Colour TeChnology of CoaTings

Wilhelm Kettler et al.

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Colour TeChnology of CoaTings

Walter Franz | Peter Gabel | Stephan Gauss | Uwe Hempelmann | Rainer Henning Wilhelm Kettler | Hans-Jörg Kremitzl | Gerhard Rösler/translated by Manfred Binder

Sandra Weixel | Gerhard Wilker

Colour Technology of Coatings

Wilhelm Kettler et al.: Colour Technology of Coatings© Copyright 2016 by Vincentz Network, Hanover, Germany

Wilhelm Kettler et al.Colour Technology of CoatingsHanover: Vincentz Network, 2016 EuropEan Coatings library ISBN 3-86630-600-8ISBN 978-3-86630-600-4

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EuropEan Coatings library

Walter Franz | Peter Gabel | Stephan Gauss | Uwe Hempelmann | Rainer Henning Wilhelm Kettler | Hans-Jörg Kremitzl | Gerhard Rösler/translated by Manfred Binder

Sandra Weixel | Gerhard Wilker

Colour Technology of Coatings


PrefaceLike many other textbooks, this one has its origins in the classroom and is the culmination of more than 10 years’ experience of teaching courses on colorimetry to engineers and technicians from various branches of industry.

The VDMI, the VdL and the FPL asked various experts at pigments, paints, and instrument makers in 2000 to design various training modules for colourists working on industrial, plastics, paint, and print applications of colour.

The objective was to provide a technically correct and up-to-date introduction to those many aspects of colour and colour applications in industry. The target group was engineers deeply involved in colour applications in various industrial sectors. The first set of training modules was offered in 2001. Until retiring in 2014, Dr. Tasso Bäurle successfully steered the ship for almost 14 years, taking charge of the training modules. He continually refined and modified the underlying concept to meet the demands of the participants and participating industries and also acted as publisher of the German edition of this book. The latest version comprises just two modules covering elementary concepts of colorimetry and in-depth insights in colorimetry. Although the main focus is on paint applications, all the methods presented can be readily adapted to colour applications in other industries.

The main purpose of this book is to provide a comprehensive survey of relevant industrial colour applications and numerous concepts of physical and physiological pigment optics in order that a written record may be preserved of the specialist knowledge of all the lectur-ers involved in the coloristic training course. The colour problems discussed in this book include optics and chemistry of solid-colour and effect pigments, colourant formulation, optical microscopy of effect colour shades for pigment identification, methods of elementary and advanced colorimetry, measurement and visual assessment of solid and effect colour shades, colour tolerances and acceptability, and colour-order systems. Compared to the first German edition, this English edition contains a further chapter devoted to the newly emerging area of visual texture assessment of effect colour shades. Although technology for measuring visible texture is already available on the market, no mathematically rigorous definition of texture parameters and their dimensionality has been formulated so far that would be accepted throughout the colour community. However, a combination of multidi-mensional texture and colour information is the appropriate paradigm to adopt for proper physical characterisation of the visual appearance of effect colour shades. The contents of this book are a mix of objective detachment on one hand and a detailed first-hand knowledge and practical relevance on the other.

The mathematics throughout the book have been kept to a minimum, even though the technical treatment of colour problems is being driven more and more by mathematical models. Many references at the ends of chapters cite original papers. The reader is encour-aged to consult these as further sources of information and to supplement the physical and physiological basics presented in this book with more general mathematics and rigor, as necessary.

As this English edition of the training course for colourists was being prepared, one of the authors, Dr. Gerhard Rösler, unexpectedly passed away in December 2012. I believe that Gerhard would be pleased with the expanded English edition of our joint project.

Wilhelm H. Kettler

Wülfrath, Germany, January 2016

Contents 7

Contents

I Fundamentals of colour perception Stefan Gauss ....................................................................................................... 15

1 Human colour vision ............................................................................................................... 151.1 The human eye ......................................................................................................................... 161.1.1 Optical structure ..................................................................................................................... 171.1.2 Signal processing and special features .............................................................................. 171.2 The photoreceptor cells in the human eye ......................................................................... 181.2.1 Spectral sensitivity of the receptors ................................................................................... 191.2.2 Visual defects ........................................................................................................................... 201.3 Colour perception .................................................................................................................... 211.3.1 Chromophoric attributes ........................................................................................................ 211.3.2 Colour constancy ..................................................................................................................... 212 Light as Electromagnetic Radiation .................................................................................... 223 Colour mixing .......................................................................................................................... 243.1 Additive colour mixing .......................................................................................................... 243.2 Subtractive colour mixing ..................................................................................................... 244 Interaction of light and matter ............................................................................................. 255 Standard illuminants and light sources ............................................................................ 275.1 Standard illuminants .............................................................................................................. 275.2 Light sources ............................................................................................................................ 296 Standard observer ................................................................................................................... 307 CIE 31 system ........................................................................................................................... 327.1 Calculation of tristimulus values ......................................................................................... 327.2 Chromaticity coordinates ...................................................................................................... 348 CIELAB system ........................................................................................................................ 358.1 The L*, a*, b* coordinates ...................................................................................................... 378.2 The L*, C*, h coordinates ....................................................................................................... 388.3 Colour differences ................................................................................................................... 388.4 Colour tolerances and MacAdam ellipses ......................................................................... 419 Metamerism .............................................................................................................................. 429.1 Colour constancy ..................................................................................................................... 439.2 Metameric pairs ....................................................................................................................... 439.3 Special metameric index ....................................................................................................... 45

II Colour measurement, colour measurement systems and visual colour assessment ................................................................................ 47

1 Principles behind measuring coloured surfaces Gerhard Rösler, translated by Manfred Binder .................................................................... 47


Contents8

1.1 Analytical and visual characterisation of colour ............................................................. 471.1.1 Method A: Tristimulus colorimeter ..................................................................................... 471.1.2 Methods B and C: Spectrophotometer with polychromatic illumination ................... 491.1.3 Method D: Spectrophotometer with monochromatic illumination ............................... 491.1.4 Method E: Bi-spectral measurement .................................................................................. 501.1.5 Spectral measuring range, resolution and illumination ................................................ 511.1.6 Spectrometers, monochromators and detectors ............................................................... 522 Measuring geometries Gerhard Rösler, translated by Manfred Binder ................................................................... 522.1 Sphere geometries for reflectance measurements .......................................................... 552.2 Directional geometries for reflectance measurements .................................................. 562.3 Measuring geometries for different sample types and sample properties ................ 582.4 Recommended geometries for transmission measurements ............................................ 582.5 Notes on choosing the right geometry ............................................................................... 602.6 Multi-angle geometries .......................................................................................................... 603 Measuring geometries for special effect pigments Peter Gabel ................................................................................................................................. 613.1 Optical principles behind special effect pigments .......................................................... 613.2 Measuring geometries for metallic pigments ................................................................... 623.3 Measuring geometries for special effect pigments ......................................................... 623.4 New measuring geometries – applications for special effect pigments ..................... 643.4.1 New measuring geometries – new developments ........................................................... 654 Sample preparation Gerhard Rösler, translated by Manfred Binder ................................................................... 665 Recommended colourimetric conditions Gerhard Rösler, translated by Manfred Binder ................................................................... 685.1 Calibration of the colour-measuring instrument ............................................................. 695.2 Ambient conditions ................................................................................................................. 695.3 Black calibration ...................................................................................................................... 695.4 White calibration .................................................................................................................... 695.5 Calibration function ............................................................................................................... 695.6 Control measurement ............................................................................................................. 705.7 Storage of calibration standards .......................................................................................... 705.8 Laboratory report ..................................................................................................................... 706 Influence of the surface Gerhard Rösler, translated by Manfred Binder ................................................................... 707 Special case: optical brighteners and fluorescence Gerhard Rösler, translated by Manfred Binder ................................................................... 728 Sources of error in colour measurements Stefan Gauss ............................................................................................................................. 738.1 Errors in sample preparation ................................................................................................ 748.2 Instrument error ...................................................................................................................... 748.3 Experimental error .................................................................................................................. 759 Profiling of measuring instruments and colour management Gerhard Rösler, translated by Manfred Binder ................................................................... 76

Contents 9

9.1 Commonly used colour standards for profiling colour-measurement instruments ............................................................... 769.2 Set of colour standards for colour-measuring instruments ............................................ 779.3 Instrument maker accuracy and profiling ........................................................................ 7710 Non-contact colour measurement Gerhard Rösler, translated by Manfred Binder ................................................................... 77

III Visual colour assessment Gerhard Rösler, translated by Manfred Binder ....................................................... 79

1 Colour perception and colour deficiency ............................................................................ 792 Light booths .............................................................................................................................. 803 Visual colour assessment of samples with effect coatings ............................................ 813.1 Method: Sample modulation ................................................................................................. 823.2 Method: Illumination modulation ....................................................................................... 823.3 Method: Observer modulation .............................................................................................. 833.3.1 Assessment of special effect pigments by the method of observer modulation ....... 843.4 Summary of visual assessment of effect-coated samples .............................................. 84

IV Colour-order systems Wilhelm Kettler ................................................................................................... 85

1 Introduction and definition ................................................................................................... 852 Psychometric scales ................................................................................................................ 853 Colour scales ............................................................................................................................. 864 Colour notation systems ......................................................................................................... 864.1 CIELAB, CIELUV, and DIN99 ................................................................................................ 864.2 Munsell colour system ........................................................................................................... 874.3 DIN colour system ................................................................................................................... 884.4 NCS colour system .................................................................................................................. 894.5 OSA-UCS colour system ......................................................................................................... 904.6 RAL design system ................................................................................................................. 925 Colour-naming systems and colour-card collections ...................................................... 935.1 RAL system ............................................................................................................................... 935.2 British Standards Institution (BSI) ...................................................................................... 935.3 Pantone colour system ........................................................................................................... 946 Link between colour-order systems ................................................................................... 947 Decisions ................................................................................................................................... 95

V Instrumental colour difference assessment Wilhelm Kettler ................................................................................................... 98

1 Introduction .............................................................................................................................. 982 Geometric structure of colour difference models ....................................................................... 983 Colour difference model CMC(kL : kC)................................................................................... 994 Colour difference model BFD(kL : kC) .................................................................................... 1015 Colour difference model CIE94 ............................................................................................ 1026 Colour difference model CIEDE2000 .................................................................................. 102

Contents10

7 Colour space DIN99 ................................................................................................................ 1048 Parametric effects ................................................................................................................... 1069 Comparative analyses of the performance of modern colour difference models ..... 1079.1 Normalisation ........................................................................................................................... 1089.2 Lightness scale ......................................................................................................................... 1089.3 Chroma and hue scales .......................................................................................................... 1109.4 Total colour difference ............................................................................................................ 11010 Appraisal of the current state of colour difference metric ............................................ 11211 Model extensions for goniochromatic colours .................................................................. 113

VI Definition and application of colour tolerances Wilhelm Kettler ................................................................................................... 117

1 Tolerance and acceptance ..................................................................................................... 1171.1 Euclidean colour space ........................................................................................................... 1171.2 Non-Euclidean colour space .................................................................................................. 1182 Psychophysical measurements ............................................................................................ 1183 Visual colour assessment ...................................................................................................... 1194 Statistical threshold determination ..................................................................................... 1195 An experiment to define colour tolerances ....................................................................... 1216 Significance of colour measurement results ..................................................................... 1276.1 Multivariate statistics ............................................................................................................. 1276.2 Statistics for 3-dimensional colour spaces ........................................................................ 1286.3 The scatter ellipsoid ................................................................................................................ 1307 Tolerances for solid colours: DIN 6175 P1 .......................................................................... 1338 Tolerances for gonioapparent colours: DIN 6175 P2 ........................................................ 134

VII Pigment optics – physical processes Uwe Hempelmann ............................................................................................... 137

1 Colour-generating processes ................................................................................................ 1372 Reflection, refraction, diffraction, interference ................................................................ 1403 Mie theory ................................................................................................................................. 1434 Kubelka-Munk function for opaque layers ....................................................................... 1495 Saunderson correction: how surfaces influence the outcome of reflectance measurements ........................................................................ 1516 Kubelka-Munk equation for transparent layers .............................................................. 1567 Multi-flux theory ..................................................................................................................... 1577.1 Criticism of the Kubelka-Munk model ............................................................................... 1577.2 Radiative transfer equation................................................................................................... 159

VIII Practical applications Uwe Hempelmann ............................................................................................... 165

1 Tinting strength....................................................................................................................... 1652 Hiding power ............................................................................................................................ 171

Contents 11

IX Measuring the texture of effect finishes Sandra Weixel ..................................................................................................... 172

1 Sparkle and graininess .......................................................................................................... 1721.1 Visual evaluation of sparkle and graininess..................................................................... 1721.2 Instrumental measurement of sparkle and graininess .................................................. 1741.2.1 Sparkle measurement under direct illumination ............................................................ 1741.2.2 Graininess measurement under diffuse illumination .................................................... 1752 Sparkle and graininess applications .................................................................................. 1752.1 Influence of flake size on sparkle and graininess ........................................................... 1752.2 Influence of flake orientation on total colour impression ............................................... 1763 Conclusions ............................................................................................................................... 176

X Characterisation of pigments ......................................................................... 180

1 Inorganic pigments – characterisation Rainer Henning ......................................................................................................................... 1821.1 White (P.W. 6) and black (P.Bl. 7) ......................................................................................... 1821.2 Important inorganic colouredpigments ............................................................................. 1821.2.1 Inorganic yellow and red pigments ..................................................................................... 1831.2.2 Inorganic green and blue pigments .................................................................................... 1842 Organic pigments – characterisation Rainer Henning ......................................................................................................................... 1852.1 Red pigments ............................................................................................................................ 1852.2 Orange pigments ..................................................................................................................... 1892.3 Yellow pigments ...................................................................................................................... 1902.5 Blue organic pigments ........................................................................................................... 1942.6 Violet organic pigments ......................................................................................................... 1953 Characterisation of aluminium pigments Hans-Jörg Kremitzl ................................................................................................................... 1983.1 The metallic effect and its cause .......................................................................................... 1983.1.1 Leafing and non-leafing properties .................................................................................... 1983.1.2 Particle-size and diameter .................................................................................................... 1993.1.3 Particle shape, thickness, and topography ........................................................................ 1993.1.4 Orientation of pigments in the paint film .......................................................................... 2003.2 Comparison of various pigment grades ............................................................................. 2013.3 Chemical and mechanical properties ................................................................................. 2014 Characterisation of pearlescent pigments and special effect pigments Peter Gabel and Gerhard Pfaff ............................................................................................... 2024.1 Manufacture, properties and types of special effect pigments .................................... 2034.1.1 Metal oxide mica pigments ................................................................................................... 2044.1.1.1 Titanium dioxide mica pigments ......................................................................................... 2064.1.1.2 Titanium dioxide mica pigments with multi-layers ........................................................ 2094.1.1.3 Iron(III) oxide mica pigments ............................................................................................... 2114.1.1.4 Combination pigments based on metal oxide and mica ................................................ 2124.1.2 Effect pigments based on alumina flakes .......................................................................... 2134.1.3 Metal oxide pigments based on borosilicate flakes ........................................................ 214

Contents12

4.1.4 Metal oxide pigments based on silica flakes ..................................................................... 2164.1.5 Metal oxide pigments based on iron oxide flakes ............................................................ 2174.1.6 Multi-layer pigments with a Fabry-Perot structure ........................................................ 2184.1.7 Effect pigments based on liquid-crystal polymers (cholesteric effect pigments) .... 2194.1.8 Structured effect pigments ................................................................................................... 220

XI Recipe prediction Uwe Hempelmann ............................................................................................... 224

1 Recipe prediction for solid colours ...................................................................................... 2242 Calibration of colourants ........................................................................................................ 2263 Computer-aided correction of colour recipes .................................................................... 2284 Practical colour-recipe prediction of gonioapparent colours ........................................ 2314.1 Topology of effect pigments in surface coatings .............................................................. 2354.2 The limitations of colour recipe prediction ....................................................................... 2365 The profitability of colour recipe calculation .................................................................... 2375.1 Review ........................................................................................................................................ 2375.2 General savings potential afforded by colour recipe calculation ................................. 2385.2.1 Swift feasibility analysis ...................................................................................................... 2385.2.2 Low-cost metamerism-free recipes .................................................................................... 2385.2.3 Accurate corrections in production ..................................................................................... 2385.2.4 Fewer complaints .................................................................................................................... 2395.2.5 Computer algorithms for saving costs in specific applications .................................... 2395.2.6 Profitability analysis – summary ........................................................................................ 2426 Guidelines for formulating and matching object colours ............................................... 2426.1 Rules for mixing pigments .................................................................................................... 2436.2 Solid colours ............................................................................................................................. 2446.3 Gonioapparent colours ........................................................................................................... 2476.4 Hiding power and pigmentation level................................................................................. 2497 Recipe dosability ...................................................................................................................... 2508 Structure of colour mixing systems ................................................................................... 2518.1 Introduction .............................................................................................................................. 2518.2 Alternative methods of paint production .......................................................................... 2518.2.1 Production of OEM paint material ....................................................................................... 2518.2.2 Advantages of colour mixing systems ............................................................................... 2518.2.3 Paint mixing systems ............................................................................................................ 2528.2.4 Universal mixing systems .................................................................................................... 2538.3 Coloristic demands on colour mixing systems ................................................................. 2539 Optimisation of colour mixing systems ............................................................................. 2559.1 CIELAB colour maps of colour mixing systems .............................................................. 2559.2 Colouring characteristics of pigments ............................................................................... 25610 Colour gamuts and the limits of colour matching............................................................ 258

XII Microscopic analysis of effect pigments Gerhard Wilker ................................................................................................... 263

1 Matching of effect colour shades ......................................................................................... 263

15Human colour vision

I Fundamentals of colour perception

Stephan Gauss

1 Human colour visionThe term “colour” has different meanings. It is used variously to describe a characteristic of an object perceived by the eye, a paint or surface coating applied to a garden fence, and it can refer to a printing ink. Consider also the various materials, which give these products their colour. Such colourants are either pigments (insoluble in the paint or coating medium), or dyes (soluble in the medium), see Chapter IX. Colourants can be further ordered according to their “colour index” (C.I.), which more accurately should be called “colourant index.”

A question that often arises during daily work in the paint and coatings industry is: “is today’s production batch the same colour as the reference sample?” This book will show how we can obtain useful numbers from this perception in the brain to help us answer that question.

Any description of the term “colour” must distinguish between the physical processes that lead to a sensory stimulus in our eyes and our own subjective evaluation of that stimulus, which is transmitted from the eye to the brain. This is shown in Table I.1.

The left column in Table I.1 is more bio-physical in nature and is discussed in more detail in Sections I.1.1 and I.1.2. The right column is explained in Section I.1.3.

It is important to understand that the colour of an object exists only in our mind; different people may describe the colour of the same object differently. For this reason, it is common practice in the commercial world to talk instead about the colour differences from a given reference standard and not about the colour itself. Naturally, the coloristic attributes of the reference standard must be close to those of the sample being assessed. Because colour dif-ferences are usually small, people usually find it easier to agree on them. These colours are also known as related colours.

German standard DIN 5033 (Part 1) [1] defines the term colour in very sober terms:

Colour is the sensation of a part of the visual field, which the eye perceives as having no struc-ture and by which this part can be distinguished alone from another structureless and adjoining region when viewed with just one motionless eye.

Colour as a sensory perception

In this book the term colour will be used to describe the sensory perception which the brain asso-ciates with a given attribute of an object. This perception is affected

External, objective Subjective

Physical stimulus, colour stimulus (φ)

Colour perception

Colorimetry spectrum Colouring perceptions

Table I.1: Objective and subjective colour perception.


I Fundamentals of colour perception16

by many other influences, in addition to the given attribute of the object. Thus, although the particular perception may vary from person to person, the environment and our own physi-cal condition influence our colour perception, too. This is echoed in the phrase “looking at the world through rose-tinted glasses”.

If we wish to perceive the colour of a non-luminous object, we must first illuminate it with a light source. Alternatively, we could observe the colour of a light source (luminous colour). These two ways are equivalent as regards the actual colour stimulus and the associated colour perception. Thus, our colour perception results from the interaction of three things:

• Light source• Coloured object• Detector (eye and brain)

This is illustrated in Figure I.1.

1.1 The human eyeIn humans, the eye is the most important sensory organ [2, 3]. It is the origin of most of the signals processed in our brain. Because of this importance, nature has provided it with

Figure I.1: Our impression of the colour of an object is the outcome of interaction between a light source, the observed object, the human eye, and signal processing in our brain. Source: Clariant Produkte (Deutschland) GmbH

Figure I.2: Schematic diagram of the human eye. Source: Klett Verlag [4]

Human colour vision 17

several protective mechanisms. It is located in the eye-socket, where it is protected by the nasal bone, the cheekbone and the frontal bone. It has eyelashes to keep dust and dirt from entering and it features an eyelid-closure reflex for coping with an emergency. Its surface is continually cleaned by lachrymal fluid. Over the course of evolution, the eye as a means of providing vision has passed through a number of designs. A schematic diagram of a human eye is shown in Figure I.2.

1.1.1 Optical structure

Light passes through the cornea and the jelly-like lens of the eye into the vitreous humour (vitreous body). In front of the lens is the iris, which acts as an aperture that enables the eye to adapt to varying lightness levels. The iris is capable of capturing three aperture stops, in the manner of a camera diaphragm. This corresponds to a correction factor of 1 : 8, but does not quite reflect the geometric conditions of 2 to 8 mm diameter for the pupil (aperture) (Stiles-Crawford effect). The ciliary muscle around the lens enables the latter to adapt to dif-ferent focal distances in the manner of a telephoto lens by making the lens more spherical for focusing on near objects and flatter for distant objects. At the back of the vitreous humour is the retina, which contains the cells responsible for processing the sensory signals. Its structure is also shown in Figure I.3.

1.1.2 Signal processing and special features

The retina is a light-sensitive, irregularly structured layer of tissue that lines the back of the vitreous humour [6]. Colour vision is only possible at angles of up to 40° around the optical axis. At greater angles, only monochromatic (black and white) vision is possible, while at smaller angles, colour vision and resolution increase. Close to the optical axis and located on the retina is the fovea, the area of highest resolution and greatest visual acuity. It has a visual angle of around 2° and is located within the macula, which contains pigments believed to provide the photoreceptors with additional protection against intense exposure to illumination. Surprisingly, the fovea is not on the optical axis, but is offset above it by about 4°. On the other side of the optical axis, offset by 10°, is another special feature of the human eye: the blind spot. This is an area where all the nerve fibres come together and leave the retina. Consequently, there are no photoreceptors in the blind spot.

Figure I.3: Schematic diagram of the human retina showing the arrangement of rods and cones. The light is incident from the bottom. Source: Cornelsen Verlag [5]


Rods and cones

There are essentially two types of photoreceptor cells in the retina, namely rods and cones, which derive their name from the shape of their sensitive areas. Rods and cones are not evenly distributed across the retina – for example, the macula contains a high concentration of just cones. The proportion of rods increases with distance from the optical axis, with the result that areas of the retina outside the visual angle of 40° contain only rods. Aside from this variation in rod and cone distribution, the concentration of all sensory cells decreases with increase in distance away from the retina and so too does the density of connections between these sensory cells and the brain. Inside the macula, each cell (here: cone) is con-nected to the visual centre by one nerve fibre, but with increase in distance from the centre more and more sensory cells are connected to a single nerve fibre. Ultimately, in the outer part of the retina, more than 100 rods and cones are connected to one nerve fibre. As a result, there are only 1 million nerve fibres for the roughly 6 million cones and 100 million rods in the human eye. Apart from photoreceptors and nerve fibres, the retina contains cells for processing the electrical signals, and fine blood vessels.

Individual images make up the overall picture

One consequence of the structure of the human eye is that we mostly see using just 0.02 % of the retina, i.e. that portion which is located in the macula. Recent studies have shown that the images of the viewed object are not created on the retina in the manner of a camera, but rather that small eye-movements cause the macula to keep refocusing on new areas. The brain composes all these individual images, which are captured every split second, to form a steady image. In other words, our vision is not due to our eyes fixating on an object, but instead is a composite of a large number of individual images.

1.2 The photoreceptor cells in the human eye

Electromagnetic waves in the visible wavelength range trigger a chemical reaction in the photoreceptor cells, generating an electrical signal, which is transmitted by the nerve fibres to the visual centre in the brain (colour stimulus). Photoreceptor cells are around 40 µm in length and 2 µm in diameter. Surprisingly, they are located in the retina in such a way that the light ray from the lens has to pass through the entire cell to reach the photosensi-tive area. All the nerve fibres stretch between the retina and the vitreous humour and

Figure I.4: Schematic diagram of the (a) rods and (b) cones in the human retina. The light is incident from the left. Source: Clariant Produkte (Deutschland) GmbH


exit to the brain via the blind spot. A schematic diagram of the structure of rods and cones is shown in Figure I.4.

Rods and cones have different functions. Rods do not convey any sense of colour, but rather are responsible for our ability to distinguish between light and dark in low light conditions (scotopic vision). They are sensitive to luminance levels down to below 0.1 cd/cm2, and are responsible for our night vision. A single photon is enough to make them fire. As the light intensity increases, the rods become less responsive and the cones start to take over. Cones are two orders of magnitude less photosensitive and are responsible for our day vision. The stimuli experienced by the cones ultimately give rise to our colour vision (photopic vision). There are three types of cones in the eye, which sense light in different bands of the visible spectrum. The photosensitive protein molecules (opsins) located in the outer parts of the cones (and the rods) differ in their maximum sensitivity. The stimuli experienced by the three types of cones are processed separately. It is the different stimulation of the three types that creates the perception of colour in our brain.

1.2.1 Spectral sensitivity of the receptors

The advent of micro-spectrometers after 1960 permitted the spectral sensitivity of the rods and the three types of cones to be analysed directly. The cones are named according to their sensitivity to wavelength ranges: short (S), medium (M) and long (L). In the literature, they are also referred to as R (red), G (green) and B (blue). They exhibit maximum sensitivity at 420 nm (S), 530 nm (M) and 560 nm (L). These spectral sensitivities are shown in Figure I.5, and are relative to the overall sensitivity of the fovea. It should be noted that this fun-damental data, which will later be used in colorimetric calculations (see Section I.7), was determined on a small number of test persons (17) in the 1920s [7, 8]. The chosen persons were deliberately younger than 30 years old in order that incipient yellowing of the eye-lens and age-related macular degeneration could be ruled out. This shows that everyone’s eyes are different and that everyone’s colour perception is subjective. The eye sensitivities discussed here are therefore average values for a fairly small test sample (people with normal vision; emmetropia). New tests to broaden the data base are planned.

Lightness and colourfulness

The three sensitivity ranges, which are shown in Figure I.5, overlap extensively. Rather than being a drawback, this allows us to see colour. The stimuli to which the photorecep-tors respond need to be differentiated on the basis of light intensity and wavelength. A strong signal fired by an L photoreceptor may be caused by light of high intensity at 500 nm (range of low sensitivity) or of low intensity at 560 nm (range of high sensitivity). At each individual wavelength, the signals emitted from the three types of cones are in a fixed ratio. Any signal-processing system can derive information about the light intensity from the absolute signal height while the spectral composition can be determined from the ratio of the different signals. This is how we are able to perceive lightness and colour separately. Details on human signal processing during vision can be found in Kaiser and Lee [3, 9]. Our current understanding is that the three cones and the rods generate three types of signal (pairs of parameters) that are transmitted to the brain. These three signals together are called colour stimulus and can be represented mathematically as a vector.

It should also be noted that there are substantial differences in the number of S, L and M cones in the retina. That is to say, there is no defined pattern equivalent to the red, green and blue pixels on TV screens. Recent studies assume that the frequency distribution


for the S, M und L receptors is 1 : 3 : 6 [10]. One reason for the rela-tively small number of S cones might be chromatic aberration of the eye-lens. As its focal point var-ies with the wavelength, it cannot accurately focus the entire visible spectrum on the retina. The eye is likely optimised for the M and L cones whose maxima are closer together while a blurry image will reach the S cones. Conse-quently, a fine distribution is not needed for this wavelength range.

Consider now the rods, which fire even at low light levels. Their light-sensitive pigment is rhodop-sin, which mainly absorbs in the blue-green range. Its sensitiv-ity (response) curve is shown in Figure I.6. The colour stimulus of the rods cannot distinguish between light of high intensity in the low-sensitivity region (e.g. above 600 nm) and of low inten-sity at the sensitivity maximum at 507 nm. As a result, humans cannot see colour differences in low light, which is why “at night all cats are grey”. In the transi-tion region from rod vision to cone vision (twilight), the greater sensitivity of the rods in the blue range gives rise to the belief that blue objects become brighter than in broad daylight. This is called the Purkinje effect.

While humans possess trichromatic vision, there are animals which have just 2-cone and even 4-cone sensors (tetrachromatic vision). Some birds, reptiles and fish have cones, which are sensitive in the UV range.

1.2.2 Visual defects

Our vision changes as we get older: the eye-lens tends to lose its elasticity and become yel-lowish. Again, some 8 % of males and 0.5 % of females are born with visual defects (ametro-pia). Their cones may exhibit different sensitivity or even none at all. This kind of colour blindness can be diagnosed with the aid of special colour displays called Ishihara plates that exploit the effect of colour confusion . Another way to test for colour blindness is to use an anomaloscope. With this instrument, the patient uses a set of three primary-colour light sources to match a given reference colour (additive colour mixing, see Section I.3.1).

Figure I.5: Sensitivity of the three types of cones S (blue), M (green) and L (red) as a function of wavelength. Types M and L overlap extensively, with their maxima occurring at 540 nm (M) and 650 nm (L).

Figure I.6: Spectral sensitivity of human photoreceptor rods. Their maximum sensitivity occurs at 498 nm, i.e. between those of the S cones and the M cones.


1.3 Colour perception

1.3.1 Chromophoric attributes

The names we give to colours and our perceptions of them are mainly determined by our cultural history. This would explain why different cultures have different views on colour.

As early as 1850, Grassmann and von Helmholtz described three distinct characteristics of human colour perception. This three-colour theory or trichromatic model1 was posited around one hundred years before experimental proof of the existence of three different cones in the retina was found. Some years later, in 1878, K. Hering published his opponent colour theory according to which our colour perception is based on pairs of hues, namely red/green and blue/yellow, as well as on black and white. This model has since been veri-fied experimentally.

Our perception of colour is based upon three fundamental attributes:

• Brightness or, even more important, lightness • Hue • Chroma or colourfulness (chromaticness), and saturation

Current definitions of all these terms are presented in more detail in ISO 11664 Part 1 and DIN 5033 Part 1 [1, 12]

Chroma is a measure of how much a colour stimulus differs from that of an uncoloured surface of the same lightness. As in the case of lightness, the human eye evaluates sensory stimuli on a relative basis. Chroma can therefore be thought of as relative colourfulness. Finally, by expressing chroma as a ratio of lightness, we obtain the saturation.

1.3.2 Colour constancy

The human brain does not convert transmitted colour stimuli directly into a colour percep-tion, but instead automatically takes account of the conditions in which the observations are made. The raw colour signals themselves do not penetrate into our consciousness.

Colour vision incorporates the concept of colour constancy. Our visual interpretation of a hue always takes ambient lightness into account. For example, the illumination level outdoors in direct sunlight is several orders of magnitude greater than it is inside. The stimuli reach-ing our eyes outdoors should lead to white-out of any object, which would only appear white. The combined input from the eyes and the brain leads to a recognition that the different stimulus triggered by a green apple, say, stems from a change in lighting conditions and not from a change in the hue of the apple itself. Colour constancy is therefore based on the notion that the lightness of an object is determined by the lightness of the environment or of a reference object, and our colour perception changes accordingly (“related colours”, see also Section I.1). For a good example, consider our visual perception of the moon. If we look at it in the afternoon, it appears pale yellow against the blue of the sky. Some hours later, in the dark, we perceive it as being bright yellow even though its lightness, which is the result of illumination by the sun, has not changed much during this short time. A further special effect is that we can make out the details of the moon’s surface at night.

1 The basic idea behind this concept was published in 1807 by Thomas Young


2 Light as Electromagnetic Radiation The light perceived by the human eye consists of electromagnetic radiation. Figure I.7 shows the different wavelength and frequency ranges of electromagnetic radiation on a logarithmic scale, along with several applications in these ranges. The human eye can only detect radiation in the tiny wavelength range of 370 to 700 nm (1 nm = 10-9 m). Due to the low sensitivity of our eyes outside this range, colour measurements are made over the range 400 to 700 nm by international agreement.

Figure I.7: Wavelength and frequency of electromagnetic radiation on a logarithmic scale, along with different applications. Only wavelengths in the narrow range from 400 to 700 nm are detectable by the human eye. Source: Clariant Produkte (Deutschland) GmbH

Figure I.8: A prism disperses the white light into its components: light of different colours and wavelengths (Newton, 1766). A diffraction grating could be used instead to achieve the same dispersion. Source: Clariant Produkte (Deutschland) GmbH

23Light as Electromagnetic Radiation

The relative spectral power S (λ) of the radiation usually differs from wavelength to wavelength, and gives rise to different colour perceptions. In colorimetry, the relative spectral power is usu-ally referenced to 560 nm. Back in 1766, Isaac Newton demon-strated in his prism experiment (see Figure I.8) that white sun-light is a combination of different colours. In other words, there is no wavelength for white light. Light of different wavelengths creates different colour percep-tions in humans. Radiation with a wavelength ranging from 450 to 490 nm appears blue, while that in the range 490 to 560 nm is seen as green, and wavelengths longer than 630 nm are perceived as red. The adjacent wavelength ranges are ultraviolet (<400 nm) and infrared (>700 nm). Electromag-netic radiation itself is colourless; it is human sensory perception, which converts the various spec-tral power distributions into the concept of colour in our brains. The spectral power distribu-tion emitted by an object carries the information about the colour of the object.

The intensity distribution of the radiation is referred to as a spectrum. The curve of this distribution is called a remission curve or reflectance curve R(λ). Further details on reflection and remission are presented in Sections I.4 and VII.2. Only the term reflectance curve is used in daily life. Figure I.9 shows the intensity distributions of a blue (Figure I.9b) and a yellow (Figure I.9d) hue. The colour distribution in a rainbow or in Isaac Newton s prism experiment is created by shifting an intensity maximum along the visible wave-length range, whereby the colour curve changes from blue to green to yellow and finally red. The steeper or greater the slope of the intensity maximum, the purer is the hue. In theory, a spectrum could be based on an abrupt, rectangular distribution where the intensity jumps from 0 % to 100 %. Such (theoretical) colours are called ideal colours.

A constant intensity distribution from 400 to 700 nm would be perceived as achromatic. Depending on the absolute intensity value, an ideal white, ideal grey or black would be cre-ated (Figure I.9a). A single intensity maximum is not the only way to create a colour – the radiation intensity in the spectrum can be any shape. That is how other colours are created in addition to the colours of the rainbow. For example, violet is not seen in the rainbow but is created by a spectrum having maxima in both the blue and the red wavelength ranges. Figure I.9c shows an example of such a spectral power distribution containing a mixture of blue and red. This intensity distribution is perceived as violet. The two methods for mixing colours are presented in Section I.3.

Figure I.9: Schematic diagram of a spectrum of ideal white (a), blue (b) and yellow (d). Mixing blue and red creates violet (c). A bar chart symbolizes the colorimetric measurement of the radiation distribution at discrete sampling points. Measurements, e.g., every 20 nm yield 16 sampling points in the range 400 to 700 nm. Source: Datacolor AG


3 Colour mixingColour mixing is important in daily life because commercial colourants (pigments and dyes) do not cover the full range of all perceptible colours. The technical background to matching colours to a reference sample (recipe calculation) is discussed in detail in Section XI. The basics are presented below.

3.1 Additive colour mixingThere are basically two kinds of colour mixing. In additive colour mixing, different hues add up to eventually yield white. This is shown schematically in Figure I.10. Probably the easiest example of this type of mixing occurs in every TV set. The screen consists of sepa-rate red, green and blue pixels. Varying the intensity of these three pixels allows nearly every colour perception to be created in the viewer’s brain. Adding red and green together, for example, produces yellow. Additive colour mixing is thus the concept behind the mixing of coloured light. Where all three pixels have the same intensity, white is created. Red (R), green (G) and blue (B) are called the primary colours (primaries) of additive col-our mixing. Complementary colours are two coloured lights that add up to white (see also Grassmann’s 1st law).

3.2 Subtractive colour mixingWhen we experimented with paints as children, however, we had a different experience. Mixing red and green produces a grey-brown colour – not yellow as described above. Mix-ing of pigments, i.e. colourants, usually follows the rules of subtractive colour mixing. Consider the fundamentals of colour creation. The red colour of a pigment is based on the absorption of blue and green wavelengths whereas green is created by the absorption of red and blue wavelengths. Mixing of red and green colourants will lead to absorption of nearly

Figure I.10: In additive colour mixing, a combination of all three primaries yields white. This type of colour mixing occurs most often with coloured lights. Source: Clariant Produkte (Deutschland) GmbHa

25Interaction of light and matter

the whole visible spectrum, leaving only grey-brown. The term “subtractive” is somewhat misleading. A better term would be “the addition of the absorption effect of colourants.” The primaries for subtractive colour mixing are cyan (C), magenta (M) und yellow (Y), and are especially important in the printing industry. They add up to yield a kind of black (in the printing industry a really good black (K [for key]) is included by way of a fourth colour component). Subtractive mixing of cyan und yellow is illustrated in Figure I.11. This reveals another important aspect of this type of colour mixing, namely that subtractive mixing mostly yields spectra of low intensity (darker colours) and flatter slope. As a result, the mixed colour has less chroma (and less brilliance) than the original colours.

Both kinds of colour mixing can take place simultaneously. For example, coloured pictures can be created by printing several colours on top of each other (subtractive mixing) and by printing pixels of several colours beside each other. We must remember that the process behind colour perception in the human brain is different from the technique used in col-our reproduction. In daily technical communications involving colour, the RGB/sRGB and CMYK colour spaces spanned by their primaries play an important role. They are not the same as the general colour spaces presented in Chapter V. The range of colours that can be produced by a colourant system is called the colour gamut.

4 Interaction of light and matterWhen light strikes matter (material), the interaction that occurs depends on the nature of the matter. And this will also influence the colour of the light.

Transmission: Transmission occurs when the light beam passes through the material without modification. Such a material is said to be transparent. In addition, if the material is also colourless, the light beam will exit the material with the same intensity distribution (spectrum). Only a small quantity is reflected at the point where it enters and exits the mate-rial (interface of material and air or vacuum).

Figure I.11: Subtractive mixing of cyan and yellow pigments yields green (a). The spectra (b) illustrate how the green reflectance curve is generated by the subtractive mixing of cyan and yellow reflectance curves. Source: Clariant Produkte (Deutschland) GmbH

291Index

Symbole

2,9-dimethylquinacridone 197

2-flux concept 159

2-flux model 158, 159

β-naphthol 189

A

absorbing power 165

absorption 26, 143, 151

by carbon black 147

coefficient 226

accurate corrections in production 238

achromatic axis 38

achromatic pigment 182

achromatic point 257

adjustment 229

aging process 241

albedo 160

alumina flakes 213

aluminium pigment 233

amount of paste 253

analysis, feasibility 238

angular distribution 149

anthraquinone 186

appearance 114

coarseness 114, 115

illumination conditions 115

sparkle 114

visual texture 114

approximation method 232

azimuth angle 233

B

base colours 253

batch, correction 231

benzimidazolone 190, 192

BFD formula 101

rotation term 101

bismuth vanadate yellow 184

bi-spectral measurement 50

blackbody radiator 28

blank-sample 75

blends with white 250

blind spot 17

blue organic pigments 194

blue pigments 184

BON pigment Mn lake 187

borosilicate flakes 214

boundary layer 161

brightness 21, 201

BSI 93

colour standards 94

C

calculation of reflectance 152

calibration

black 69

of colorants 226

samples 226

white 69

carbon black 182

car repair 241

characterisation

of aluminium pigments 198

of pigments 180

cholesteric effect pigments 219

cholesteric phase 219

chroma 21, 38

chromaticity

coordinates 34

diagram 34

chromium oxide green 185

chrome rutile yellow 184

chromophore 182

CIE31 system 32

CIE94 equation 102

CIE 1976 colour space 35

CIEDE2000 formula 102

rotation term 103

CIELAB system 35

CIELAB unit 37

Index


Index292

CIELUV system 36

CMC-formula 99

cobalt blue pigment 185

cobalt green pigment 185

colour 15

colour assessment 119

hue 119

observer 119

viewing conditions 119

visual 79

colour deficiency 79

colour notation systems

CIELAB 86

CIELUV 86

DIN99 86

DIN99o 87

colour characterisation, visual 47

colour constancy 21, 43

colour depth 166, 167

colour depth adjustment 254

colour difference 38, 225, 228

colour difference assessment 98

colour difference metric

effect colours 113

solid colours 98

influence of texture 114

lightness weighting for effect colour shades 113

colour difference models 98

BFD(kL:kC)

chroma and hue scales 110

CIE94 102

CIEDE2000 102

colour difference equation CMC(kL:kC)

comparison 107

comparison of total colour difference 110

DIN99 104

DIN99o 104

Euclidean 99

geometric structure 98

goniochromatic colours 99, 101, 113

lightness scale 108

non-Euclidean 99

normalisation 108

parametric effects 106

performance criteria 112

Riemannian space 99

scaling parameter 99

state of development 112

weighting function 99

colour distance 38

colour formulation system 238

colour gamut 245

CIELUV colour space 260

computation 259

effect pigments 259

hull 260

interference pigments 260

optimal colours 258

solid colours 258

Colour Index 15, 180

colour location 166, 182

colour matching 228, 246, 259

limits 258

colour matching function 32

colour measurement 127, 180

covariance ellipsis 128

covariance matrix 128

F-test 129

Gaussian distribution 127

measurement error 130

multivariate statistics 127

probability integral 129

scattering ellipsoid 128, 132

significance of measurement results 127

significance test 129

statistics for 3-dimensional colour spaces 128

colour measurement, non-contact 77

colour measurement, source of error 73

colour mixing

additive 24

subtractive 24

colour mixing system 251, 251, 253

advantages 251

change of type 255

CIELAB colour map 255

coloristic demands 253

colouring characteristics of pigments 256

concept of pigment pastes 251

metamerism 256

optimisation 255

paint mixing systems 252

performance 252

rationalisation effect 253

tolerance solid 254

type conformity 254

type mutation 255

universal 253

colour recipe calculation, profitability 237

Index 293

colour, caused by absorption and emission 138

colourant 15

colourant assortment 251

colour-card collections 93

colour-difference metric

colour-matching functions 31

colour-measurement instrument, profiling 76

colour-measuring instrument, calibration 69

colour naming systems 93

BSI 93

electronic 96

Pantone 94

RAL 93

transformations 94

colour notation systems 86

DIN colour system 88

NCS colour system 89

OSA-UCS 90

RAL design 92

colour perception 15, 21

colour position 243

colour recipe prediction 251, 256

colour scales

chroma 86

hue 86

lightness 86

colour space 182, 243

colour stimulus 18, 19

colour temperature

correlated 29

colour tolerance 41, 117

colour travel 247

colour, causes 137

colour, luminous 16

colour, nonluminous 16

colour, related 15

colour-order systems 85

colour scales 86

definition 85

psychometric scales 85

combination pigments 212

complementary colours 24

cumulative frequencies 124

definition 121

DIN 6175 P1 133

DIN 6175 P2 134

Euclidean colour space 117

interval of uncertainty 125

lower limit 125

non-Euclidean colour space 118

pass-fail assessment 124

pass-fail decisions 121

point of subjective equality 124

three-level acceptance model 126

tolerance 117

tolerance ellipse 126

upper limit 125

cones 18, 19

conservation of radiation energy 159

construction paints 240

continuum theory 157

control measurement 70

copper phthalocyanine 195

cornflake pigment 201

correction

algorithm 231

process 231

crispening effect 110

D

defect evaluation 74

degree of deorientation 235

degrees of freedom 232

delamination effect 250

deorientation agent 248

determination of reflectance 156

dichloroquinacridone 197

difference equation 159

differences in colour position 254

diffraction 140

diffractive pigments 221

diffuse transmission 59

diketopyrrolopyrrole 186, 187

DIN99 equation 104

DIN99 formula

chroma 104

colour coordinates 104

hue-angle 104

DIN99o equation

lightness 104

DIN99o formula

chroma 104

colour coordinates 104

lightness 104

DIN colour system 88

darkness 88

hue 88

saturation level 88

Index294

dioxazine violet 196

dosing and weighing devices 251

downward flux 150

dry hiding film thickness 250

dye 15

E

effect pigment 232, 247

topology 235

manufacture 203

properties 203

equidistance 35

equidistance/lack of equidistance 42

error propagation 73

experimental error 227

extinction 145

eye 16

eye sensitivity 19

F

Fabry-Perot structure 218

feasibility analysis 238

flop control agent 236

fluctuations of film-build 250

fluorescence 72

fovea 17

Fresnel equations 152

Fresnel reflection 158

frost effect 249

G

Gamut 25

gauge samples 75

gloss trap 155

graininess 241

Grassmann, Hermann 21

Grassmann’s laws 228

gravimetric dosing systems 250

green pigment 184, 193

grinding resin 253

H

halogenated copper phthalocyanine 193

Helmholtz reciprocity principle 155

Hering, Konstantin 21

hiding power 146, 171, 199

high brilliance 199

holographic pigment 220

homogeneous hydrolysis 207

hue 21, 38, 244

I

ideal-white 23

illumination modulation 82

indanthrone blue 195

inhomogeneity, sample 74

inner reflection 158

inorganic coloured pigment 182

inorganic pigment, characterisation 182

inorganic pigment, characteristics 147

interference 140

interference colours 208

interference pigment 202, 259

intermediate products 251

intersection point 257

iridescent effects 210

Iron(III)-oxide mica pigment 211

iron oxide flake 217

iron oxide red 184

Ishihara plates 20

isoindoline, opaque 190

isoindoline, transparent 191

isoindolinone 191

K

Kubelka-Munk equation 156

Kubelka-Munk function 149, 166, 225

L

laboratory report 70

Lambert-Beer law 26

Lambert cosine law 152

LCh coordinates 38

leafing pigment 198

LED, light emitting diode 30

light

booths 80

fluorescent 28

source 29

stray 75

light-absorbing pigments 202

lightness 21, 38, 165, 166, 167, 182

axis 182

criterion 167

value 182

liquid-crystal polymers 219

Index 295

Lorenz-Debye-Mie theory 143

luminosity 33

M

MacAdam ellipses 35, 41

macula 17

manganese-laked azo pigment 197

manual weighing systems 250

matching of object colours 242

colouristic criteria 242

compensative mixtures 245

deorientation agent 248

ecologic demands 242

economic criteria 242

effect colour shades 247

flop killer 249

formulating guideline 242

frost effect 249

hiding power 249

isomeric colour match 246

metameric colour match 246

metamerism 245

metamerism-free 242

mixing rules 243

non-hiding coatings 245

parameric colour match 246

pigmentation level 250

process reliability 242

scope of shading 244

solid colours 244

subtractive colour mixture 243

tintability 244

weathering fastness 246

material parameters 232


measuring accuracy 73

measuring geometries 52

measuring system analyses 73

metal-free phthalocyanine 194

metallic effect 198

metal oxide mica pigments 204

metal oxide mica pigments, structural principles 204

metamerism 42

index 45

metamerism-free recipes 238

Mie scattering for an organic model pigment 146

Mie scattering for soot-like particles 146

Mie scattering for white pigments 145

Mie theory 143

minimum tinting quantity 238

mixed-phase pigment 185

mixing function 228

mixing series 229, 257

mixing system 253

molybdate red 184

monoazo 191, 192

multi-angle geometries 60

multi-layer pigments 218

multi-step correction procedure 228

Munsell 87

chroma 87

hue 87

notation 87

saturation level (chroma) 87

value 87

N

n2-law of radiance 161

naphthol AS 187

NCS 89

blackness 89

chromaticness 89

elementary colours 89

hue 89

notation 89

whiteness 89

new colormetric solution 241

Newton, Isaac 23

nickel rutile yellow 184

non-leafing pigment 198

O

oblique view 247

observer

standard 30

2°- 31

10°- 31

observer, normative 19

opponent colour theory 21

optimal colours 259

orange pigments 189

organic pigments – characterisation 185

organic pigments, characteristics 147

OSA-UCS 90

lightness axis 91

ref-green axis 91

yellow-blue axis 91

outer reflection 158

Index296

P

paint production 251

alternative methods 251

formula completion 251

intermediate products 251

OEM original paints 251

pigment pastes 252

Pantone 94

colour formula guide 94

process colour imaging guide 94

particle shape 199

particle-size 199, 202

pearlescent pigments 202

perylene 186

perylene, transparent 188

phase boundary 158

photoreceptor 18

physical vapour deposition, PVD 198

pigment 15

pigment combination 245

pigment load calculation 239

pigment mixtures 243

binary 243

gonioapparent 244

quaternary 243

ternary 243

pigment orientation 200

pigment particle dimensions 205

Planck’s law 28

power distribution

spectral 28

practical applications 165

precision of measurement 73

psychometric scales 85

interval scale 86

nominal scale 85

ordinal scale 85

ratio scale 86

psychophysical experiments 118

colour assessment 119

method of constant stimuli 118

method of equal stimuli 118

paired comparisons 118

method of judgement scale 118

statistical threshold determination 119

thresholds 118

Purkinje effect 20

purple line 34

pyrazoloquinazolone 189

Q

quality assurance 73

quinacridone red 196

quinophthalone 192

R

radiation balance 159

radiation, electromagnetic 22

radiative transfer equation 159

method of discrete ordinates 161

radiation field 160

spectral radiance 160

radiative transfer model 228

radiator

luminescence 29

thermal 29

RAL 93

register 93

RAL-design-system 92

notation 92

Raman scattering 143

Rayleigh scattering 148

receptors 18

recipe calculation

critique of the Kubelka-Munk model 157

effect colour 157

effect colour shades

recipe simulation 234

effect colour shade

high-solid system 237

coherent scattering 237

deorientation 235

dependent scattering 237

gonioapparent colour shades 231

influence of polarisation 237

limitations 236

pigment calibration 232

practical examples 234

reflection indicatrix 235

scattering function 233

topology 235

recipe correction 228, 234

action matrix 229

batch correction 231

colour match 228

concentration increments 229

constraint 229

risk minimisation 231

Index 297

spectral match 228

Taylor expansion 228

tinting vector 229

vectorial notation 229

recipe

dosability 250

ingredients 229

prediction 224

prediction program

system of rules 243

recipe calculation

effect colour shades 157, 231

optical material parameters 232

red pigments 183, 185

reduction 75

reduction in complaints 239

reference illumination 45

reference state 233

refinish mixing system 249

reflectance curve 182

reflectance, maximum 182

reflectance measurements 55

directional geometries 56

reflectance spectra 227

reflection 26, 140

reflection indicatrix

simulation 234

reflectance curve 23

refraction 140

refractive index 26

refractive index of inorganic and organic pigments 147

regular reflection 158

remission curve 23

reproducibility 255

residual colour difference 165, 168

calculated 168

measured 168

retina 17

rods 18

S

sample preparation 74

saturation 21

Saunderson correction 151, 154

scattered light, intensity 148

scattering 26, 143, 151

isotropic 160

scattering coefficient 226

scattering ellipsoid 130

application error 130

homogeneity error 130


scattering function 160

asymmetry parameter 162

Henyey-Greenstein 162

Rayleigh scattering 162

scattering index 26

scattering power 165

sensitivity, spectral 19

shade 21

shading process 244

silica flakes 216

single-step correction procedure 228

SKM model 244

software for colour measurement 75

special effect pigment 202

specific application 239

spectral matching 224

spectral method 47

spectral power 23

spectrum 23

spectrum locus 34

sphere geometries 55

staining and glazing, recipe calculation 240

standard depth (SD) 180

standard illuminant 27

standardised sample 180

statistical threshold determination 119

cumulative Gaussian function 120

Gaussian function 120

logistic function 120

logit transformation 120

probit transformation 120

psychometric function 120

Stiles-Crawford effect 17

stimulus, sensory 15

structured effect pigment 220

surface, influence on measurement 70

T

temperature

colour 28

Thénard´s blue 185

thickness 199

three-colour-theory 21

tinting step 231

tinting strength 165, 166, 167

Index298

tinting strength adjustment 76, 254

titanium dioxide mica pigment 204, 206

with multi-layers 209

titanium dioxide, rutile-anatase modification 207

titration method 207

tolerance frame 253

topography 199

total extinction 145

total transmission 59

transmission 25

transmission measurements

geometry recommendations 58

transparency of sample 75

trichromatic model 21

tri-coats 245

tristimulus match 229, 231

tristimulus value 32, 168

V

violet organic pigment 195

vision, photopic 19

vision, scoptic 19

visual colour assessment 40

visual defects 20

volumetric dosing system 250

von Helmholtz, Hermann 21

W

weathering fastness 246

Y

yellow pigment 183,190

The Mission: The latest knowledge on the colour technology under-pinning coatings in a single book – from the fundamentals of colour perception, to colour measurement and colour-order systems, through to the characterisation and practical application of pigments. The book serves as a vehicle for acquiring a solid grounding in the key principles underpinning the application of colour technology to coatings. Indispensable for coatings experts and those with aspirations in that direction.

The Audience: Trainees, students and newcomers to the profession who are seeking to acquire a solid grounding in colour technology, as well as experts wishing to deepen, extend or refresh their knowledge.

The Value: This book provides a comprehensive state-of-the-art sur-vey of relevant industrial colour applications and explains the various physical physiological aspects of pigment optics. Colour problems discussed in this book include the optics and chemistry of solid-colour and effect pigments, colorant formulation, optical microscopy of effect colour shades for pigment identification, methods of elemen-tary and advanced colorimetry, measurement and visual assessment of solid and effect colour shades, colour tolerances and acceptability, and colour-order systems. There is also a special chapter devoted to the newly emerging area of visual texture assessment of effect colour shades.

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Wilhelm Kettler et al.Colour TeChnology of CoATings

ISBN 978-3-86630-600-4

Colour TeChnology of CoaTings

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