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12/4/2013 Newton’s Color Experiments 1671 Newton’s Conclusions • White light has seven constituent components: red, orange, yellow, green, blue, indigo and violet. • Dispersed light can be recombined to form white light. • Magenta and purple can be obtained by combining only portions of the spectrum. 1 12/4/2013 Color and Color Vision The perceived color of an object depends on four factors: 1. Spectrum of the illumination source 2. Spectral Reflectance of the object 3. Spectral response of the photoreceptors (including bleaching) 4. Interactions between photoreceptors Light Sources Spectral Spectral Spectral Output Output Output nm nm nm 400 700 400 700 400 700 White Monochromatic Colored 2 12/4/2013 Pigments Spectral Spectral Spectral Reflectance Reflectance Reflectance nm nm nm 400 700 400 700 400 700 Red Green Blue Subtractive Colors Pigments (e.g. paints and inks) absorb different portions of the spectrum Spectral Spectral Spectral Multiply the spectrum Output Output Output of the light source by the spectral reflectivity of the object to find nm nm nm the distribution 400 700 400 700 400 700 entering the eye. nm nm nm 400 700 400 700 400 700 3 12/4/2013 Blackbody Radiation For lower temperatures, blackbodies appear red. As they heat up, the shift through the spectrum towards blue. Our sun looks like a 6500K blackbody. Incandescent lights are poor efficiency blackbodies radiators. Gas‐Discharge & Fluorescent Lamps A low pressure gas or vapor is encased in a glass tube. Electrical connections are made at the ends of the tube. Electrical discharge excites the atoms and they emit in a series of spectral lines. We can use individual lines for illumination (e.g. sodium vapor) or ultraviolet lines to stimulate phosphors. 4 12/4/2013 CIE Standard Illuminants Illuminant A - Tungsten lamp looking like a blackbody of 2856 K Illuminant B - (discontinued) Noon sunlight. Illuminant C - (discontinued) Noon sunlight. Illuminant D55 - (Occasionally used) 5500 K blackbody Illuminant D65 - 6500 K blackbody, looks like average sunlight and replaces Illuminants B and C. Illuminant D75 - (Occasionally used) 7500 K blackbody Illuminants A and D65 300 250 200 Illuminant A 150 Illuminant D65 100 Spectral Radiance 50 0 300 500 700 Wavelength (nm) 5 12/4/2013 Additive Colors Self-luminous Sources (e.g. lamps and CRT phosphors) emit different spectrums which combine to give a single apparent source. Spectral Spectral Spectral Add the spectrums of Output Output Output the different light sources to get the spectrum of the apparent source entering the eye. nm nm nm 400 700 400 700 400 700 Color Models • Attempt to put all visible colors in a ordered system. • Mathematics based, art based and perceptually based systems. 6 12/4/2013 RGB Color Model A red, green and blue primary are mixed in different proportions to give a color (1,0,0) is red (0,1,0) is green (0,0,1) is blue 24 bit color on computer monitors devote 8 bits (256 values) to each primary color (i.e. red can take on values (0…255) / 255) (1,1,1) is white (0,0,0) is black HSB (HSV) Color Model Hue – color is represented by angle Saturation – amount of white represented by radial position Brightness (Value) – intensity is represented by the vertical dimension 7 12/4/2013 HLS Color Model Hue – color is represented by angle Lightness – intensity is represented by the vertical dimension Saturation – amount of white represented by radial position CIE • 90 year old commission on color • Recognized as the standards body for illumination & color. • Has defined standard illuminants and human response curves. 8 12/4/2013 Luminosity Functions The spectral responses of the eye are called the luminosity functions. The The V() curve (photpic response) is for cone vision and the V’() curve (scoptopic response) is for rod vision. V() was adopted as a standard by the CIE in 1924. There are some errors for <500 nm, that remain. The V’() curve was adopted in 1951 and assumes an observer younger than 30 years old. 1 0.9 V() 0.8 0.7 0.6 V’() Photopic 0.5 Scotopic 0.4 0.3 0.2 0.1 Relative Spectral Sensitivity Spectral Relative 0 380 480 580 680 780 Wvaelength (nm) Purkinje Shift Note how the brightness of reds and blues change with decreasing illumination. This is due to the sensitivity of the eye shifting from photopic to scoptic 9 12/4/2013 Human Color Models Observer adjusts the Luminances of R, G and B R lights until they match C G B R = 700.0 nm G = 546.1 nm B = 435.8 nm C r’(R + g’(G + b’(B C 1931 CIE Color Matching Functions 0.4 0.35 b'() 0.3 g'( ) 0.25 r'( ) 0.2 0.15 0.1 0.05 0 Chromaticity Coordinates -0.05380 430 480 530 580 630 680 730 780 -0.1 435.8 546.1 700.0 -0.15 Wavelength (nm) 10 12/4/2013 Color Matching Functions What does a negative value of the Color Matching Function mean? Bring one light to the other side of the field. Observer now G adjusts, for example, the Luminances of G and B lights until they match C + R B R = 700.0 nm G = 546.1 nm B = 435.8 nm R C C +r’(R g’(G + b’(B 1931 CIE Color Matching Functions The CIE defined three 2 1.8 z'() theoretical primaries 1.6 x’, y’ and z’ such that 1.4 1.2 the color matching y'( ) 1 functions are everywhere x'() 0.8 positive and the “green” 0.6 matching function is the 0.4 same as the photopic Chromaticity Coordinates 0.2 0 response of the eye. -0.2380 430 480 530 580 630 680 730 780 Wavelength (nm) 11 12/4/2013 Conversion x’y’z’ to r’g’b’ r' 0.41846 0.15860 0.08283x' g' 0.09117 0.25243 0.01571 y' b' 0.00092 0.00255 0.17860 z' The x’, y’, z’ are more convenient from a book-keeping standpoint and the relative luminance is easy to determine since it is related to y’. The consequence of this conversion is that the spectral distribution of the corresponding primaries now have negative values. This means they are a purely theoretical source and can not be made. Tristimulus Values X, Y, Z X P()x'()d The tristimulus values are coordinates 0 in a three dimensional color space. They are obtained by projecting the spectral Y P()y'()d distribution of the object of interest P() 0 onto the color matching functions. Z P()z'()d 0 12 12/4/2013 Chromaticity Coordinates x, y, z X The chromaticity coordinates x are used to normalize out the X Y Z brightness of the object. This way, the color and the brightness can be Y y spearated. The coordinate z is not X Y Z independent of x and y, so this is a two dimensional space. Z z 1 x y X Y Z Chromaticity Coordinates Light Source Object Spectral Spectral Distribution Spectral x Reflectance or = of Light entering the Distribution Transmittance eye. Project onto x’, y’, z’ Normalize Calculate Plot (x, y) Calculate Tristimulus Chromaticity Values X, Y, Z Coordinates 13 12/4/2013 Example ‐ Spectrally Pure Colors Suppose P() = (-o) X ( )x'()d x'( ) o o x'(o ) 0 x x'(o ) y'(o ) z'(o ) Y ( )y'()d y'( ) o o y'(o ) 0 y x'(o ) y'(o ) z'(o ) Z ( )z'()d z'( ) o oz 1 x y 0 Plotting x vs. y for spectrally pure colors gives the boundary of color vision CIE Chromaticity Chart 1 0.8 0.6 0.4 y chromaticity coordinate chromaticity y 0.2 0 0 0.2 0.4 0.6 0.8 1 x chromaticty coordinate 14 12/4/2013 Example ‐ White Light Suppose P() = X x'()d 1 1 x 0.33 0 111 1 Y y'()d 1 y 0.33 0 111 z 1 x y 0.33 Z z'()d 1 0 CIE Chromaticity Chart 15 12/4/2013 Example ‐ MacBeth Color Checker 0.4 0.35 0.3 0.25 0.2 0.15 0.1 Spectral Reflectance 0.05 0 380 430 480 530 580 630 680 730 780 Wavelength (nm) Blue Sky Example ‐ Blue Sky Patch The Macbeth color 140 120 checker assumes that 100 Illuminant C is used 80 Illuminant C for illumination. 60 After Reflection 40 Relative Radiance 20 Multiply the spectral 0 distribution of Illuminant 380 480 580 680 780 Wavelength (nm) C by the spectral reflectance of the color patch to get the light entering the eye. 16 12/4/2013 Example Blue Sky Patch X P()x'() 188.1 Y P()y'() 192.6 Z P()z'() 379.9 x 188.1/(188.1192.6 379.9) .247 y 192.6 /(188.1192.6 379.9) .253 Example ‐ MacBeth Color Checker 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 Spectral Reflectance 0.05 0 380 430 480 530 580 630 680 730 780 Wavelength (nm) Dark Skin 17 12/4/2013 Example ‐ Dark Skin Patch 140 Dark skin is much darker 120 than the blue sky. The 100 Illuminant C chromaticity coordinates, 80 Dark Skin 60 however, remove the Blue Sky 40 luminance factor and only Relative Radiance 20 look at color. 0 380 480 580 680 780 Wavelength (nm) Example Dark Skin X P()x'() 113.6 Y P()y'() 98.8 Z P()z'() 66.2 x 188.1/(188.1192.6 379.9) 0.41 y 192.6 /(188.1192.6 379.9) 0.35 18 12/4/2013 CIE Chromaticity Diagram Think of this as a distorted version of the HSB color model.
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