COLOR and the human response to light Contents Introduction: The - PowerPoint PPT Presentation

COLOR and the human response to light

Contents  Introduction:  The nature of light  The physiology of human vision  Color Spaces:  Linear  Artistic View  Standard  Distances between colors  Color in the TV 2

How many dimension ?  RBG  Lab  CMY  … 3

Introduction 4

Electromagnetic Radiation - Spectrum Short- AC Ultra- Gamma X rays violet Infrared Radar FM TV wave AM electricity -12 -8 -4 4 8 10 10 10 1 10 10 Wavelength in meters (m) Visible light 600 nm 400 nm 500 nm 700 nm Wavelength in nanometers (nm) 5

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

The Human Retina cones rods horizontal bipolar amacrine ganglion light 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       l    l l L , M , S L P L d l 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

3D Color Spaces  Three types of cones suggests color is a 3D quantity. How to define 3D color space? Cubic Color Spaces Polar Color Spaces Opponent Color Spaces Brightness black-white Hue G blue-yellow B R red-green 19

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 : is then given by: (a, b, c) 20

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

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

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) 23

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

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

Combining Colors Additive (RGB) Subtractive (CMYK) 26

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

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

Example 29

Example 50 100 150 200 50 100 150 200 250 30

Example 50 100 150 200 50 100 150 200 250 31

Example 50 100 150 200 50 100 150 200 250 32

Example 50 100 150 200 50 100 150 200 250 33

From RGB to CMY             R 1 C C 1 R                             G 1 M M 1 G                   B 1 Y       Y 1 B 34

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 white 35

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) Example: Value: 0 ... 10 (dark ... pure white) 5YR 8/4 Chroma: 0 ... 20 (neutral ... saturated) 36

Munsell Book of Colors 37

Munsell Book of Colors 38

HSV/HSB Color Space HSV = Hue Saturation Value HSB = Hue Saturation Brightness Saturation Scale Brightness Scale 39

HSV Value Saturation Hue 40

HLS Color Space HLS = Hue Lightness Saturation V green 120 ° yellow cyan 0.5 red 0 ° Blue 240 ° magenta H 0.0 S black 41

Back to RGB  Problem 1: RGB differ from one device to another 42

Color Matching Experiment test match - + + - + -  Three primary lights are set to match a test light 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 43

Back to RGB  Problem 2: RGB cannot represent all colors RGB Color Matching Functions 44

CIE Color Standard - 1931  CIE - Commision Internationale d ’ Eclairage  1931 - defined a standard system for color representation.  XYZ tristimulus coordinate system. X Y Z 45

XYZ Spectral Power Distribution XYZ Color Matching Functions  Non negative over the 1.8 visible wavelengths. z( l ) Tristimulus values  The 3 primaries associated 1.4 with x y z spectral power y( l ) distribution are unrealizable 1 x( l ) (negative power in some of the wavelengths). 0.6  The color matching of Y is equal to the spectral 0.2 luminous efficiency curve. 400 500 600 700 Wavelength (nm) 46

RGB to XYZ  RGB to XYZ is a linear transformation X R 0.490 0.310 0.200 = 0.177 0.813 0.011 Y G 0.000 0.010 0.990 Z B 47

A linear transformation 48

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

Color Naming 0.9 520 530 540 510 550 y 505 560 green 570 yellow- 500 green 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 50 x

Blackbody Radiators and CIE Standard Illuminants CIE Standard Illuminants: 2500 - tungsten light (A) 4800 - Sunset 10K - blue sky 6500 - Average daylight (D65) 51

Chromaticity Defined in Polar Coordinates 0.8 Given a reference white. Dominant Wavelength – wavelength of the spectral 0.6 color which added to the reference white, produces the given color. 0.4 reference white 0.2 0 0 0.2 0.4 0.6 0.8 52

Chromaticity Defined in Polar Coordinates 0.8 Given a reference white. Dominant Wavelength 0.6 Complementary Wavelength - wavelength of the spectral color which 0.4 added to the given color, produces the reference reference white white. 0.2 0 0 0.2 0.4 0.6 0.8 53

Chromaticity Defined in Polar Coordinates 0.8 Given a reference white. Dominant Wavelength 0.6 Complementary Wavelength purity 0.4 Excitation Purity – the ratio of the lengths reference white between the given color 0.2 and reference white and between the dominant wavelength light and 0 reference white. 0 0.2 0.4 0.6 0.8 Ranges between 0 .. 1. 54