Color

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© 1998-2008 by Glenn Elert -- A Work in Progress
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Discussion

red green blue

Color is a function of the human visual system, and is not an intrinsic property. Objects don't "have" color, they give off light that "appears" to be a color. Spectral power distributions exist in the physical world, but color exists only in the mind of the beholder.

Start with monochromatic light — that is, light of a single frequency. The visible spectrum ranges from roughly 700 to 400 nm. If I shine light of a single frequency at your eye and dial the wavelength from 700 nm to 400 nm this is roughly what you'd see.

How many colors are there in this swatch? How many were you taught in elementary school?

red orange yellow green blue violet

The simple named colors are mostly monosyllabic in English — red, green, blue, brown, black, white, gray. (Yellow is the one exception to this rule, but it's still pretty simple.) Brevity indicates a pre-English, Anglo-Saxon origin. Monosyllabic words are generally the oldest words in the English language — head, eye, nose, foot, cat, dog, cow, eat, drink, man, wife, house, sleep, rain, snow, sword, sheath, God, and the "four letter words" — words that go back a thousand years. Some of the names for colors are loan words from French — orange and beige, since the "zh" sound doesn't exist in pure English (garage is a very french word) and violet and purple, since they just sound too fancy to be anglo-saxon.

That raises an interesting point. Did the English (or the Angles and the Saxons) "see" orange before the French told them about it? Did the French see orange before the Spanish told them about it? Did the Spanish see orange before the Arabs told them about it? Why does Islam identify with green? Why do Russians identify with red? Why do the Dutch groove on orange? (These are rhetorical questions. Please don't email me your answers.) Where do I put black, white, gray, purple, and brown? What the hell is indigo?

Enough about language, this is a physics book. Here's the point. There is no physical significance in these colors. It's all a matter of culture and culture depends on where you live, what language you speak, and what century it is. There is nothing special about these colors. We humans who speak English and live at the dawn of the Twenty First Century have identified the following six frequency bands (well, wavelength bands actually, since wavelength is easier to measure than frequency) in the electromagnetic spectrum as being significant enough to warrant designation with a special name. They are: red, orange, yellow, green, blue, and violet.

Where one monochromatic color ends and another begins is a matter of debate as you will see in the table below.

Wavelength Ranges for Monochromatic Light (nm)
color   1 2 3 4
red   647-700 647-760 630-700 620-800
orange   585-647 585-647 590-630 590-620
yellow   575-585 575-585 570-590 560-590
green   491-575 491-575 500-570 480-560
blue   424-491 424-491 450-500 450-480
violet   400-424 380-424 400-450 400-450
  1. CRC Handbook of Chemistry and Physics. 1966.
  2. Hazel Rossotti. Color. Princeton University Press, 1983.
  3. Edwin R. Jones. Physics 153 Class Notes. University of South Carolina, 1999.
  4. Deane B. Judd. Goethe's Theory of Colors. MIT Press, 1970.

But wait, it gets worse. How many of you reading this learned about "Roy G. Biv" (Americans, I presume) or that "Richard of York Gave Battle In Vain" (Britons, I presume)? Who among you leaned that between blue and violet there was this special color called indigo? Oooo, indigo. Yeah, there's a word I use a lot in everyday conversation. The only time I ever hear it is when students recite the visible spectrum. Let me state that anyone who says indigo is a color equal in importance to blue or green is a thoughtless idiot. Indigo is a color of relatively little importance. If indigo counts as a color then so should canary, and mauve, and puce, and brick, and teal, and … well, you get the idea.

rubeus aureus flavus viridis cæruleus indicus violaceus

If you believe that indigo is an important color, then here's a set of spectral tables for you.

Wavelength Ranges for Monochromatic Light (nm) with Indigo
color   5 6 7 8
red   650-800 630-750 650-750 620-740
orange   590-640 590-630 590-640 585-575
yellow   550-580 570-590 550-580 575-858
green   490-530 490-570 490-530 500-575
blue   460-480 450-490 460-480 445-500
indigo   440-450 420-450 440-450 425-445
violet   390-430 380-420 390-430 390-425
  1. S. Eskinazi. Numerical Modeling of Color. Leonardo On-Line, 1996.
  2. Howard L. Cohen. AST 1002 Study Guide. University of Florida, 1999-2003.
  3. J.L. Morton. Color Matters, 1995-2002.
  4. A Dictionary of Science. Oxford University Press, 2000.

Did Richard of York give battle in vain so that future citizens in the dismantled British Empire would forever remember indigo? Did Mr. and Mrs. Biv conceive little Roy G. so that generations of fast food stuffed Americans might learn the true nature of light? Where the hell did indigo come from?

When Newton attempted to reckon up the rays of light decomposed by the prism and ventured to assign the famous number seven, he was apparently influenced by some lurking disposition towards mysticism, If any unprejudiced person will fairly repeat the experiment, he must soon be convinced that the various coloured spaces which paint the spectrum slide into each other by indefinite shadings: he may name four or five principal colors, but the subordinate spaces are evidently so multiplied as to be incapable of enumeration. The same illustrious mathematician, we can hardly doubt, was betrayed by a passion for analogy, when he imagined that the primary colours are distributed over the spectrum after the proportion of the diatonic scale of music, since those intermediate spaces have really no precise defined limits. -- Treatises on Various Subjects of Natural and Chemical Philosophy, p. 59.

The human eye can distinguish something on the order of 7 to 10 million colors -- that's a number greater than the number of words in the English language (the largest language on earth).

The retina …

The rods, which far outnumber the cones, respond to wavelengths in the middle portion of the spectrum of light. If you had only rods in your retina, you would see in black and white. The cones in our eyes provide us with our color vision. There are three types of cone, identified by a capital letter, each of which responds primarily to a region of the visible spectrum: L to red, M to green, and S to blue.


Cone response curves [magnify]. The peak sensitivities are 580 nm for red (L), 540 nm for green (M), and 440 nm for blue (S). Red and green cones respond to nearly all visible wavelengths, while blue cones are insensitive to wavelengths longer than 550 nm. The total response of all three cones together peaks at 560 nm -- somewhere between yellow and green in the spectrum.

Paraphrase …

While red, green, and blue are spaced somewhat equally across the visible spectrum, the specific sensitivities of the L, M, and S cones are not. This might seem a little confusing, especially since the L cones aren't even closely centered on the red area of the spectrum. Fortunately, the spectral sensitivity of the cones is only one part of how the brain decodes color information. Additional processing takes these sensitivities into account


A Commission Internationale de l'Eclairage (CIE) chromaticity diagram [magnify]. The relative response of the red and green cones to different colors of light are plotted on the horizontal and vertical axes, respectively. Values on the tongue shaped perimeter are for light of a single wavelength (in nanometers). Values within the curve are for light of mixed frequency. The point in the center labeled D65 corresponds to light from a blackbody radiator at 6500 K — the effective temperature of daylight at midday, a generally accepted standard value of white light.

white & black

text

Temperature (or Effective Temperature) of Selected Radiant Sources
kelvin
temperature		 radiant energy source
3 cosmic background radiation
306 human skin
500 household oven at its hottest
660 minimum temperature for incandescence
770 dull red heat
1400 glowing coals, electric stove, electric toaster
1900 candle flame
2000 kerosene lamp
2800 incandescent light bulb, 75 W
2900 incandescent light bulb, 100 W
3000 incandescent light bulb, 200 W
3100 sunrise or sunset (effective)
3200 professional studio lights
3600 one hour after sunrise or one hour before sunset (effective)
4000 two hours after sunrise or two hours before sunset (effective)
5500 direct midday sunlight
6500 daylight (effective)
7000 overcast sky (effective)
20-30,000 lightning bolt

Transition paragraph

Metal Temperature by Color   Color Scale of Temperature
color approximate temperature   color Temperature
°F °C K   °C K
faint red 930 500 770   incipient red heat 500 - 550 770 - 820
blood red 1075 580 855   dark red heat 650 - 750 920 - 1020
dark cherry 1175 635 910   bright red heat 850 - 950 1120 - 1220
medium cherry 1275 0690 0965   yellowish red heat 1050 - 1150 1320 - 1420
cherry 1375 0745 1020   incipient white heat 1250 - 1350 1520 - 1620
bright cherry 1450 0790 1060   white heat 1450 - 1550 1720 - 1820
salmon 1550 0845 1115   "This table is the result of an effort to interpret in terms of thermometric readings, the common expressions used in describing temperatures. It is obvious that these values are only approximations."
dark orange 1630 0890 1160
orange 1725 0940 1215
lemon 1830 1000 1270
light yellow 1975 1080 1355
white 2200 1205 1480
Sources: Process Associates of America & Handbook of Chemistry & Physics, 1924

additive color mixing

the absence of light is darkness, add light to it

talk talk talk

red + green = yellow green + blue = cyan blue + red = magenta

the color wheel

The basic rules of additive color mixing The additive color wheel
red  +  green  =  yellow
green  +  blue  =  cyan
blue  +  red  =  magenta
red + green + blue  =  white
 

more talk

subtractive color mixing

the color wheel

The basic rules of subtractive color mixing The subtractive color wheel
cyan  +  magenta  =  blue
magenta  +  yellow  =  red
yellow  +  cyan  =  green
cyan + magenta + yellow  =  black
 
A four color press: yellow, magenta, cyan, black

more talk

historical junk

The painter's color wheel is a historical artifact that refuses to die. The primary colors are not red, yellow, and blue. Painters and art teachers promote this scheme. It is a convenient way to understand how to mimic one color by mixing red, yellow, and blue. But these colors do not satisfy the definition of primary colors in that they can't reproduce the widest variety of colors when combined. Cyan, magenta, and yellow have a greater chromatic range as evidenced by their ability to produce a reasonable black. No combination of red, yellow, and blue pigments will approach black as closely as do cyan, magenta, and yellow.

Johann Wolfgang von Goethe (17949-1832), student of the arts, theatrical director, and author (Iphigenia at Taurus, Egmont, Faust). Lots of interesting descriptive information on the subjective nature of color, which many physicists of his day ignored, but does not propose a physical model of color.

The theory of colors, in particular, has suffered much, and its progress has been incalculably retarded by having been mixed up with optics generally, a science which cannot dispense with mathematics; whereas the theory of colors, in strictness, may be investigated quite independently of optics.

Colour is a law of nature in relation with the sense of sight … [It] is an elementary phenomenon in nature adapted to the sense of vision …

It is not light, in an abstract sense, but a luminous image that we have to consider.

Yellow, blue, and red, may be assumed as pure elementary colors, already existing; from these, violet, orange, and green, are the simplest combined results.

That all the colours mixed together produce white, is an absurdity which people have credulously been accustomed to repeat for a century, in opposition to the evidence of their senses.

Color Mixing Rules from Theory of Colors [Zur Farbenlehre] (1810) by Johann Wolfgang von Goethe (17949-1832) Germany. The painter's color wheel.

hmmm

color production

methods

color spaces

Summary

Problems

practice

  1. Write something.
    • Answer it.
  2. Write something else.
    • Answer it.
  3. Write something different.
    • Answer it.
  4. Write something completely different.
    • Answer it.

conceptual

  1. Imagine that our sun was a red star instead of a yellow star, but that everything else about the solar system was unchanged.
    1. What would clouds look like?
    2. What would a rainbow look like?
    3. What would the midday sky look like?
    4. What would a sunset look like?
    5. What would a full moon look like?

Resources

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