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Let's Talk About Colorimetric Basics

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Let's Talk About Colorimetric Basics Let‘s talk about Colorimety Let‘s talk about Colorimetric Basics Textile Effects Electro-Magnetic Waves visible light Textile Effects Different technics of color mixtures Additive color mixture e.g. used for TV or computer screens or paper graphics Subtractive color mixture e.g. used for textile dyeing or printing Textile Effects Relation between reflectance and absorbance light source human eye textile sample The light is either reflected or absorbed by the sample Textile Effects Reflectance Reflectance Reflectance factor beta in % I r %R 100 I0 The reflectance factor is the ratio of the reflected light Ir to the illuminating light I0 Textile Effects Absorbance Absorbance Absorbance factor in % A 1 %A 1 100 The absorbance factor is the difference between the total, illumuninating light and the reflected light Both, reflectance and absorbance, are relative factors and independant from the energy distribution of the illuminant Textile Effects Reflectance Curves/1 Reflectance curce of a yellow shade Reflectance curve of a red shade 1 1 0.9 absorbed light 0.8 0.8 0.7 0.6 0.6 0.5 absorbed light reflected light 0.4 0.4 Reflectancebeta factor Reflectancebeta factor 0.3 reflected 0.2 0.2 light 0.1 0 0 400 500 600 700 400 500 600 700 Wavelength [nm] Wavelength [nm] Textile Effects Reflectance Curves/2 Reflectance curve of a blue shade Reflectance Curve of a Green Color 0.6 0.6 0.5 0.5 0.4 absorbed light 0.4 absorbed light 0.3 0.3 Reflectancebetafactor Reflectancebeta factor 0.2 0.2 0.1 0.1 reflected light reflected light 0 0 400 500 600 700 400 500 600 700 Wavelength [nm] Wavelength [nm] Textile Effects Commission International de l‘Éclairage (CIE) The CIE has defined several standard illuminants: D65 = Average daylight (6500 K) A = Tungsten (incandescent) light (2854 K) But also fluorescent lamps like: F2 = Cool White Fluorescent CWF (4200 K) F11 = Fluorescent light Philips TL84 (4000K) The illuminants are defined by its color temperature and the relative spectral energy distribution. Textile Effects CIE Standard Illuminant D65 Daylight (6500 K) 150 100 50 Relative spectral energyspectralRelative distribution 0 350 400 450 500 550 600 650 700 750 Wavelength in nm Textile Effects CIE Standard Illuminant A Incandescent light (2854 K) 250 200 150 100 50 Relative spectral energy distribution energy Relative spectral 0 350 400 450 500 550 600 650 700 750 Wavelength in nm Textile Effects CIE Illuminant F2 Cool White Fluorescent (4200 K) 50 40 30 20 10 Relative spectral energy distribution energy Relative spectral 0 350 400 450 500 550 600 650 700 750 Wavelength in nm Textile Effects CIE Illuminant F11 Fluorescent light Philips TL84 (4000 K) 100 80 60 40 20 Relative spectral energy distribution energy spectral Relative 0 350 400 450 500 550 600 650 700 750 Wavelength in nm Textile Effects Human Eye Cross section of the human Cross section of the retina eye Textile Effects Human Eye/2 In the retina there are two different types of optical sensors: Rods for light/dark sensation Cones for color sensation Cones are sensitive to red or green or blue. CIE has defined the sensitivity of the cones as standard observers for: 10° (large field) 2° (small field) Textile Effects CIE Standard Observer 10° 2.5 _ z 2.0 1.5 _ _ x y 1.0 Tristimulus values illuminant E illuminant values Tristimulus 0.5 0.0 350 450 550 650 750 Wavelength in nm Textile Effects The color vision process CIE Standard Illuminant D65 Reflectance curce of a yellow (Daylight) 1 shade 150 absorbed light 0.8 textile sample 100 0.6 reflected light 0.4 Reflectancebeta factor 50 Relative spectral energy distribution energy spectral Relative 0.2 0 0 350 400 450 500 550 600 650 700 750 400 500 600 700 Wavelength in nm Wavelength [nm] light source Reflectance curve CIE Standard Observer Red CIE Standard Observer Green CIE Standard Observer Blue 1.5 1.5 2.5 _ _ z x 2.0 _ y 1.0 1.0 1.5 1.0 Tristimulus values Tristimulus Tristimulus values Tristimulus Tristimulus values Tristimulus 0.5 0.5 0.5 0.0 0.0 380 430 480 530 580 630 680 730 0.0 380 430 480 530 580 630 680 730 380 430 480 530 580 630 680 730 Wavelength in nm Wavelength in nm Wavelength in nm eye sensors Textile Effects The color vision process/2 3 color signals from a Yellow color Red Signal of a Yellow Color Green Signal of a Yellow Color Blue Signal of a Yellow Color 100 100 100 80 80 80 60 60 60 40 Y 40 40 X Tristimulus values Tristimulus D65/10 Tristimulus values Tristimulus D65/10 Tristimulus values Tristimulus D65/10 Z 20 20 20 0 0 0 400 500 600 700 400 500 600 700 400 500 600 700 Wavelength in nm Wavelength in nm Wavelength in nm Textile Effects CIE Color Coordinates Calculation of CIE Calculation of CIE Tristimulus Values chromaticity coordiantes 700 _ X X k S x x 400 X Y Z 700 _ Y y Y k S y X Y Z 400 _ Z 700 z Z k S z X Y Z 400 S = illuminant e.g D65 The sum of x+y+z x,y,z = CIE standard observer is always 1. = reflectance factor k = normalization factor Textile Effects CIE 1931 chromaticity diagram Textile Effects Let‘s talk about Color difference and tolerances Textile Effects MacAdam Ellipses The MacAdam ellipses define the visible color difference perceptibilty within the CIE diagram. They show, that the CIE x,y,Y color space is not equidistant to our visual perception. Textile Effects Uniform Color Spaces To improve the non-uniformity of the CIE 1964 color space, several transformation from x,y,Y color space to systems with better uniformity have been developed, e.g. : Hunter : L, a, b Adams-Nickerson : ANLAB CIE 1976 : L*, a*, b* Textile Effects CIE 1976 Color Space Lightness L* L* 116 Y*16 Red-Green axis a* a* 500(X*Y*) Yellow-Blue axis b* b* 200(Y*Z*) 2 2 1/2 Chroma C*ab Cab* (a * b * ) Hue h b * ab hab arctan( ) a * Textile Effects Calculation of CIELab difference dL* = L*sample - L*standard Lightness + = lighter - = darker difference: da* = a*sample - a*standard Red - Green difference : + = redder - = greener Yellow-Blue difference: db* = b*sample - b*standard + = yellower - = bluer 2 2 2 1/2 Total colour difference:: dEab* ((dL*) (da*) (db*) ) approximate perceptibility = 0.2 - 0.5 Textile Effects Calculation of CIELCH difference dL* = L*sample - L*standard Lightness difference : + = lighter - = darker dCab* = Cab*sample - Cab*standard Chroma difference : + = brighter - = duller 2 2 1/2 Hue difference : dH* ((dEab * (dL*) (dC*) ) Red + = yellower - = bluer The interpretation is Yellow + = greener - = redder depending on the hue, Green + = bluer - = yellower e.g. : Blue + = redder - = greener Textile Effects Production Control - Color tolerances The CIE-Lab color space is, like the CIE-x,y,Y color space, visually not equidistant. The uniformity has been improved, but is not yet perfect. Therefore tolerances have to be defined for each color. Also the different visual sensitivity of lightness- chroma- or hue-differences has to be considered. To define tolerance limits, sufficient and reliable visual jugements are necessary. Around each standard color up to 50 trials should be distributed evenly and a minimum of 20 to 30 colorists should assess the differences - this means a total of 1000 visual assessments for each colour. Textile Effects Production Control - Pass/fail formulas To avoid the huge effort of defining tolerances for each color, the CIELab color space has been modified by weighting the lightness- chroma- and hue difference in the formula, using large data sets from the industry. Some of this modified formulas are: JPC79 (J.P.Coats) CMC (l:c) (Colour Measurement Committee, UK) BFD (l:c) (Bradford university, UK) (l:c is the ratio of lightness and chroma, recommended is 2:1) M&S89 (Marks & Spencer) Datacolor ISO recommends the CMC (l:c) formula for small color differences (ISO/DP 105-J01).The CMC (l:c) formula has some disadvantages in the blue range of the color space. This has been improved with the BDF(l:c) formula, but there is not enough experience to tell if BDF is generally better than CMC. Textile Effects CIE-Lab Tolerancing CIE-Lab defines a cubic color space, which does not always correlate with our visual perception Textile Effects CIE-LCH Tolerancing CIE-LCH coordinates define a cylindrical color space. The correlation with our visual perception is much better. Textile Effects CMC Tolerancing CMC uses tolerance ellipsoids, which have different shapes within the color space. They have been adapted to visual color perception Textile Effects Let‘s talk about Metamerism and Color Constancy Textile Effects Metamerism Same colors under Different colors under daylight illumination shop illumination Textile Effects Metameric colors under D65 Brown 1 Brown 2 Textile Effects Metameric colors under TL84 Brown 1 Brown 2 Textile Effects Metamerism If a pair of samples looks identical under a particular illuminant, but different under another light source, the samples are described as metameric. They have different reflectance curves and produce different sets of color co-ordinates under different light sources. In colorimetry, metamerism is defined using a metamerism index. Textile Effects Reflectance Curves of metameric Colors 0.8 Brown No. 1 0.6 Brown No. 2 0.4 Reflectancebetafactor 0.2 0 400 450 500 550 600 650 700 Wavelength [nm] Textile Effects Metamerism Index lluminant Co-ordinates Brown 1 Brown 2 D65/10 L* 35.1I 34.4 a* 5.8 4.3 b* 10.0 8.8 CIELab difference dE Standard 2.0 A/10 L* 51.9 52.0 a* 7.8 15.8 b* 7.0 5.5 Metamerism Index dE Standard 6.5 TL84/10 L* 51.6 50.2 a* 9.0 15.0 b* 4.0 2.9 Metamerism Index dE Standard 10.1 Textile Effects Color Constancy Same colors under Same colors incandescent under daylight lamp, but different illumination from daylight Textile Effects Color Inconstancy of a green color Green color under Beige color under illuminant D65 illuminant A Textile Effects Color Constancy If one sample changes color appearance when it is illuminated using a different light source, we talk about good or inadequate color constancy of the sample.
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