COLOR and the human response to light Idit

COLOR and the human response to light Idit Haran

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

Introduction 3

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

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 Relative Power n 1 0. 5 0 400 500 600 700 Wavelength ( ) 5

Examples 6

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

The Physiology of Human Vision 8

The Human Eye 9

The Human Retina cones rods horizontal bipolar amacrine ganglion light 10

The Human Retina 11

Retinal Photoreceptors 12

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

3 Types of Cones n n n 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) 14

Cones Spectral Sensitivity 15

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

History n 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. “ n 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. “ n Trichromatic: “Tri”=three “chroma”=color 17

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

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

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

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

Color Matching Experiment test match - + - Three primary lights are set to match a test light Test light Match light 1 1 ~ = 0. 75 0. 25 0 - + n + 400 500 600 700 0. 75 0. 25 0 400 500 600 700 22

CIE-RGB n n n 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) 23

RGB Image 111 10 14 126 36 111 36 200 36 1712 111 200 36 14 200 111 1414 36 1217 126 128 36126 36200 111111 14 17 12 36 14 36 12 17 111 36 3636 12 36111 14 14 126 36 126 17 36 1736 127236 126 200 36111 12 14 126 72 17 17 200 3617 12111 36200 12 126 200 7236 1212 17126 111 14 17 36 200 111 14 36 72 36 12 17 72 106 155 36 111 36 10 200 111 17 200 36 12 36 17 14 126 200 17 36 72 12 128 36 14 36 111 17 36 111 200 126 36 200 36 111 12 12 126 36 14 36 126 111 200 36 72 12 111 12 14 17 17 200 36 36 14 126 12 17 36 36 12 126 36 14 36 126 72 111 36 12 111 14 36 12 36 36 72 17 111 111 200 14 36 36 12 126 17 17 111 14 36 36 72 12 126 17 111 106 155 14 36 36 12 24

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

Combining Colors Additive (RGB) Subtractive (CMYK) 26

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

CMY + Black C + M + Y = K (black) n n n 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 = 100 C 50 M 70 Y + 50 K 50 C 0 M 20 Y 28

Example 29

Example 30

Example 31

Example 32

Example 33

From RGB to CMY 34

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

The Artist Point of View n n n 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 white 36

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

Munsell Book of Colors 38

Munsell Book of Colors 39

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

HSV Saturation Value Hue 41

HLS Color Space HLS = Hue Lightness Saturation V green 120° cyan yellow 0. 5 red 0° Blue 240° magenta H 0. 0 black S 42

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

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

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

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

XYZ Spectral Power Distribution n n 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. 1. 8 Tristimulus values n 1. 4 z( ) y( ) 1 x( ) 0. 6 0. 2 400 500 600 Wavelength (nm) 700 47

RGB to XYZ n X Y Z RGB to XYZ is a linear transformation = 0. 490 0. 310 0. 200 0. 177 0. 813 0. 011 0. 000 0. 010 0. 990 R G B 48

CIE Chromaticity Diagram 0. 9 520 530 540 550 510 y X 505 Y 560 570 580 500 0. 5 Z 590 600 610 650 495 490 X =x X+Y+Z Y =y X+Y+Z Z =z X+Y+Z x+y+z = 1 485 480 0. 0 470 450 0. 5 x 1. 0 49

Color Naming 0. 9 520 530 540 550 510 y 505 green yellow- 570 green 580 yellow 500 0. 5 490 cyan 485 blue 480 purple 0. 0 560 white pink 590 orange 600 610 red 650 magenta 470 450 x 0. 5 1. 0 50

Blackbody Radiators and CIE Standard Illuminants: 2500 - tungsten light (A) 4800 - Sunset 10 K - blue sky 6500 - Average daylight (D 65) 51

Chromaticity Defined in Polar Coordinates Given a reference white. 0. 8 Dominant Wavelength – wavelength of the spectral color which added to the reference white, produces the given color. 0. 6 0. 4 reference white 0. 2 0 0 0. 2 0. 4 0. 6 0. 8 52

Chromaticity Defined in Polar Coordinates Given a reference white. Dominant Wavelength 0. 8 Complementary Wavelength - wavelength of the spectral color which added to the given color, produces the reference white. 0. 6 0. 4 reference white 0. 2 0 0 0. 2 0. 4 0. 6 0. 8 53

Chromaticity Defined in Polar Coordinates Given a reference white. Dominant Wavelength 0. 8 Complementary Wavelength 0. 6 Excitation Purity – the ratio of the lengths between the given color and reference white and between the dominant wavelength light and reference white. Ranges between 0. . 1. purity 0. 4 reference white 0. 2 0 0 0. 2 0. 4 0. 6 0. 8 54

Device Color Gamut n n We can use the CIE chromaticity diagram to compare the gamut of various devices: Note, for example, that a color printer cannot reproduce all shades available on a color monitor 55

Color Spaces n n n Linear (RGB, CMYK) Artistic View (Munsell, HSV, HLS) Standard (CIE-XYZ) Perceptual (Luv, Lab) Opponent (YIQ, YUV) – used in TV 56

Luminance v. s. Brightness (Lightness) V in HSV Equal intensity steps: Equal brightness steps: Luminance (intensity) vs Y in XYZ DI 2 DI 1 I 1 < I 2, DI 1 = DI 2 57 I 2

Weber’s Law DI = constant I (I is intensity, DI is change in intensity) Weber’s Law: Perceived Brightness = log (I) Perceived Brightness In general, DI needed for just noticeable difference (JND) over background I was found to satisfy: Intensity 58

Munsell lines of constant Hue and Chroma 0. 5 0. 4 y 0. 3 0. 2 0. 1 Value =1/ 0 0 0. 1 0. 2 0. 3 x 0. 4 0. 5 0. 6 59

Mac. Adam Ellipses of JND (Just Noticeable Difference 0. 8 0. 6 y (Ellipses scaled by 10) 0. 4 0. 2 0 0 0. 2 0. 4 x 0. 6 60

Perceptual Color Spaces n n An improvement over CIE-XYZ that represents better uniform color spaces The transformation from XYZ space to perceptual space is Non Linear. Two standard adopted by CIE are L*u’v’ and L*a*b* The L* line in both spaces is a replacement of the Y lightness scale in the XYZ model, but it is more indicative of the actual visual differences. 61

Munsell Lines and Mac. Adam Ellipses plotted in CIE-L*u’v’ coordinates 100 Value =5/ 100 50 v* 50 0 v* -50 -100 -150 -100 0 -50 0 u* 50 100 150 200 -150 -100 -50 0 u* 50 100 150 200 62

Distance should be measured in perceptual color spaces 63

Color Spaces n n n Linear (RGB, CMYK) Artistic View (Munsell, HSV, HLS) Standard (CIE-XYZ) Perceptual (Luv, Lab) Opponent (YIQ, YUV) – used in TV 64

Opponent Color Spaces + black-white + blue-yellow - + red-green 65

YIQ Color Model n n YIQ is the color model used for color TV in America (NTSC= National Television Systems Committee) Y is luminance, I & Q are color (I=red/green, Q=blue/yellow) n n Note: Y is the same as CIE’s Y Result: backwards compatibility with B/W TV! Convert from RGB to YIQ: The YIQ model exploits properties of our visual system, which allows to assign different bandwidth for each of the primaries (4 MHz to Y, 1. 5 to I and 0. 6 to Q) 66

YUV Color Model n n YUV is the color model used for color TV in Israel (PAL), and in video. Also called YCb. Cr. Y is luminance as in YIQ. U and V are blue and red (Cb and Cr). The YUV uses the same benefits as YIQ, (5. 5 MHz for Y, 1. 3 for U and V). n Converting from RGB to YUV: n n n Y = 0. 299 R + 0. 587 G + 0. 114 B U = 0. 492(B – Y) V = 0. 877(R – Y) 67

YUV - Example Y U V 68

Summary n Light Eye (Cones, Rods) [l, m, s] Color n Many 3 D color models: n n Reproducing Metamers to Colors Different reproduction Gamut More / Less intuitive CIE standards 69