Measuring Colour

Robert Hunt, Independent Colour Consultant and Visiting Professor, University of Leeds, UK Formerly Assistant Director of Research, Kodak Limited, Harrow; Dr. Hunt is now an independent colour consultant and a Visiting Professor at the University of Leeds' Department of Colour Science. He has written more than a hundred papers on colour vision, colour reproduction, and colour measurement. He has previously published the 6th edition of The Reproduction of Colour (2004) with Wiley which is viewed as a classic text on the topic. He has been awarded the Newton Medal of the Colour Group (Great Britain) (1974), the Progress Medal of the Royal Photographic Society (1984), the Judd-AIC Medal of the International Colour Association (1987), the Gold Medal of the Institute of Printing (1989), the Johann Gutenberg Prize of the Society for Information Display (2002), and the Godlove Award of the Inter-Society Color Council (U.S.A.) (2007). In 2009 he was given the award of Officer of the British Empire (O.B.E.). Michael Pointer,  Independent Colour Consultant and Visiting Professor, University of Leeds, UK & University of the Arts London, UK Formerly Senior Scientist at the National Physical Laboratory, Teddington; Dr. Pointer is an independent colour consultant and a Visiting Professor at the University of Leeds' Department of Colour Science and the University of the Arts, London, in their School of Communication. In 1997, he received the Fenton Medal, The Royal Photographic Society's award for services to the Society. In 2004, he received a Silver Medal from the Society of Dyers and Colourists for 'contributions to colour science.' He has authored more than 95 scientific papers, is a Fellow of The Royal Photographic Society and the Institute of Physics, Secretary of CIE Division 1 Vision & Colour and UK Associate Editor of the journal, Color Research & Application.

• About the Authors xv Series Preface xviiPreface xixAcknowledgements xxi1 Colour Vision 11.1 Introduction 11.2 The spectrum 11.3 Construction of the eye 31.4 The retinal receptors 41.5 Spectral sensitivities of the retinal receptors 51.6 Visual signal transmission 81.7 Basic perceptual attributes of colour 91.8 Colour constancy 101.9 Relative perceptual attributes of colours 111.10 Defective colour vision 131.11 Colour pseudo-stereopsis 152 Spectral Weighting Functions 192.1 Introduction 192.2 Scotopic spectral luminous efficiency 192.3 Photopic spectral luminous efficiency 212.4 Colour-matching functions 262.5 Transformation from R, G, B to X, Y, Z 322.6 CIE colour-matching functions 332.7 Metamerism 382.8 Spectral luminous efficiency functions for photopic vision 393 Relations between Colour Stimuli 413.1 Introduction  413.2 The Y tristimulus value 413.3 Chromaticity 423.4 Dominant wavelength and excitation purity 443.5 Colour mixtures on chromaticity diagrams 463.6 Uniform chromaticity diagrams 483.7 CIE 1976 hue-angle and saturation 513.8 CIE 1976 lightness, L  523.9 Uniform colour spaces 533.10 CIE 1976 colour difference formulae 573.11 CMC, CIE94, and CIEDE2000 color difference formulae 613.12 An alternative form of the CIEDE2000 colour-difference equation 643.13 Summary of measures and their perceptual correlates 643.14 Allowing for chromatic adaptation 653.15 The evaluation of whiteness 663.16 Colorimetric purity 673.17 Identifying stimuli of equal brightness 673.18 CIEDE2000 worked example 694 Light Sources 734.1 Introduction 734.2 Methods of producing light 744.3 Gas discharges 744.4 Sodium lamps 754.5 Mercury lamps 764.6 Fluorescent lamps 784.7 Xenon lamps 814.8 Incandescent light sources 824.9 Tungsten lamps 864.10 Tungsten halogen lamps 874.11 Light emitting diodes 884.12 Daylight 894.13 Standard illuminants and sources 914.14 CIE standard illuminant A 914.15 CIE illuminants B and C 924.16 CIE sources 934.17 CIE illuminants D 944.18 CIE indoor daylight 944.19 Comparison of commonly used sources 965 Obtaining Spectral Data and Tristimulus Values 995.1 Introduction 995.2 Radiometry and photometry 995.3 Spectroradiometry 1005.4 Tele-spectroradiometry 1005.5 Spectroradiometry of self-luminous colours 1015.6 Spectrophotometry of non-self-luminous colours 1015.7 Reference whites and working standards 1025.8 Geometries of illumination and viewing 1035.9 CIE Geometries of illumination and measurement 1045.10 Spectroradiometers and spectrophotometers 1085.11 Choice of illuminant 1105.12 Calculation of tristimulus values from spectral data 1115.13 Colorimeters using filtered photo-detectors 1146 Metamerism and Colour Constancy 1176.1 Introduction 1176.2 The cause of metamerism 1176.3 The definition of metamerism 1186.4 Examples of metamerism in practice 1196.5 Degree of metamerism 1216.6 Index of metamerism for change of illuminant 1226.7 Index of metamerism for change of observer 1226.8 Index of metamerism for change of field size 1246.9 Colour matches and geometry of illumination and measurement 1246.10 Correcting for inequalities of tristimulus values 1256.11 Terms used in connection with metamerism 1266.12 Colour inconstancy 1276.13 Chromatic adaptation transforms 1296.14 The Von Kries transform 1306.15 The CAT02 transform 1316.16 A colour inconstancy index 1346.17 Worked examples 1357 Colour Rendering by Light Sources 1437.1 Introduction 1437.2 The meaning of colour rendering 1447.3 CIE colour rendering indices 1457.4 Spectral band methods 1477.5 Other methods for assessing the colour rendering of light sources 1507.6 Comparison of commonly used sources 1518 Colour Order Systems 1558.1 Introduction 1558.2 Variables 1558.3 Optimal colours 1578.4 TheMunsell System 1598.5 TheMunsell Book of Color 1648.6 Unique hues and colour opponency 1688.7 The Natural Colour System (NCS) 1708.8 Natural Colour System Atlas 1728.9 The DIN System 1798.10 The Coloroid System 1828.11 The Optical Society of America (OSA) System 1838.12 The Hunter Lab System 1878.13 The Tintometer 1908.14 The Pantone System 1918.15 The RAL System 1918.16 Advantages of colour order systems 1928.17 Disadvantages of colour order systems 1929 Precision and Accuracy in Colorimetry 1979.1 Introduction 1979.2 Sample preparation 1989.3 Thermochromism 1999.4 Geometry of illumination and measurement 1999.5 Reference white calibration 2009.6 Polarisation 2009.7 Wavelength calibration 2029.8 Stray light 2029.9 Zero level and linearity 2029.10 Use of secondary standards 2039.11 Bandwidth 2039.12 Correcting for errors in the spectral data 2049.13 Calculations 2079.14 Precautions to be taken in practice 21410 Fluorescent Colours 21910.1 Introduction 21910.2 Terminology 21910.3 Use of double monochromators 22010.4 Illumination with white light 22110.5 Correcting for differences between an actual and the desired source 22210.6 Two-monochromator method 22410.7 Two-mode method 22510.8 Filter-reduction method 22610.9 Luminescence-weakening method 22610.10 Practical considerations 22711 RGB Colorimetry 23111.1 Introduction 23111.2 Choice and specification of matching stimuli 23111.3 Choice of units 23311.4 Chromaticity diagrams using r and g 23311.5 Colour-matching functions in RGB systems 23411.6 Derivation of XYZ from RGB tristimulus values 3511.7 Using television and computer displays 23912 Colorimetry with Digital Cameras 24112.1 Introduction 24112.2 Camera characterisation 24212.3 Metamerism 24412.4 Characterisation methods 24412.5 Practical considerations in digital camera characterisation 24912.6 Practical example 25112.7 Discussion 25413 Colorant Mixtures 25713.1 Introduction 25713.2 Non-diffusing colorants in a transmitting layer 25713.3 Non-diffusing colorants in a layer in optical contact with a diffusing surface 25913.4 Layers containing colorants which diffuse and absorb light 26213.5 The use of multi-spectral analysis to reduce metamerism in art restoration 26414 Factors Affecting the Appearance of Coloured Objects 26714.1 Introduction 26714.2 Measuring optical properties 26714.3 Colour 26814.4 Gloss 27114.5 Translucency 27914.6 Surface texture 28114.7 Conclusions 28915 The CIE Colour Appearance Model CIECAM02 29315.1 Introduction 29315.2 Visual areas in the observing field 29415.3 Chromatic adaptation in CIECAM02 29415.4 Spectral sensitivities of the cones in CIECAM02 29515.5 Cone dynamic response functions in CIECAM02 29715.6 Luminance adaptation in CIECAM02 29715.7 Criteria for achromacy and for constant hue in CIECAM02 29915.8 Effects of luminance adaptation in CIECAM02 30015.9 Criteria for unique hues in CIECAM02 30315.10 Redness-greenness, a, and yellowness-blueness, b, in CIECAM02 30315.11 Hue angle, h, in CIECAM02 30515.12 Eccentricity factor, e, in CIECAM02 30515.13 Hue quadrature, H, and hue composition, Hc, in CIECAM02 30615.14 The achromatic response, A, in CIECAM02 30815.15 Correlate of lightness, J, in CIECAM02 30815.16 Correlate of brightness, Q, in CIECAM02 30915.17 Correlate of chroma, C, in CIECAM02 31015.18 Correlate of colourfulness, M, in CIECAM02 31115.19 Correlate of saturation, s, in CIECAM02 31115.20 Comparison of CIECAM02 with the natural colour system 31115.21 Testing model CIECAM02 31215.22 Filtration of projected slides and CIECAM02 31415.23 Comparison of CIECAM02 with CIECAM97s 31515.24 Uniform colour space based on CIECAM02 31515.25 Some problems with CIECAM02 31615.26 Steps for using the CIECAM02 model 31615.27 Steps for using the CIECAM02 model in reverse mode 31915.28 Worked example for the model CIECAM02 32116 Models of Colour Appearance for Stimuli of Different Sizes 32516.1 Introduction 32516.2 Stimuli of different sizes 32516.3 Room colours 32516.4 A model for predicting room colours 32616.5 Steps in using the model for predicting room colours 32717 Model of Colour Appearance for Unrelated Colours in Photopic and Mesopic Illuminances 32917.1 Introduction 32917.2 A model for predicting unrelated colours 33017.3 Input data required for the model 33117.4 Steps in using the model for unrelated colours 33217.5 Worked example in the model for predicting unrelated colours 333Appendices 335Appendix 1 Radiometric and Photometric Terms and Units 337A1.1 Introduction 337A1.2 Physical detectors 337A1.3 Photometric units and terms 338A1.4 Radiant and quantum units and terms 340A1.5 Radiation sources 340A1.6 Terms for measures of reflection and transmission 341A1.7 Other spectral luminous efficiency functions 343A1.8 Mesopic photometry 343Reference 344Appendix 2 Spectral Luminous Efficiency Functions 345Appendix 3 CIE Colour-Matching Functions 347Appendix 4 CIE Spectral Chromaticity Co-Ordinates 351Appendix 5 Relative Spectral Power Distributions of Illuminants 355A5.1 Introduction 355A5.2 CIE illuminants 355A5.3 Representative fluorescent lamps 359A5.4 Planckian radiators 368A5.5 Gas discharge lamps 371A5.6 Method of calculating D illuminant distributions 374Appendix 6 Colorimetric Formulae 379A6.1 Chromaticity relationships 379A6.2 CIELUV, CIELAB, and U*V*W* relationships 379Appendix 7 Calculation of the CIE Colour Rendering Indices 383A7.1 Spectral radiance factors of test colours 383A7.2 Worked example of the CIE colour rendering indices 388Appendix 8 Illuminant-Observer Weights for Calculating Tristimulus Values 393Appendix 9 Glossary of Terms 431Reference 453Index 455