among which electricity, light, sound, hertzian waves or sea waves are

included. Their characteristics are determined by studying sine waves. This is the reason why the concept of wave radiationand

characteristics to define them is used.

1.2. Wave characteristics

Wavelength ()

It is defined as the distance travelled by a wave in a period. For a transversal wave, it may be defined as the distance between two

consecutive maximums or between any other two points located in the same phase( Fig. 1) .

Figure 1. Wavelength .

* Wave: Graphic expression of a periodic variation represented in ampli tude and time. Amplitude is the maximum value or ordenate taken by the wave.

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Chapter 1. THE LIGHT


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Wavelength is a highly important characteristic in order to classify the visible radiation spectrum, object of study in this section of

LIGHTING ENGINEERING 2002.

This parameter is determined by the result of propagation velocity ( ) , multiplied by the time it takes to cover one cycle ( T Period) :

= ( m/s s = m)

Frequency ( f)It is defined as the number of periods that take place in a time unit.

Since period is inverse to frequency, , the equation above is transformed into:

(m/s 1/s-1 = m)

and, therefore, frequency is directly proportiona l to propagation velocity, and inversely proportional to wavelength.

( s-1 = cycles/second = Hz)

Wavelength decreases when frequency increases.

Frequency is stable and independent from the medium through which the wave propagates. This constitutes an important characteristic

to classify electromagnetic waves.

Propagation velocity ()

Propagation velocity depends on wave type, elasticity of the medium and rigidity. If the medium is homogeneous and isotropic,

propagation velocity is the same in all directions.

For example, sound propagation velocity in the air, at 20 C , is that of 343.5 m/ s, whereas electromagnetic waves propagation velocity

in the vacuum is equivalent to 300 000 km /s = 3 108 m/s.

The fundamental equation which relates propagation velocity to wavelength and frequency is

= f (m s-1 = m/s)

1.3. Frequency spectrum

Given the fact that electromagnetic radiations share the same nature and they all propagate in the vacuum at the same velocity

( = 3 108 m/s) , those characteristics that make them di fferent are their wavelength, that is to say, their frequency( = f) .

Electromagnetic radiations are the following: gamma rays, X-rays, ultraviolet radiation, light, Infrared rays, microwaves, radio waves and

other radiations. The human eye is sensitive to electromagnetic radiation with wavelengths ranging approximately between 380 and 780

nm. This interval is known as visible light. Shortest wavelengths of the visible spectrum correspond to violet light, and the longest, to

red light. Between these two extremes are all the colours found in the rainbow ( Fig. 2) . Electromagnetic waves have slightly shorter

wavelengths when compared to visible light and are known as ultraviolet rays. Those with slightly longer wavelengths are known as

infrared waves. Thermal radiation emitted by bodies at a normal temperature is placed in the infrared region of the electromagnetic

spectrum. There are no lim its in electromagnetic radiation wavelength, which is the same as stating that all wavelengths ( or frequencies)

are possible from a theoretical point of view.

It m ust be taken into account that those wavelength intervals (or frequency ones) in which the electromagnetic spectrum divides

sometimes are not well defined and often, they overlap. For example, electromagnetic waves with wavelengths of the order of 0.1 nm.

are frequently named X-rays. Nevertheless, if originated from nuclear radioactivity, they are called Gamma rays.

f

=

= f

= 1f


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Figure 2. Classification of visible spectrum.

Lamp manufacturers usually give radio spectrometrical curves with values raging between 380 nm. and 780 nm. As we have shown,

apart from the meter, nanometer (nm.) is also used in order to express wavelengths, as well as other units like Angstrom ( ) and micron

(m.).

1 m. = 10-60 m

1 nm . = 10-90 m

1 . = 10-10 m

Radiation of a continuous spectrum source

All bodies radiate energy in an ample field of wavelength at any temperature except for absolute zero. This radiation is known as

incandescenceor temperature radiation. Sources of incandescent artificial light are:

- A flame from combustion, like a candle, oil candle, etc.

- A red-hot ingot or steal bar.- An incandescentlamp filament, as the most common source to produce artificial light.

Incandescence is applied to types of radiation associated with temperature.

The spectroradiometer is used to know how the radiated potency is distributed between wavelengths. The spectroradiometrical function

or spectral distribution curve obtained is indicated in Fig. 3. Wavelengths in nm. are placed in the abscissas, and values related to energy,

with respect to the maximum radiated understood as 100% , are placed in the ordinates.

300 nm.Black light

Infrared

Violet

Indigo

Blue

Green - Blue

Green

Green - Yellow

Yellow

Orange

Red

Ultraviolet rays790x1012 Hz

400x1012 Hz

384x1012 Hz

370x1012 Hz

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360

380

400 nm.420

440

460

480

500 nm.

520

540

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580

600 nm.

620

640

660

680700 nm.

720

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760

780

800 nm.

Vis

iblelightspectraldistribution

DSpectral

distributionaccordingtolampmanufacturer





Figure 3

Radiation of a discontinuous spectrum source

Radiant energy of a gaseous discharge source, such as the ones of high pressure sodium, high pressure mercury, argon, neon, etc.,

consists in a radiation integrated by small wavelength intervals which may be called emission peaks.

Each gas has a wavelength characteristic of its own radiation which depends on the gas molecular structure through which discharge

takes place. This kind of discharge is usually called luminescence and it is characterised by temperature independent radiation types.

The most common luminous sources or discharge lamps are fluorescent tubes: high pressure mercury, high pressure sodium and

induction ones.

As for incandescence, the spectroradiometer is used to obtain the spectral distribution curve. The spectroradiometer function obtained

is indicated in Fig. 4. Wavelengths in nm. are placed in the abscissas, and values related to energy, with respect to the maximum radiated

understood as 100% , are placed in the ordinates.

Also, the specific potency in mW/nm.wavelength is usually given in the ordinates.

Figure 4

1.4. Dual nature of light

Light has intrigued humankind for centuries. The most ancient theories considered light as something emittedby the human eye. Later

on, it was understood that light should come from the objects seen and that it entered the eye producing the feeling of vision. The

question of whether light is composed by a beam of particles or it is a certain type of wave movement has frequently been studied in

the history of science. Between the proponents and defendants of the corpuscular theory of light , the most influential was undoubtedly

Newton. Using the above mentioned theory, he was able to explain the laws of reflection and refraction. Nevertheless, his deduction of

the law of refraction was based on the hypothesis that light moves more quickly in water or in glass than in air. Some time later, the

hypothesis was proved to be wrong. The main proponents of the wave theory of light were Christian Huygens and Robert Hooke. Using

380nm.

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400nm.

500nm.

600nm.

700nm.

780nm.

380nm.

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400nm.

500nm.

600nm.

700nm.

780nm.

380nm.

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400nm.

500nm.

600nm.

700nm.

780nm.

380nm.

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%%

400nm.

500nm.

600nm.

700nm.

780nm.

Spectral distribution for a cold white coloured fluorescent lamp Spectral distribution for a high pressure mercury lamp

of corrected colour

380nm.

20

40

60

80

100

400nm.

500nm.

600nm.

700nm.

780nm.

380nm.

20

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400nm.

500nm.

600nm.

700nm.

780nm.

Spect ral dist ribut ion fo r a normal day l ight Spect ral dist ribu tion for an incandescent lamp

380nm.

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100

400nm.

500nm.

600nm.

700nm.

780nm.

380nm.

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780nm.

%%



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their own theory of wave propagation, H uygens was able to explain reflection and refraction supposing that light travels more slowly in

glass or in water than in air. Newton realized about the advantages of the wave theory of light, particularly because it explained colours

formed by thin films, which he had studied very thoroughly. Notwithstanding, he rejected the wave theory due to the apparent rectilinear

propagation of light. In his time, diffraction of the luminous beam, which allows to evade objects, had not yet been observed.

Newton's corpuscular theory of light was accepted for more than a century. After some time, in 1801, Thomas Young revitalized the

wave theory of light. H e was one of the first scientists to introduce the idea of interference as a wave phenomenon present both in thelight and in the sound. His observations of interferences obtained from light were a clear demonstration of their wave nature.

Nevertheless, Young's research was not known by the scientific community for more than ten years. Probably, the most important

breakthrough regarding a general acceptance of the wave theory of light is due to the French physicist Augustin Fresnel ( 1782-1827) ,

who conducted thorough experiments on interference and diffraction. He also developed a wave theory based on a solid mathematical

foundation. In 1850, Jean Foucault measured the speed of light in water and checked that it is slower than in air. Thus, he finally

destroyed Newton's corpuscular theory of light. In 1860, James Clerk M axwell published his electromagnetic mathem atical theory which

preceded the existence of electromagnetic waves. These waves propagated with a calculated speed through electricity and magnetism

laws which was equivalent in value to 3 x 108 m /s, the same value than the speed of light. M axwell's theory was confirmed by Hertz

in 1887 who used a tuned electric circuit to generate waves and another similar circuit to detect them. In the second half of the 19th

century, Kirchoff and other scientists applied M axwell's laws to explain interference and diffraction of light and other electromagnetic

waves and support Huygens' empirical methods of wave construction on a solid mathematical basis.

Although wave theory is generally correct when propagation of light is described ( and of other electromagnetic waves) , it fails when other

light properties are to be explained, specially the interaction of light with matter. Hertz, in a famous experiment in 1887 confirmed

M axwell's wave theory, and he also discovered the photoelectric effect. Such an effect can also be explained by means of a model of

particles for light, as Einstein proved only a few years later. This way, a new corpuscular model of light was introduced. The particles of

light are known as photons and energy E of a photon is related to frequency f of the luminous wave associated by Einstein's famous

ratio E = h f ( h = Planck's constant) . A complete understanding of dual nature of light was not achieved before the 20's in the 20th

century. Experiments conducted by scientists of the time ( Davisson, Germer, Thompson and others) proved that electrons ( and other

"particles" ) also had a dual nature and presented interference and diffraction properties besides their well-known particle properties.

In brief, the modern theory of quantum mechanics of luminous radiation accepts the fact that lightseems to have a dual nature. O n the

one hand, lightpropagation phenomena find a better explanation within M axwell's electromagnetic theory ( electromagnetic wave

fundamental nature). O n the other hand, mutual action between lightand matter, in the processes of absorption and emission, is a

photoelectric phenomenon ( corpuscular nature) .





Chapter 2.

THE EYE

2.1. Human eye as a light reception organ . . . . . . . . . . . . . . . . . . . . . . . . 23

2.2. Structural description of the eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.3. Image formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.4. Eye sensitivity curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.5. Accommodation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.6. Contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.7. Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.8. G lare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


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2.1. Human eye as a light receptor organ

The eye is the physiological organ of sight through which light and colour feelings are experienced. For the lighting process to take place,

as action and effect of illuminate and see, three agents are required:

1) A source producing light or luminous radiation.

2) An object to be illuminated so that it is visible.

3) The eye, which receives luminous energy and transforms it into images which are sent to the brain for their interpretation.

The study and description of eye components, together with the process which takes place since the moment in which light arrives and

goes through the paths and visual centers, until it is interpreted by the brain, would take us to the field of neurophysiology. Some

behaviour and concepts related to the sense of sight will be described and exposed in the present chapter. Their knowledge is

indispensable and contributes to a better design of lighting installations.

2.2. Structural description of the eye

In Fig. 1, a schematic longitudinal section of the human eye is represented, where its anatomic constitution may be observed.

Figure 1. Human eye constitution.

The eye is mainly constituted by the following elements:

a) Eye globe: whose primary function is to form the image on the retina.

b) Cornea: receives and transmits visual impressions and constitutes the eye fundamental optical refractor component.

c) Crystaline lens: is a biconvex, transparent and colorless lens located behind the iris. This elastic membrane changes its form to focus

objectives.

d) Iris: circular lamina located in front of the crystalline lens, and highly pigmented. It can contract the pupil controlling the amount of

light that passes to the crystalline lens.

e) Pupil: circular orifice situated in the center of the iris, and through which light rays pass. The opening of this orifice is controlled by

the iris. Its contraction is called meiosisand its extension, mydriasis.

f) Retina: is the eye inner back film constituted by a nervous membrane, expansion of the optical nerve, whose function is to receive

and transmit visual images or impressions. It contains an extremely thin layer of photosensitive cells, cones and rods, which diverge

from the optical nerve and which are in the external layer, next to the pigmented layer.

Visual axis

Crystalline lens

Vitreous humor

Upper eyelid

Aqueous humor

Cornea

Iris

Ciliary muscle

Lower eyelid Blind spot

Yellow spot

O phthalmic muscles

O phthalmic muscles

O ptical nerve

Retina

ChoroidsSclera


Chapter 2. THE EYE



g) Cones: photosensitive or photoreceptive cells of the retina which are mainly located in the fovea. They are very sensitive to colours

and almost insensitive to light. Hence, their function is to discrim inate fine details and to perceive colours ( Fig. 2) .

h) Rods: photosensitive or photoreceptive cells of the retina which are only outside the fovea and more concentrated in the periphery.

They are very sensitive to light and movement, and almost insensitive to colour. Thus, their function is to perceive more or less

brightness with which objects are illuminated (Fig. 2) .

i) Macule: yellow spot situated in the rear part of the retina, on the optical axis, where a precise and sharp fixation of details and colourstake place. The fovea is in its center which is only formed by cones.

j) Blind spot: a spot in the retina through which the optical nerve drives images or feelings of light to the brain. At this point, there are

no photoreceptors.

Practical consequences of the cone and rod function

When we look at a dimly illuminated space, for example, in the twilight at night, visual acuity is low, because cones do not function and

neither colours nor details are distinguished. This is the reason for the famous saying "no-one will notice in the dark" . This type of night

vision is called scotopicand essentially rods intervene, which collect the greater or lesser amount of light and objects movement with

extreme sensitivity.

This justifies the fact that some public lighting of avenues, roads, and department stores is done with high pressure sodium lamps which

reproduce colours badly, but contribute with a great amount of light.

O n the contrary, with daily light or when illum ination level increases the necessary amount, objects are seen with precision and detai l

also cones, mainly. This way, colours may be distinguished. Daily light is called photopicvision. In this case the quantity requires to be

accompanied by quality, since only quantity would produce irritability in eyes and very disturbing glares.

Figure 2. Eye photosensitive part. Behaviour of cones and rods.

2.3. Image formation

Human beings visual field is limited by an angle of about 130 degrees in a vertical way and about 180 degrees in a horizontal way.

From illuminated objects or those with their own light located in the visual field, luminous rays emerge that go through the cornea and

the aqueous humor. The iris, by means of the opening of the pupil, controls the amount of light which is refracted through the crystalline

lens to reach the retina finally. In this place, the photosensitive pigment of photoreceptors registers in inverted images much smaller

than in reality, as it happens in the photographic camera. O nce images are received and formed in the retina by means of the optical

nerve, they are sent to the brain, which is in charge of interpreting them and modifying their position ( Fig. 3) .

Pigmented cellCone

Rod

Pigment grains

Nerve cell

Retina enlargement

Eye globe

Chapter 2. THE EYE


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Figure 3. Image formation and its rectification in the brain.

The following chart compares the human eye to the photographic camera.

Chart 1

2.4. Eye sensitivity curve

Wavelength radiations ranging between 380 nm. ( ultraviolet) and 780 nm. ( infrared) are transformed by the eye into light. O ut of this

range, the eye cannot see: it is blind and does not perceive anything. All luminous sources have their own radiation or a mixture of them

included within such limits.

A sunny midday white ligh t is the sum of all wavelengths of the visible spectrum. If we try to make them reach the eye independently

and with the same amount of energy, a curve like the one in Fig. 4 is obtained. It has been elaborated by the C.I.E.* measuring a great

number of people.

* C.I.E.: International Comm ission on Illumination (Comm ission Internationale de lEclairage) .

Human eye Photographic camera

Crystalline lens ( controls accommodation) Lens ( adjusts distance between lens and film)

Pupil (controls adaptation) Diaphragm - shutter (adapts exposition and amount of light)

Pigment of photoreceptors Film emulsion

Retina ( creates images) Film ( creates images)


Chapter 2. THE EYE



Figure 4. Eye sensitivity curve to monochromatic radiations.

In this curve, the maximum eye sensitivity for day white light ( photopic) corresponds to a 555 nm. wavelength and to the yellow colour.

The minimum sensitivity corresponds to the red and violet colours.

Hence, luminous sources whose wavelength corresponds to yellow - green are the ones with highest efficacy and worst quality, the

reason being that such light is not appropriate for our eye, which is accustomed to the sun white light. Thus, in premises where there

is a high illumination level orange and red colours are highlighted.

In the case of night light (scotopic) , the maximum of sensitiveness moves towards shorter wavelengths (Purkinje'seffect) . Consequently,

those radiations with a shorter wavelength ( blue- violet) produce greater intensity of sensation with low i llumination. Such an effect is

very important when illuminating premises with a low illumination level where blue and violet colours can be seen better.

2.5. AccommodationIt is the eye capacity to adjust automatically to different distances of objects, and, this way, to obtain sharp images on the retina. This

adjustment takes place by modifying the crystalline curvature and, thus, the focus distance by contracting or relaxing ciliary muscles.

Provided that the objective is close to the eye, the crystalline curvature is greater than when it is far. In the photographic camera, the

lens and the film.

Accommodation or focus is easier with high luminances * ( lighting) which oblige the pupil to adapt or modify the diaphragm towards a

closing position. The common result of this action is the increase of the field depth, or what is the same, a sharp vision of objects at

different distances from the eye or camera.

The eye accommodation capacity decreases with age, as a result of a hardening of the crystalline.

2.6. Contrast

All objects are perceived by contrasts of colour and luminance which different parts of their surface present among themselves and in

relation to the background in which the object appears.

* Luminance: Luminosity effect which a surface produces on the eye retina, whether it comes from a primary source of light or a reflecting surface.

20

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NIGHT DAY

%

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Chapter 2. THE EYE


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For high enough lighting levels, the normal eye is colour sensitive, whereas for low lighting ones, objects are fundamentally perceived

by luminance contrast which is present against the background. The luminance difference between the observed object and its

immediate space is known as contrast.

Figure 5

In Fig. 5, the surface of the object has a luminance " L0" and the background surface has a luminance " L

f" . Therefore, contrast "K " is the

difference between these two luminances, divided by their background one, that is to say:

"K"is, thus, a relative value between luminances.

As we have commented, the visibility of an object over a background, depends on the luminance difference between the object and

the background. For a light coloured object over a dark background, its contrast will be positive ( values between 0 and infinitum) .

However, an object darker than its background will be seen as a silhouette, and its contrast will be negative, varying between 0 and ( -

1) .

Contrast K may be positive or negative:

If L0

> Lf

K > 0 contrast is positive ( the object is lighter than its background) .

If L0

< Lf

K < 0 contrast is negative ( the object is darker then its background) .

Contrast K may acquire the following values:

Positive contrast ( light object) 0 < K < e

N egative contrast (dark object) -1 < K < 0

Example a) in Fig. 6 presents an easily distinguished contrast, whereas b) and c) offer greater difficulty.

Figure 6

There is also a colour contrast. Chart 2 shows some examples.

a b c

K=L0

Lf

Lf

Lf

Lo


Chapter 2. THE EYE



Chart 2. Colour contrasts.

Contrast sensitivity

It is a concept derived from the former one which is equivalent to the minimum contrast of luminances that may be perceived by the

human eye. M athematically speaking, it would be the inverse of contrast.

Therefore, the greatest sensitivity to contrast possible is approximately:

However, in normal practical conditions, sensitivity to contrasts is quite smaller because of the reasons exposed above.

2.7. AdaptationIt is the ability of the eye to adjust automatically to different lighting degrees for objects. It consists of the adjustment of the size of the

pupil so that luminance projected in the retina is equal to a value bearable by sensitive cells. If compared to a photographic camera, it

would be the greater or lesser opening of the diaphragm.

If lighting is very intense, the pupil contracts, decreasing the amount of light that reaches the crystalline. If lighting is scarce, it expands

to capture more of it.

In high value illuminations, the pupil reduces to a diameter of approximately 2 mm. In very low value illuminations, the pupil expands

up to about 8 mm.

When a person moves from a place with high illuminance to another which is completely dark, the eye undergoes an adaptation process.

In order to adjust totally to the new situation, the eye needs 30 minutes. The opposite process, when a person goes from a completely

dark place into another with high illuminance, the adaptation period lasts for only a few seconds (Fig. 7) .

G=1

= 1000.01

G=Lf

=1

L0L

fK

O bject colour Background colour

black yellow

green white

red white

blue white

white blue

black white

yellow black

white red

white green

white black

Chapter 2. THE EYE


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Figure 7. Eye relative photosensitive curve regarding adaptation time.

2.8. Glare

It is a phenomenon that produces disturbance or decrease in the capacity to distinguish objects, or else, both things at the same time.

This could be due either to an inadequate luminance distribution or phasing or to excessive contrasts in space or time.

This phenomenon affects the retina of the eye: an energetic photochemical reaction is produced which desensitizes it for a certain period

of time, after which, it recovers.

Effects produced by glare may be classified as psychological (discomfort) or physiological ( disability) . It may be produced in different

ways: di rect glare, like the one from sources of light ( lamps, luminaires or windows) , which are located within the field of vision. Reflected

glare specially from surfaces with great reflectance, specular surfaces like polished metal.

Sources of light generally give rise to a disability glare which is proportional to the lighting produced by the source of light on the eye

pupil, as well as to a factor dependent on the q angle. Such an angle is formed by both the straight line R which joins the eye withthe F focus and the H horizontal plane which goes through the eye in a working position. In Fig. 8, different glares are indicated,

depending on the angle function. A m inimum value of 30 has been taken as admissible.

Figure 8. Glare according to the q angle.

0 10 20 30 40 50 60

Values for the angle

Glare

H

R

F

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0 10 20 30 40 50

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Adaptation time (min.)

Relativephotosensitivity

%


Chapter 2. THE EYE



Surfaces which are not completely matte give rise to more or less sharp images of their sources of light due to light reflection. Even if their

luminance is not excessive, such images are almost always discomforting when found in the field of vision, and specially, in its central area.

According to these lines, all unnecessary polished surfaces will be avoided as far as possible ( glass over tables, for example.) . In case semi-

polished surfaces are used ( blackboards) , sources of light will have the least possible luminance and their position will be calculated bearing

in mind reflexes that may occur ( filters, grids, diffusers, etc.) . In special cases, images which provide reflection will be useful ( silhouette effect

vision, flaw inspection in polished surfaces, typesetting, etc.) .

Figure 9. Surfaces which reflect light.

Chapter 2. THE EYE



Chapter 3.

MATTER OPTICAL

PROPERTIES

3.1. General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.2. Reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3. Transmmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.4. Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.5. Refraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37


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When a light ray propagates along a medium and reaches the lim it which separates it from the second one, i t may return to it ( reflection) ,

it may strike it and become part of the second medium, where it will be converted into a different form of energy ( absorption) , and

some will not change ( transmission) .

O ut of these phenomena, two or three take place simultaneously. Following the fundamental principle of energy, the sum of reflected,

absorbed and transmitted radiation must equal the incident radiation.

Therefore, the use of light in the most convenient way requires control and distribution achieved by modifying its characteristics through

the physical phenomena of light reflection, absorption and transmission, without leaving aside the fourth factor known as refraction.

3.2. Reflection

When any type of waves strikes a flat surface like a mirror, for example, new waves that move away from the surface are generated. This

phenomenon is known as reflection.

When light is returned by a surface, a certain amount of light is lost due to the absorption phenomenon. The ratio between the reflected

flux and the incident flux is called surface reflectanceAny surface which is not completely dark may reflect light. The amount of reflected light is determined by the surface reflection

properties. There are four kinds of reflection, namely: specular, composed, diffused and mixed. Reflector systems are based on these

reflection properties.

Specular reflection( Fig. 1) : It takes place when the reflecting surface is flat. This kind of reflection is based on two fundamental laws:

1. The incident ray, the reflected ray and the normal to the surface at the point of incidence lie in the same plane.

2. The angle of incidence ( i) is the same as the angle of reflection ( r) .

Figure 1. Specular reflection.

Composed reflection( Fig. 2) : Contrary to specular reflection, there is no mirror image of the light source, but the maximum angle of

reflected intensity is the same as the angle of incidence. This type of reflection takes place when the surface is irregular or rough.

i r

N


Chapter 3. MATTER OPTICAL PROPERTIES



Figure 2. Composed reflection.

Diffused reflection( Fig. 3) : This takes place when the light that strikes a surface is reflected in all directions, the normal ray to the surface

being the most intense one.

This kind of reflection takes place on surfaces such as matt white paper, walls, plaster flat ceilings, snow, etc.

Figure 3. Diffused reflection.

Mixed reflection( Fig. 4) : This is an intermediate kind of reflection between the specular and the diffused reflection, in which some of

the incident beam is reflected and some, diffused. This kind of reflection takes place with non polished metals, glossy paper and

barnished surfaces.

Figure 4. M ixed reflection.



23/259

Chart 1. Reflection coefficient for white daylight.

3.3. Transmmission

Radiation passes through a medium without a change in the frequency of monochromatic radiations. This phenomenon can be seen

on certain kinds of glass, crystal, water and other liquids, and air, of course.

However, when passing through the material, some of the light is lost due to the reflection on the medium surface and through

absorption. The relation between the transmitted light and the incident light is known as material transmittance.

Transmission falls into three categories: spread, diffused and mixed.Spread transmission( Fig. 5) : The beam strikes a medium and passes through it. The media which fulfill this property are called

transparent materials and allow a sharp view of objects on the opposite side.

Reflecting surface % reflection index

Gloss silver 92 - 97

Gold 60 - 92

M atte silver 85 - 92

Polished nickel 60 - 65

Polished chrome 60 - 65

Polished aluminium 67 - 72

Electropolished aluminium 86 - 90

Vaporised aluminium 90 - 95

Copper 35 - 80

Iron 50 - 55

Enamelled porcelain 60 - 80

M irrors 80 - 85

M atte white paint 70 - 80

Light beige 70 - 80

Yellow and light cream 60 - 75

Accoustic ceilings 60 - 75

Light green 70 - 80

Light green and pink 45 - 65

Light blue 45 - 55

Light grey 40 - 50

Light red 30 - 50

Light brown 30 - 40

Dark beige 25 - 35

Dark brown, green and blue 5 - 20

Black 3 - 4




24/259


25/259

3.4. Absorption

Process by which radiant energy is converted into a different form of energy, mainly in the form of heat. This phenomenon is

characteristic both of all surfaces which are not completely reflective and of materials which are not totally transparent. The ratio between

absorbed flux to incident flux is known as absorptance.

Absorption of certain light wavelengths is called selective absorption. G enerally speaking, objects take their color from selective

absorption.

3.5. Refraction

The direction of the light beam may change when passing from one medium to the other. This is a result of a change in the light speed

of propagation. Speed decreases if the new media density is higher, and increases if it is lower. This change in speed and direction is

known as refraction.

There are two laws of refraction:

1. When the wave goes from one medium to another, the incident ray, the reflected ray and the normal to the separating surface of

the media on the incidence point, are on the same plane.2. The ratio between the incidence angle sine and the refraction angle sine is a constant for the given pair of media.

The above mentioned constant is known as the index of refraction n, for the given media. The second law of refraction is usually known

asSnells law.

Figure 8. Refraction in the boundary bewtween two media.

n1* = angle of refration for the first medium.

n2* = angle of refraction for the second medium.

a1

= angle of incidence.

a2

= angle of refraction.

When the first medium is the air, n1

= 1 and the formula is:

sin a1

= n2

sin a2

The distance D in figure 8 is known as displacement. Such a displacemnt depends on the angle of incidence and on the index ofrefraction. When the incident ray is perpendicular to the surface, refraction and displacement equal zero.

n1 sin a1 = n2 sin a2 c sina

1 = n2 = nsin a

2= n

1

2

1

1

D

n1

n2

n1





Refraction varies according to wavelength. Short waves ( like blue and violet) are transmitted better than long waves ( for example red) .

This phenomenon is used to decompose white light into its component colours when passing through a refraction prism. The degree

to which color is decomposed depends on the angle of incidence and the refraction properties of the prism material. This is called

dispersion.

* ni is calculated by the quotient between the speed of light in the air and the speed of light in the medium i.




Chapter 4.

THECOLOUR

4.1. General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.2. Colour classification according to the C.I.E. chromatic diagram . . . . . . 41

4.3. Colour temperature (Tc) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.4. Colour rendering index ( R) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.5. Colour and harmony psychic effects . . . . . . . . . . . . . . . . . . . . . . . . . . 44


29/259


Colour is a subjective psycho physiologic interpreta tion of the visible electromagnetic spectrum.

Luminous sensations or images, produced in our retina, are sent to the brain and interpreted as a set of monochromatic sensations

which constitute the colour of the light.

The sense of sight does not analyze each radiation or chromatic sensation individually. For each radiation there is a colour designation,

according to the frequency spectrum classification.

It is important to indicate that objects are distinguished by the colour assigned depending on their optical properties. Objects neither

have nor produce colour. They do have optical properties to reflect, refract and absorb colours of the light they receive, that is to say:

the set of additive monochromatic sensations that our brain interprets as colour of an ob ject depends on the spectral composition of

the light that illum inates such an object and on the optical properties possessed by the object to reflect, refract or absorb.

Newtonwas the first one to discover the decomposition of white light in the group of colours that forms a rainbow. When a white light

beam went through a prism, the same effect as that indicated in Fig. 1 was obtained.

Figure 1. White light decomposition in the rainbow spectrum.

4.2. Colour classification according to the C.I.E. chromatic diagram

Subjective evaluations of object surfaces, in the same way they are perceived by the human eye, are interpreted bearing in mind colour

attributes or qualities. They are the following:

a) Lightness or brightness: Luminous radiation received according to the illuminance possessed by the object. The further from black in

the grey scale, the lighter the colour of an object. It refers to intensity.

b) Hue or tone: common name for colour (red, yellow, green, etc.) . It refers to wavelength.

c) Purity or saturation: proportion in which a colour is mixed with white. It refers to spectral purity.

In order to avoid a subjective evaluation of colour there exists a chromaticity diagram in the shape of a triangle, approved by the C.I.E.

It is used to treat sources of light, coloured surfaces, paints, luminous filters, etc. from a quantitative point of view.

All colours are ordered following three chromatic coordinates, x, y, z, whose sum is always equivalent to the unit ( x + y + z = 1) . When

each of them equals 0.333, they correspond to the white colour. These three coordinates are obtained from the specific potencies for

each wavelength. It is based on the fact that when three radiations from three sources of different spectral composition are mixed, a

radiation equivalent to another with a different value may be obtained. The result is the triangle in Fig. 2, in which any two coordinates

are enough to determine the radiation colour resulting formed by the additive mixture of three components.

White light

Prism

380 nm.400 nm.

500 nm.

600 nm.

700 nm.

780 nm.


Chapter 4. THE COLOUR



Figure 2. C.I.E. Chromaticity diagram

4.3. Colour temperature (TC)

In the C.I.E. chromaticity diagram in Fig. 2, a curve has been drawn representing the colour emitted by a black body according to its

temperature. It is known as black body colour temperature curve, TC..

Colour temperature is an expression used to indicate the colour of a source of light by comparing it with a black body colour, that is to

say, a " theoretical perfect radiant" ( object whose light emission is only due to i ts temperature) . As any other incandescent body, the

black body changes its colour as its temperature increases, acquiring at the beginning, a red matte tone, to change to light red later on,

orange, yellow and finally white, bluish white and blue. For example, colour of a candle flame is similar to the one of a black body heated

at about 1 800 K *. Then, the flame is said to have a "colour temperature" of 1 800 K.

Incandescent lamps have a colour temperature which ranges from 2 700 to 3 200 K, depending on their type. Their fleck is determined

by the corresponding coordinates and is located virtually on the black body curve. Such temperature bears no relation at all with that of

an incandescent filament.

Therefore, colour temperatureis, in fact, a measure of temperature. It only defines colour and it can be applied exclusively to sources

of light which have a great colour resemblance with the black body.

The practical equivalence between colour appearanceand colour temperatureis established arbitrarily according to Chart 1.

* K = Kelvin. Temperatures of Kelvins scale exceed in 273 C the corresponding ones in the centigrade scale.

520

510

500

490

480

470460450

400-380

530

540

550

560

580

590

24.000

10.000 6.500

5.000

3.200

2.500800

600

610620

630650700750

570



31/259

Chart 1

4.4. Colour rendering index (R)

Colour temperature datum is only referred to the colour of light, but not to its spectral composition which is decisive for colour

reproduction. Thus, two sources of light may have a very similar colour and possesses, at the same time, very different chromatic

reproduction properties.

The colour rendering index (R) characterizes the chromatic reproduction capacity of objects illum inated with a source of light. The R

offers an indication of the capacity of the source of light to reproduce normalized colours, in comparison with the reproduction provided

by a light as reference pattern.

Chart 2

Luminous sources Tc ( K) R.C.

Blue sky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 000 a 30 000 85 to 100 (group 1)

Cloudy sky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 000 85 to 100 (group 1)

Daylight. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 000 85 to 100 (group 1)

Discharge lamps (except for Na) . . . . . . . . . . . . . .

Daylight (halogene) . . . . . . . . . . . . . . . . . . . . . . . . 6 000 96 to 100 (group 1)

Neutral white . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 000 a 5 000 70 to 84 (group 2)

Warm white . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lower than 3 000 40 to 69 (group 3)

D ischarge lamp ( N a) . . . . . . . . . . . . . . . . . . . . . . . 2 900 Lower than 40

Incandescent lamp . . . . . . . . . . . . . . . . . . . . . . . . . 2 100 a 3 200 85 to 100 (group 1)

Photographic lamp . . . . . . . . . . . . . . . . . . . . . . . . . 3 400 85 to 100 (group 1)

Candle flame or oil candle . . . . . . . . . . . . . . . . . . . 1 800 40 to 69 (group 3)

Colour appearance group Colour appearance Colour temperature ( K)

1 Warm Below 3 300

2 Intermediate From 3.300 to 5 300

3 Cold Above 5 300





Lamps colour rendering groups

In order to simplify the specifications for lamp colour rendering indexes of those used in lighting, colour rendering groups have been

introduced as indicated in Chart 3.

Chart 3. Lamp colour rendering groups.

4.5. Colours and harmony psychic effects

It has been proved that colour in the environment produces psychic or emotional reactions in the observer. Hence, using colours in the

adequate way is a very relevant topic for psychologists, architects, lighting engineers and decorators.

There are no fixed rules for choosing the appropriate colour in order to achieve a certain effect, since each case requires to be given a

particular approach. However, there are some experiences in which different sensations are produced in the individual by certain colours.

O ne of the first sensations is that of heat or coldness. This is the reason why the expression " hot colours" and "cold colours" is

mentioned. H ot colours are those which go from red to greenish yellow in the visible spectrum; cold colours the ones from green to

blue.

A colour will be hotter or colder depending on its tendency towards red or blue, respectively.

O n the one hand, hot colours are dynamic, exciting and produce a sensation of proximity. O n the other hand, cold colours calm and

rest, producing a sensation of distance.

Likewise, colour clarity also produces psychological effects. Light colours cheer up and give a sensation of lightness, while dark colours

depress and produce a sensation of heaviness.

When two or more colours are combined and produce a comfortable effect, it is said that they harmonize. Thus, colour harmony is

produced by means of selecting a colour combination which is comfortable and even pleasant for the observer in a given situation.

From all the above mentioned, it may be deduced that a knowledge of the spectral distribution curve of sources of light is necessary to

obtain the desired chromatic effect.

Rendering group Rendering range in Colour appearance Examples for preferible uses Examples for acceptable usein colour

colour ( R or Ra)

WarmColour equalness, medical

1 A R 90 Intermediateexplorations, art galleries

Cold

Warm H ouses, hotels, restaurants,

Intermediate shops, offices, schools, hospitals1 B 90 > R 80

Intermediate Printing, painting and textile industry,

Warm industrial work

Warm

2 80 > R 60 Intermediate Industrial work O ffices, schools

Cold

3 60 > R 40 Rough industries Industrial work

Rough work, industrial work

4 40 > R 20 with low requisitesfor

colour rendering




Chapter 5.

LUMINOUS MEASUREMENTS

5.1. Luminous flux ( luminous output) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.2. Amount of light ( luminous energy) . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.3. Luminous intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.4. Illuminance ( luminous level) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.5. Luminance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.6. O ther interesting luminous measurements . . . . . . . . . . . . . . . . . . . . . 51

5.7. Luminous measurement graphic representation . . . . . . . . . . . . . . . . . 52

5.8. Luminous measurement summary chart . . . . . . . . . . . . . . . . . . . . . . 56


35/259

Two basic elements intervene in lighting engineering: both the source of light and the object to be illuminated.

In the present chapter, we will deal with fundamental measurements and units used to evaluate and compare the quality and effects of

sources of light.

5.1. Luminous flux (luminous output)Energy transformed by light sources cannot be totally taken advantage of for light production. For example, an incadescent lamp

consumes a certain amount of electric energy which is transformed into radiant energy. O ut of this, only a small amount ( about 10% )

is perceived by the human eye as light, while the rest of it is lost as heat.

A luminous flux produced by a source of light is the total amount of light, either emitted or radiated in all directions in one second.

M ore precisely, a source of light luminous flux is radiated energy received by the human eye depending on its sensitivity curve, and

which is transformed into light for a second.

Luminous flux is represented by the Greek letter F and is measured in lumens ( lm) . Lumen is the luminous flux of the monochromatic

radiation characterised by a value frequency of 540 1012 Hz. and a radiant power flux of 1/ 683 W. O ne 555 nm. wavelength radiant

energy watt in the air equals 683 lm approximately.

Luminous flux measurement

Luminous flux measurement is conducted by means of an adjusted photoelement depending on the phototopic sensitivity curve of the

standard eye to the monochromatic radiations, incorporated to a hollow sphere known as Ullbrichts sphere( Fig. 1) . The source to be

measured is placed inside it. M anufacturers provide lamp flux in lumens for nominal potency.

Figure 1. Ullbrichts sphere.

Luminous performance (Luminous efficacy)

Luminous performance of a source of light indicates the flux emitted by this source per unit of electrical output consumed to obtain it.

It is represented by the Greek letter e, and it is measured as lumen/ watt (lm/W).

The formula which expresses luminous efficacy is:

(lm/W)

If a lamp was to be manufactured which transformed all the consumed electrical output into light at one 555 nm. wavelength without

losses, such a lamp would have the highest performance possible. Its value would be 683 lm/W.

=


Chapter 5. LUMINOUS MEASUREMENTS



5.2. Amount of light (Luminous energy)

In a similar way to electrical energy, which is determined by the electrical output in the time unit, the amount of light or luminous energy

is determined by the luminous output or luminous flux emitted by the time unit.

The amount of light is represented by the letter Q, and is measured as lumen per hour ( lm h) .

The formula which expresses the amount of light is the following:

Q = F t ( lm h)

5.3. Luminous intensity

This measurement is solely understood as referred to a specific direction and contained in a w solid angle.

In the same way that a plane angle measured in radians corresponds to a surface, a solid or stereo angle corresponds to a volume

measurement and is measured in stereoradians.

The radian is defined as the plane angle within an arc of a circle, equal to the radius of the circle. ( Fig. 2) .

Figure 2. Plane angle.

The stereoradian is defined as the solid angle which corresponds to a spherical cap whose surface equals the square of the sphere

radius ( Fig. 3) .

Figure 3. Solid angle.

Luminous output of a source of light in one specific direction equals the ratio between the lum inous flux contained in whatever solid

angle whose axis coincides with the considered direction . Its symbol is , and its unit of measurement is the candela ( cd) . The formula

which expresses it is the following:

( lm/sr)

Candela is defined as the luminous intensity of a specific source which em its luminous flux equal to one lumen in a solid angle per

stereoradian (sr) .

=

r = 1m.

1cd

1cd

= 1 LmE= 1 LuxS= 1 m2

(total) = 4 stereoradians

r = 1

= 1 radian

(total) = 2 radians

= 1



37/259

According to the I.S.*, candela may also be defined as the luminous intensity in a certain direction, from a source which emits

monochromatic radiation with a frequency of 540 1012 Hz, and whose energy intensity in the aforementioned direction is 1/683 watts

per stereoradian.

5.4. Illuminance (Luminous level)Illuminance or luminous level of a surface is the ratio between the lum inous flux received by the surface to its area. It is represented

by the letter E, and its unit is the lux ( lx) .

The formula which expresses illuminance is:

( lx = lm/m 2)

Thus, according to the formula, the higher the luminous flux incident on a surface, the higher its illuminance. Also, for the same given

incident luminous flux, illuminance will be higher as surface decreases.

According to the I.S., lux may be defined as the illum inance of a certain surface which receives a luminous flux of one lumen, spread

over one square meter of its surface.

Lighting level measurement

Luminous level measurement is conducted with a special device known as foot- candle metre. It consists of one photoelectric cell which

generates a weak eletric current when light strikes its surface, thus, increasing according to light incidence. Such current is measured by

means of an analogic or digital miliammeter, calibrated directly in lux (Fig. 4) .

Figure 4. Foot- candle metre.

5.5. Luminance

Luminance is the effect which produces a surface on the retina of the eye, both com ing from a primary source which produces light,or from a secondary source or surface which reflects light.

Luminance measures brightness for primary light sources as well as for sources constituting illuminated objects. This term has substituted

the concepts of brightness and lighting density. Nevertheless, it is interesting to remember that the human eye does not perceive colours

but brightness, as a colour attribute. Light perception is, in fact, the perception of differences in luminance. Therefore, it may be stated

that the eye perceives luminance differences but not illuminance ones ( provided that we have the same lighting, different objects have

different luminance since they have different reflection characteristics).

Luminance of an illuminated surface is the ratio between lum inance of a source of light in a given direction, to the surface of the

projected source depending on such direction.

*I.S.c International System.

1

2

3

BBAA

=

S





Figure 5. Surface luminance.

The projected area is seen by the observer in the direction of the luminous intensity. This area is calculated by multiplying the illuminated

real surface by the cosine angle forming the normal with the direction of the luminous intensity ( Fig. 5) .

Represented by the letter L, its unit is the candela/square metre called nit ( nt) , with one submultiple, the candela/square centimetre

or stilb, used for high luminance sources.

;

The formula which expresses it is the following:

where:

S cos = Apparent surface.

Luminance is independent from the observation distance.

Luminance measurementLuminance measurement is conducted by means of a special device called a luminancemetre or nitmeter. It is based on two optical

systems, directional and measurement systems, respectively. (Fig. 6) .

The di rectional system is oriented in such a way that the image coincides with the point to be measured. O nce it has been oriented,

the light that reaches it is transformed into electric current. Its values are measured in cd/m2.

Figure 6. Luminancemeter.

1

2

3

1

2

3

1

2

3

L =

S cos

1stilb=1cd

1cm21nt=

1cd

1m2

Viewed or apparent surface

Real surface

Apparent surface = Real surface x cos



39/259

5.6. Other interesting luminous measurements

5.6.1. Utilization coefficient

Ratio between the luminous flux received by a body and the flux emitted by a source of light.

Unit c %

Symbolc

Ratio c

5.6.2. Reflectance

Ratio between the flux reflected by a body ( with or without diffusion) and the flux received.

Unit c %

Symbolc

Ratio c

5.6.3. Absorptance

Ratio between the luminous flux absorbed by a body and the flux received.Unit c %

Symbolc

Ratio c

5.6.4. Transmittance

Ratio between the luminous flux transmitted by a body and the flux received.

Unit c %

Symbolc

Ratio c

5.6.5. Average uniformity factor

Ratio between minimum to medium illuminance in a lighting installation.

Unit c %

Symbolc Um

Ratio c

5.6.6. Extreme uniformity factor

Ratio between minimum to maximum illuminance in a lighting installation.

Unit c %

Symbolc Ue

Ratio c

5.6.7. Longitudinal uniformity factor

Ratio between longitudinal minimum to maximum luminance in a lighting installation.

Unit c %

Symbolc UL

Ratio c

5.6.8. Overall luminance uniformity

Ratio between minimum to medium illuminance in a lighting installation.

Unitc

%Symbolc U0

Ratio c U0 =Lmin

Lmed

UL =Llongitudinal min

Llongitudinal max

Ue =min

max

Um =min

med

=t

=a

=r

=

e





5.6.9. Maintenance factor

Coefficient indicating the preservation degree of an installation.

Unit c %

Symbol c Fm

Ratio c Fm = Fpl Fdl Ft Fe Fc

Fpl = lamp position factor

Fdl = lamp depreciation factor

Ft = temperature factor

Fe = ignition equipment factor

Fc = installation preservation factor

5.7. Luminous measurement graphic representation

The collection of luminous intensity emitted by a source of light in all directions is known as luminous distribution. The sources of light

used in practice have a more or less large luminous surface, whose radiation intensity is affected by the construction of the source itself,

presenting various values in these scattered directions.

Special devices ( like the Goniophotometer) are constructed to determine the luminous intensity of a source of light in all spatial

directions in relation to a vertical axis. If luminous intensity ( I) of a source of light is represented by vectors in the infinite spatial directions,

a volume representing the value for the total flux emitted by the source is created. Such a value may be defined by the formula below:

Photometric solid is the solid obtained. Fig. 7 shows an incasdescent lamp photometric solid.

Figure 7. Incandescent lamp photometric solid.

If a plane passes through the symmetric axis of a source of light, for example, a meridional plane, a section limited by a curve, known

asphotometric curve, or luminous distribution curve is obtained ( Fig. 8) .

020

40

80

100

120

140160180

60

= !

r

dr



41/259

Figure 8. Photometric curve for an incandescent lamp.

By reviewing the photometric curve of a source of light, luminous intensity in any direction may be determined very accurately. This data

are necessary for some lighting calculations.

Therefore, spatial directions through which luminous radiation is irradiated may be established by two coordinates. O ne of the most

frequently used coordinate systems to obtain photometric curves is the C - represented in Fig. 9.

Figure 9. C - coordinate system.

Photometric curves refer to an em itted luminous flux of 1 000 lm. G enerally speaking, the source of light emits a larger flux. Thus, the

corresponding luminous intensity values are calculated by a simple ratio.

When a lamp is housed in a reflector, its flux is distorted, producing a volume with a marked shape defined by the characteristics of the

reflector. Therefore, distribution curves vary according to different planes. The two following figures show two examples where distributioncurves for two reflectors are represented. Fig.10 reflector is symmetric and has identical curves for any of the meridional planes. This is

inclinationaxis

rotati

on

axis

"C"

planes

=180

=0

=90

Walkw

ayside

Roadwayside

C=0

C=180

C=90

C=27

0

20

40

40

60

80

180

0 30

150

90

60

120

60

80

100

120

140

cd





the reason why a sole curve is enough for its photometric identification. Fig. 11 reflector is asymmetric and each plane has a different

curve. All planes must be known.

Figure 10. Symmetric photometric distribution curve.

Figure 11. Asymmetric photometric distribution curve.

Another method to represent luminous flux distribution is the isocandela curvediagram ( Fig. 12) . According to this diagram, luminaires

are supposed to be in the center of a sphere where exterior surface points with the same intensity are linked ( isocandela curves) .

Generally, luminaires have, at least, one symmetric plane. This is the reason why they are only represented in a hem isphere.

Figure 12. Isocandela curves.GM=0 Imax=100%

10-10-20

-30

-40

-50

-60

-70

-80

-90C=0350 10 20 30 40 50 60 70 80340330320310300290280

20

30

40

50

60

70

80

90

1

510

20

30

9080

60 40

60

C= 45 C= 0C= 90

Unit = cd/1000 lm

70o

50o

30o

0o

080240320

10o

60o

30o

30o

0o

225450675900

C= 45 C= 0C= 90

Unit = cd/1000 lm



43/259

This representation is very comprehensive. However, more experience is needed to interpret it.

The flux emitted by a source of light provides surface lighting ( illum inance) whose values are measured in lux. If those values are

projected on the same plane and a line links the ones with the same value, isolux curves are formed ( Fig. 13) .

Figure 13. Isolux curves.

Finally, luminance depends on the luminous flux reflected by a surface in the observers direction. Values are measured in candelas per

square metre ( cd/m2) and are represented by isoluminance curves( Fig. 14) .

Figure 14. Isoluminance curves.

h6h 5h 4h

1 5 2030

4050

6070

80

5

10

50

1

5

3h 2h h 0 h 2h 3h

0

h

2h

3h

A

OBSERVERS: A, B AND C

B

C

ROADWAY SIDE

WALKWAY SIDERoadw ay R2Qo = 0.07

Lmax=100%

fl=0.152

h

6h 5h 4h

11

5

5

10

20

3040

5060

70

80

3h 2h h 0 h 2h 3h

0

h

2h

3h

Lmax=100%

fl=0.154

WALKWAY SIDE

ROADWAY SIDE





5.8. Luminous measurement summary chart

Chart 1. Luminous measurement summary

M easurement Symbol Unit Ratio

Luminous flux F Lumen ( lm) F = I q

Luminous efficacy Lumen per watt ( lm/ W) =

Luminous output Q Lumen per hour ( lm h) Q = F t

Candela (cd)Luminous intensity

( cd = lm/sr) =

Lux ( lx)Illuminance

( lx = lm/m 2)=

S

Nit = cd/ m2Luminance L

Stilb = cd/cm2L =

S cos

Utilization coefficient % =

e

Reflectance % =r

Absorptance % =a

Transmittance % =t

Average uniformity factor Um % Um =min

med

Extreme uniformity factor Ue % Ue =min

max

Longitudinal luminance uniformity UL

% UL=

Llongitudinal min

Llongitudinal max

O verall luminance uniformity U0 % U0 =Lmin

Lmed

M aintenance factor Fm % Fm = Fpl Fdl Ft Fe Fc




Chapter 6.

FUNDAMENTAL PRINCIPLES

6.1. Inverse square distance law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.2. Cosine law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.3. Normal, horizontal, vertical and inclined planes illumination . . . . . . . . 61

6.4. Illuminance ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

6.5. Lamberts law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65


47/259

6.1. Inverse square distance law

Since early experiments, it has been confirmed that illuminances produced by the source of light decrease inversely to the square of the

distance from the plane to illuminate the source. This ratio is expressed by the following formula:

( lx)

where is the illuminance level in lux ( lx) , is the intensity of the source in candelas (cd) , and dis the distance from the source of light

to the perpendicular receptor plane.

In this way, an illuminance ratio 1 and 2 may be established, between two planes separated by a distance dand Dfrom the source

of light, respectively:

1 d2 = 2 D 2

Figure 1. Luminous flux distribution over different surfaces.

This law is fulfilled when we are dealing with a punctual source of perpendicular surfaces to the direction of the luminous flux. However,

the law is supposed to be accurate enough when the distance undergoing measurement is, at least, five times the maximum dimension

of the luminaire ( the distance is big in relation to the size of the area of the source of light).

6.2. Cosine law

In the previous section, the surface was perpendicular to the direction of luminous rays, but when a specific angle a is formed in relation

to this, the formula for the inverse square distance lawmust be multiplied by the cosine of the corresponding angle. Such an expression

constitutes what is called the law of cosine, expressed in the formula below:

= cos ( lx)d2

F

d

D

E1

S1

S2

E2

1=

D 2

2 = d2

=

d2


Chapter 6. FUNDAMENTAL PRINCIPLES



Illuminance in any given point of a surface is proportional to the cosine of the ang le of incidence of the lum inous rays in the

illuminated point.

In Fig. 2 two sources of light F and F with the same luminous intensity ( I) and at the same distance (d) from point P are represented.

To the source of light F with cos0 = 1 corresponds an angle of incidence equal to zero. This source produces illum inance for the point

P with a value of:

Figure 2. Iluminance at a point from two sources of light with different angles of incidence.

c

Likewise, F with an angle = 60, corresponding cos60 = 0.5, will produce at the same point an illuminance valued as:

c

Therefore, p = 0.5 p, that is to say, to obtain the same illuminance at point P, the luminous intensity of the source F must double

that of the source F.

In practice, distance dfrom the source to the considered point is not known, but its height hto the horizontal of the point is. By using

a simple trigonometric relation and substituing it in the equation, a new relation where height hplays an important role is obtained:

p =

cos3 ( lx)h2

cos2 cos

h2

p=

cos =

cos =

d2

(h

)2

cos

cos =hc d =

h

d cos

p =1

( lx)2 d2

p = cos 60 = 0.5d2 d2

p =

( lx)d2

p =

cos 0 =

1d2 d2

60

h

PF

F'

d

d



49/259

6.3. Normal, horizontal, vertical and inclined planes illumination

In Fig. 3 the source F illuminates three planes situated in the following positions: normal, horizontal and vertical to the beam. Each will

have an illuminance called:

EN = Normal illuminance.

EH = Horizontal illuminance.

EV = Vertical illuminance.

Figure 3. Normal, horizontal and vertical illuminance.

Let us determine the normal, horizontal and vertical illuminance for point M in Fig. 3.

Normal illumination

The inverse square distance law is applied:

where I is the luminous intensity under the angle a. Virtually, only normal illuminance of a point is considered whenever this point is

situated in the vertical of the source on the horizontal plane ( M 1 point) . Thus, the previous formula is transformed into:

and also when it is situated in a straight line with the source on the vertical plane (M 2 point) , the illuminance is:

Horizontal illumination

If the law of cosine is directly applied, the result is:

Such a formula may be reformulated in relation to the height h between the F source and the M point ( d = h / cos) :

H =

cos3 ( lx)h2

H = N cos =

cos ( lx)d2

N = ( lx)a2

N =

( lx)h2

N =

( lx)d2

F

M2

M1M

I

d

Horizontal

illuminance

Vertical

illuminance

Norm

al

illumin

ance

a

h





Vertical illumination

In this case, the law of the cosine is also directly applied. The result is that:

V = N cos ( lx)

Between the and angles, there is a simple relation since both belong to a triangle rectangle.

+ + 90 = 180 c = 90 -

Applying trigonometric relations:

cos = cos( 90 - ) = cos90 cos + sin90 sin

Therefore, cos = sin. This value is substituted and the result is that:

V = N sin ( lx)

The equation may be expressed in relation to the height h between the F source and the M point.

Inclined planes illumination

The vertical plane may change through an angle like the one in Fig. 4. Such an angle forms the vertical plane which contains the

point P with the light incidence plane.

Figure 4. Illuminance at point P.

Taking this into account, the above mentioned expression is transformed into:

h is the vertical height of the source of light over the horizontal plane which contains point P.

6.4. Illuminance ratio

Different concepts to describe light coming from other directions different from the vertical have been proposed. These must be considered as

comfort parameters together with others like luminous level ( illuminance) .

Vertical / horizontal

The experience from high illum inance level installations with a very good glare control indicates that the ratio between vertical (EV) and

horizontal illuminance ( EH) for a good modelling* must not be lower than 0.25 in the main directions of vision.

* M odelling: Ability of light to reveal the texture and tridimensional form of an object creating light and shade contrasts.

V 0.25

H

PI =

cos2 sin cos ( lx)h2

P

h

I

V =

cos2 sin ( lx)h2

V =

sin ( lx)d2



51/259

Vectorial /Spherical

Directional lighting effects may be described partly through vectorial i lluminance and partly through the ratio between vectorial and

spherical i lluminance.

The illuminance vector at a point has a magnitude equal to the maximum difference in illuminance over those diametrically opposed

surface elements in a small disc ( Fig. 5) located in a point, their direction being from the greatest illuminance element to the lowest

one.

Figure 5. Illuminance vector E = Ef Er.

The spherical average at point is the average illuminance over all the surface of a small sphere located at such a point ( Fig. 6) .

Figure 6. Spherical medium illuminance ES.

Lighting directional intensity may be indicated by the given modelling through the ratio between vectorial illuminance and average

spherical i lluminance:

If we measure it using a sphere with a radius r which receives a beam of light with an F luminous flux, it would be:

Illuminance E of an element of the radius r surface is:

In a room with a floor, walls and a flat ceiling with diffused reflection, where there is also diffused light, we have that jj 0 ( that is to

say, there are no shadows) . Under these circumstances, the modelling index is j

/ sj 0. However, in a completely dark room where

=

r2

S =

4 r2

S

Es

E

Er

Ef





the light comes from one direction only ( for example, sunlight) , j

= ( that is to say, dark shadows) . Under these circumstances, the

modelling index is equivalent to j

/ = / s = 4.

Therefore, modelling index may vary between values such as 0 and 4.

Vector j

must have a downward direction ( preferibly between 45 and 75 to the vertical) in order to obtain a natural appearance of

human features.

Cylindrical / Horizontal

An alternative concept to describe the modelling effect is the ratio between cylindrical illuminance and horizontal illuminance at a certain

point.

The average cylindrical illuminance C at a point is average illuminance over a curved surface of a small cylinder located at the point

( Fig. 7) . Unless otherwise indicated, the cylinder axis must be vertical.

Figure 7. Average cylindrical illuminance EC.

Cylindrical illuminance at a point equals average vertical illuminance in all directions at such a point. A good modelling is achieved when

the ratio is:

Generally speaking, di rection is automatically taken into account. Therefore, i t is not necessary to specify it from an additional point of

view, like in the case of vectorial / spherical ratio: when light comes directly from above, C = 0 and C / H = 0; when light is horizontal,

H = 0 and C / H j q.

Vertical / Semicylindrical

Tests conducted in relation to lighting of pedestrian outodoor areas ( low level lighting areas) have proved that the ratio between vertical

illuminance and semicylindrical illuminance provides a useful measure of acceptance of human features modelling, for the mentioned

application area.

Semicylindrical illuminance semicyl at a point in a given horizontal direction equals the average illuminance on a curved surface of a

small vertical semicylinder located at such a point, with a curved surface focused towards the specified direction ( Fig. 8) .

0.3 C

3H

EC



53/259

Figure 8. Semicylindrical illuminance.

Well balanced lighting relief ( neither very short nor very intense) is obtained at:

Extreme ratios are:

Zero very intense modelling.

( /2) = 1.57 very short modelling.

6.5. Lamberts law

There exist emitting or diffused surfaces that, when observing them from different angles, the same brightness feeling is obtained. These

surfaces are called perfect emitters or diffusers.

If L0

is luminance according to the normal and L

is luminance according to the observation angle

, L

= L0

is verified for any given

angle .

Since and , the equation below is true:

= 0 cos

This ratio is known as Lamberts Law and only perfect emitters or diffusers comply to it.

Figure 9. Luminance invariability in relation to the incidence angle.

Io

I

Lo

L

N

Surface

L =

S cos L0 =

0

S

0.8

V

1.3semicyl

Esem





Chapter 7.

LUMINAIRES

7.1. General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

7.2. Luminaire classification according to the degree of protection

from electric contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

7.3. Luminaire classification according to working conditions . . . . . . . . . . . 70

7.4. Luminaire classification according to mounting surface flammability . 71

7.5. Luminaire classification according to service conditions . . . . . . . . . . . 72

7.6. Photometric basic data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

7.7. Luminaire efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86


57/259

General remarks

Due to the high luminanceof lamps, it is necessary to increase the emission apparent surface in order to avoid visual problems ( glare) .

Also, it is necessary to shield lamps to protect them from external agents and to direct their flux in the most convenient way for visual

task.

Thus, different studies and contemporary research place great emphasis on the combination formed by the source of light and the

luminaire.

According to the UN E-EN 60598-1* N orm, a luminaire may be defined as a lighting apparatus which spreads, filters or transforms

light em ited by a lamp or lam ps including all components necessary for supporting, fixing and protecting the lamps, (except for the

lamps themselves). Should the need arise, also the auxiliary circuits combined with the media for the connection to the power supply.

Main components

Independently from other definitions which could be more or less descriptive, a luminaire may be defined as an object formed by a

combination of elements designed to give an appropriate luminous radiation of an electric origin. M aterialization of these elements is

achieved by combining a good formal design and a reasonable economy of materials in each situation.

Formal design solves luminous control depending on needs, which is the main aim: both a thermal control which makes its functioning

stable and an electric control which offers adequate guarantees to the user. Economy of materials provides a solid and efficient product,

an easily installed luminaire, and minimum maintenance while in use.

Regarding the mo

The Light Chapter-1

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