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C O L O R and the human response to light

C O L O R and the human response to light. Idit Haran. Contents. Introduction: The nature of light The physiology of human vision Color Spaces: Linear (RGB, CMYK) Artistic View (Munsell, HSV, HLS) Standard (CIE-XYZ) Perceptual (Luv, Lab) Opponent (YIQ, YUV) – used in TV.

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C O L O R and the human response to light

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  1. COLORand the human response to light Idit Haran

  2. Contents • Introduction: • The nature of light • The physiology of human vision • Color Spaces: • Linear (RGB, CMYK) • Artistic View (Munsell, HSV, HLS) • Standard (CIE-XYZ) • Perceptual (Luv, Lab) • Opponent (YIQ, YUV) – used in TV

  3. Introduction

  4. Short- AC Ultra- wave Gamma X rays Infrared Radar FM TV AM electricity violet -12 -8 -4 4 8 10 10 10 1 10 10 Visible light 400 nm 500 nm 700 nm Wavelength in meters (m) Wavelength in nanometers (nm) 600 nm Electromagnetic Radiation - Spectrum

  5. 1 Relative Power 0.5 0 400 500 600 700 Wavelength (l) Spectral Power Distribution • The Spectral Power Distribution (SPD) of a light is a function P() which defines the power in the light at each wavelength

  6. Examples

  7. The Interaction of Light and Matter • Some or all of the light may be absorbed depending on the pigmentation of the object.

  8. The Physiology of Human Vision

  9. The Human Eye

  10. cones rods horizontal bipolar amacrine ganglion light The Human Retina

  11. The Human Retina

  12. Retinal Photoreceptors

  13. Cones • High illumination levels (Photopic vision) • Less sensitive than rods. • 5 million cones in each eye. • Density decreases with distance from fovea.

  14. 3 Types of Cones • L-cones, most sensitive to red light (610 nm) • M-cones, most sensitive to green light (560 nm) • S-cones, most sensitive to blue light (430 nm)

  15. Cones Spectral Sensitivity

  16. Metamers • Two lights that appear the same visually. They might have different SPDs (spectral power distributions)

  17. History • Tomas Young (1773-1829) “A few different retinal receptors operating with different wavelength sensitivities will allow humans to perceive the number of colors that they do. “ • James Clerk Maxwell (1872) “We are capable of feeling three different color sensations. Light of different kinds excites three sensations in different proportions, and it is by the different combinations of these three primary sensations that all the varieties of visible color are produced. “ • Trichromatic: “Tri”=three “chroma”=color

  18. G B R Cubic Color Spaces Polar Color Spaces Opponent Color Spaces Brightness Hue black-white blue-yellow red-green 3D Color Spaces • Three types of cones suggests color is a 3D quantity. How to define 3D color space?

  19. Contents • Introduction: • The nature of light • The physiology of human vision • Color Spaces: • Linear (RGB, CMYK) • Artistic View (Munsell, HSV, HLS) • Standard (CIE-XYZ) • Perceptual (Luv, Lab) • Opponent (YIQ, YUV) – used in TV

  20. Linear Color Spaces Colors in 3D color space can be described as linear combinations of 3 basis colors, called primaries = a· + c· + b· The representation of : (a, b, c) is then given by:

  21. 3 Primary Intensity 2 1 0 400 500 600 700 Wavelength (nm) RGB Color Model • RGB = Red, Green, Blue • Choose 3 primaries as the basis SPDs(Spectral Power Distribution.)

  22. - + test match - + - + Test light Match light 1 1 ~ 0.75 0.75 = 0.5 0.5 0.25 0.25 0 0 400 500 600 700 400 500 600 700 Color Matching Experiment • Three primary lights are set to match a test light

  23. CIE-RGB • Stiles & Burch (1959) Color matching Experiment. • Primaries are: 444.4 525.3 645.2 • Given the 3 primaries, we can describe any light with 3 values (CIE-RGB): (85, 38, 10) (21, 45, 72) (65, 54, 73)

  24. 36 111 14 126 12 36 36 36 111 12 17 111 200 36 12 36 14 36 36 111 200 36 12 17 200 111 14 126 17 111 14 12 36 36 14 36 10 128 126 200 12 111 36 36 111 36 14 36 17 111 14 126 17 111 17 36 36 14 36 72 17 126 72 126 17 111 12 36 12 126 200 36 12 17 126 17 111 200 200 36 12 36 12 126 14 200 36 12 126 17 72 12 17 111 14 36 126 200 111 14 36 72 128 126 200 12 111 10 36 12 17 72 106 155 200 36 12 36 14 36 36 111 14 126 12 36 17 36 36 14 36 72 200 111 14 126 17 111 36 36 111 12 17 111 12 17 126 17 111 200 36 36 111 36 14 36 36 111 200 36 12 17 14 200 36 12 126 17 17 126 72 126 17 111 14 12 36 36 14 36 126 200 111 14 36 72 200 36 12 36 12 126 17 111 14 126 17 111 36 12 17 72 106 155 72 12 17 111 14 36 12 36 126 200 36 12 RGB Image

  25. Cyan – removes Red transmit B G R Magenta – removes Green B G R Yellow – removes Blue B G R CMYK Color Model CMYK = Cyan, Magenta, Yellow, blacK Black – removes all

  26. Combining Colors Additive (RGB) Subtractive (CMYK)

  27. B G R magenta B G R + yellow B G R = red R B G R Example: red = magenta + yellow

  28. 50 CMY + Black • Using three inks for black is expensive • C+M+Y = dark brown not black • Black instead of C+M+Y is crisper with more contrast C + M + Y = K (black) = + 100 50 70 50 0 20 C M Y K C M Y

  29. Example

  30. Example

  31. Example

  32. Example

  33. Example

  34. From RGB to CMY

  35. Color Spaces • Linear (RGB, CMYK) • Artistic View (Munsell, HSV, HLS) • Standard (CIE-XYZ) • Perceptual (LUV, Lab) • Opponent (YIQ, YUV) – used in TV

  36. white The Artist Point of View • Hue - The color we see (red, green, purple) • Saturation - How far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue) • Brightness/Lightness (Luminance) - How bright is the color

  37. Munsell Color System Equal perceptual steps in Hue Saturation Value. Hue: R, YR, Y, GY, G, BG, B, PB, P, RP (each subdivided into 10) Value: 0 ... 10 (dark ... pure white) Chroma: 0 ... 20 (neutral ... saturated) Example: 5YR 8/4

  38. Munsell Book of Colors

  39. Munsell Book of Colors

  40. HSV/HSB Color Space HSV = Hue Saturation Value HSB = Hue Saturation Brightness Saturation Scale Brightness Scale

  41. HSV Value Saturation Hue

  42. V green 120° yellow cyan 0.5 red 0° Blue 240° magenta H 0.0 S black HLS Color Space HLS = Hue Lightness Saturation

  43. Color Spaces • Linear (RGB, CMYK) • Artistic View (Munsell, HSV, HLS) • Standard (CIE-XYZ) • Perceptual (Luv, Lab) • Opponent (YIQ, YUV) – used in TV

  44. CIE Color Standard • Why do we need a standard ? • RGB differ from one device to another

  45. CIE Color Standard • Why do we need a standard ? • RGB differ from one device to another • RGB cannot represent all colors RGB Color Matching Functions

  46. X Y Z CIE Color Standard - 1931 • CIE - Commision Internationale d’Eclairage • 1931 - defined a standard system for color representation. • XYZ tristimulus coordinate system.

  47. 1.8 z(l) 1.4 1 Tristimulus values x(l) 0.6 0.2 400 500 600 700 Wavelength (nm) XYZ Spectral Power Distribution • Non negative over the visible wavelengths. • The 3 primaries associated with x y z spectral power distribution are unrealizable (negative power in some of the wavelengths). • y was chosen to equal luminance of monochromatic lights. y(l)

  48. RGB to XYZ • RGB to XYZ is a linear transformation X R 0.490 0.310 0.200 0.177 0.813 0.011 0.000 0.010 0.990 = Y G Z B

  49. X = x X X+Y+Z Y = y Y X+Y+Z Z 0.9 = z Z 520 X+Y+Z 530 540 510 550 y 505 560 570 500 0.5 580 590 495 600 610 490 650 485 480 470 0.0 450 0.0 0.5 1.0 CIE Chromaticity Diagram x+y+z = 1 x

  50. 0.9 520 530 540 510 550 505 560 green 570 yellow- green 500 580 0.5 yellow 590 495 orange 600 610 white cyan 490 red 650 pink 485 magenta blue 480 purple 470 450 0.0 1.0 0.5 x Color Naming y

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