Color
Giordano Understanding Beretta Hewlett-Packard Company Palo Alto
http://www.hpl.hp.com/imaging/uc/
2000
Photonics West Showcasing the power of Light
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Table of contents
What is color? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Color theories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Color vision physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 Colorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 Objective color terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39 Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42 Famous color spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46 Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49 Measuring color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52 Color reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57 Color appearance modeling . . . . . . . . . . . . . . . . . . . . . . . . .60 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65 Short color dictionary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .74
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What is color?
Color is an illusion
Colorimetry: the art to predict an illusion from a physical measurement
Experience is much more important than knowing facts or theories
The physiology of color vision is understood only to a very small degree
Physiology: physical stimulus physiological response Psychophysics: physical stimulus behavioral response
What is essential is invisible to the eye
Antoine de Saint-Exupry (The Little Prince)
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Terminology
CIE denition 845-02-18: (perceived) color
Attribute of a visual perception consisting of any combination of chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualied by bright, dim, light, dark etc., or by combinations of such names Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure and surround of the stimulus area, on the state of adaptation of the observers visual system, and on the observers experience of the prevailing and similar situations of observation Perceived color may appear in several modes of appearance. The names for various modes of appearance are intended to distinguish among qualitative and geometric differences of color perceptions
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Color term categories
Subjective color term: A word used to describe a color attribute perceived by a human. Example: the colorfulness of a ower Objective color term: A word used to describe a physical quantity related to color that can be measured. Example: the energy radiated by a source We use objective color terms as correlates to subjective color terms
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Subjective color terms Hue
Hue: The attribute of a color perception denoted by blue, green, yellow, red, purple, and so on
hue scale
Unique hue: A hue that cannot be further described by use of the hue names other than its own. There are four unique hues, each of which shows no perceptual similarity to any of the others: red, green, yellow, and blue
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Brightness and lightness
Brightness: The attribute of a visual sensation according to which a given visual stimulus appears to be more or less intense, or according to which the visual stimulus appears to emit more or less light Objective term: luminance (L)
brightness scale
Lightness: The attribute of a visual sensation according to which the area in which the visual stimulus is presented appears to emit more or less light in proportion to that emitted by a similarly illuminated area perceived as a white stimulus Objective terms: luminance factor (), CIE lightness (L*)
Brightness is absolute, lightness is relative to an area perceived as white
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Colorfulness
Chromaticness or Colorfulness: The attribute of a visual sensation according to which an area appears to exhibit more or less of its hue. In short: the extent to which a hue is apparent Objective term: CIECAM97s M Chroma: The attribute of a visual sensation which permits a judgement to be made of the degree to which a chromatic stimulus differs from an achromatic stimulus of the same brightness. In other words, chroma is an attribute orthogonal to brightness: absolute colorfulness; we perceive a color correctly independently of the illumination level Objective term: CIE chroma (C*uv, C*ab)
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Colorfulness (cont.)
Saturation: The attribute of a visual sensation which permits a judgement to be made of the degree to which a chromatic stimulus differs from an achromatic stimulus regardless of their brightness. In other words, it is the colorfulness of an area judged in proportion to its brightness: relative colorfulness; we can judge the uniformity of an objects color in the presence of shadows and independently of the incident lights angle Objective terms: purity (p), CIE saturation (Suv)
saturation scale
Colorfulness is absolute, chroma is relative to a white area and absolute w.r.t. brightness, saturation is in proportion to brightness
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Our goal
We would like to be able to predict the color of a sample by making a measurement
Humans can distinguish about 7 to 10 million different colors just name them and build an instrument that identies them
Task: nd good correlates to the subjective color terms
Some observations:
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If you want to buy a skirt or a pair of slacks to match a jacket, you cannot match the color by memory you have to take the jacket with you Just matching in the store light is insufcient, you have to match also under the incandescent light in the dressing room and outdoors You always get the opinion of your companion or the store clerk light sources samples illuminated by them observers
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Three fundamental components of measuring color:
Spectral curves quantities we can measure
The spectral power curve gives at each wavelength the power (in watts), i.e., the rate at which energy is received from the light source The spectral reectance curve gives at each wavelength the percentage of incident light that is reected
reflectance
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
human complexion
400
450
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550
600
650
700 nm
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Spectral color reproduction
By spectral color reproduction we intend the physically correct reproduction of color, i.e., the duplication of the original object's spectrum The general reproduction methods (micro-dispersion and Lippmann) are too impractical for normal use For some special applications like painting restauration or illuminant reconstruction, the spectrum may be sampled at a small number of intervals and combined with principal component analysis Fortunately, spectral color reproduction is required only in rare cases, such as paint swatches in catalogs, and in this cases it is often possible to use identical dyes
Our aim is to achieve a close effect for a normal viewer under average viewing conditions Mathematically: build a simple model of color vision
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Color theories
800 B.C. Indian Upanishads
there are relations among colors Plato: light or re rays emanate from the eyes Epicurus: replicas of objects enter the eyes Abu Ali Mohammed Ibn al Hazen: image is formed within the eye like in a camera obscura Leonardo da Vinci: color perception color order system black & white are colors 3 pairs of opponent colors (blackwhite, redgreen, yellowblue) simultaneous contrast used color lters to determine color mixtures
400 B.C. Hellenic philosophers
First Millennium Arab school, pure science
15th century Renaissance, technology
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Opponent colors
Off-axis front views
W Y R G G B B R Y W
K
K
Top view
Y
W G R
B
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Note: rendered with chiaro-scuro technique
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Color theories (cont.)
18th century Enlightenment, physics & chemistry
Isaac Newton: spectral dispersion, white can be dispersed in a spectrum by a prism colors of objects relate to their spectral reectance light is not colored and color perception is elicited in the human visual system Thomas Young: trichromatic theory Hermann von Helmholtz: spectral sensitivity curves Ewald Hering: opponent color theory (can explain hues, saturation, and why there is no reddish green or yellowish blue) black and dark gray are not produced by the absence of light but by a lighter surround Johannes A. von Kries: chromatic adaptation G.E. Mller & Erwin Schrdinger: zone theory physiological evidence for inhibitory mec...
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