color

Color is perceived according to radiation's wavelength. Red has the longest waves, purple has the shortest. Everything else is in the middle, amazingly enough.

The phrase "of color" usually refers to non-Caucasian people, which is sort of strange, since Caucasians are sorta pink, and pink is a color, too.

Color is another term for the color medioum used to color theatre lights, often known as gel. Comes in a wide variety of colors, from several manufacturers such as Rosco, Lee, and Gam. Colors are named absurd things, such as Bastard Amber and Congo Blue(two very famous colors). Other colors, such as Tipton blue are named after designers, such as Jennifer Tipton. Colors named after designers are usually due to a big name designer requesting a special run of color for a particular show, and then the color eventually makes it's way into regular production by demand.

Color is considered the proper term these days, as all color is now made from polyester, and similar synthetics. See gel for a historical perspective.

Back to theatre lighting terminology

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Property

color

Values

<color>

Initial

UA specific

Inherited

yes

This property describes the text color of an element (often referred to as the foreground color). There are different ways to specify red:

      EM { color: red }              /* natural language */
      EM { color: rgb(255,0,0) }     /* RGB range 0-255   */

The perception of color is dependent on having a color sensor, we have three kinds of color sensors, or cone photopigments, one each for red, green and blue. About 8% of men who are color-blind typically have the cone for blue, and are missing either the green or red cones. Sometimes they will have cones that see two very slightly different red or green hues. This is possible because there are a number of genes that code for each of the red, green and blue cone photopigments. Thus not everyone has the same set of red, green and blue cones.

This suggests that not everyone has the same perception of color.

The genes for the red and green photopigments are adjacent to each other on the X chromosome; blue however is on another chromosome. Women, of course, have two X chromosomes and therefore two sets of red and green photopigment genes. Men have only one X, so they have just one shot at getting the red and green photopigment genes right. This is why color blindness occurs mostly in men. Because the genes for the red and green photopigments are right next to each other, those genes sometimes mix, and every once in awhile one of five bad combinations of the genes that code for the red and green cone photopigments can occur.

The X chromosome is missing either a red or a green photopigment gene.

The X chromosome has two identical red photopigment genes instead of both red and green ones.

The X chromosome has two slightly different red photopigment genes.

The X chromosome has two identical green photopigment genes.

The X chromosome has two slightly different green photopigment genes.

In each of the above cases the man will be color blind.

Now the interesting thing about this is that women have two sets of X chromosomes. So there is a possiblity that instead of having just three types of they may have four types cone photopigments. One X chromosome has the normal complement of red and green photopigment genes, the other has a differrent set of genes for either the red or green cone photopigments. This would very likely give her enhanced color vision.

Source: http://object.cup.org/Chapters/0521590531WSN01.pdf

In astronomy, color is a quantity comparing the brightness of a given object measured at two different wavelengths of light. It is most commonly used to determine the temperature of an individual star, the age of a composite stellar population, or the amount of extinction caused by interstellar dust.

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Definition

Astronomers use the somewhat archaic logarithmic units of magnitudes to measure brightness. The color of an object is the arithmetic difference of the brightness of an object measured in two different band-pass filters. If A is the magnitude of an object measured in one filter, and B is the magnitude measured in another filter, then the color is

(A - B) = A - B

The notation (A - B) (including the parentheses) is the standard notation for color or the color index. It is standard to measure the color with the shorter wavelength magnitude measurement first, as in

(m_blue - m_red)

If you come across a book or magazine article on astronomy, and it mentions quantities like (B - V), (V - R), or (u - y), these are colors, measured in the two filter band-passes contained within the parentheses.

Because lower magnitudes correspond to brighter objects, low and negative color indices correspond to "blue" spectra. Suppose, for example, you measure a star in the B (blue) and V (visual, green-yellow) filters, and find m_B=7.8 and m_V=8.0. Then, the (B - V) color index would be -0.2, corresponding to a very blue object. If m_B=11.4 and m_V=8.0 then the color index would be +3.4, a very red star.

It's important to note that "bluer" and "redder" are only convenient shorthand for saying "brighter at shorter wavelengths" and "brighter at longer wavelengths" -- astronomers can and do measure "colors" in everything from X-rays to the infrared.

Finally, since the color index is measured in logarithmic magnitudes, this corresponds to a ratio of brightnesses in linear units.

Temperature, luminosity and chemical abundances

The color is often used to determine the temperature of individual stars. Most stars have stellar spectra similar to that of a blackbody. The blackbody spectrum is only a function of the wavelength of light and of the temperature of the emitting object. The spectrum doesn't change shape as the temperature changes, but the peak wavelength of the spectrum -- defined by Wien's Displacement Law -- does change; as the object gets hotter, more light gets emitted at shorter (bluer) wavelengths.

The blackbody function has well-defined logarithmic slopes, depending upon whether you are blueward or redward of the peak. Redward of the peak, the function follows the Rayleigh-Jeans Law where the brightness, B_λ is inversely proportional to the wavelength to the fourth power:

B_λ,R-J = 2 c k T / λ⁴

Blueward of the peak, the blackbody curve nearly follows Wien's Law, which is a modified exponential relation,

B_λ,Wien = (2 h c / λ⁵) exp (-h c/&lambdakT)

If you want to determine the temperature of an object, then in theory you

1. measure the brightness in four or five different band-pass filters
2. determine where the slope of the spectrum changes from the Rayleigh-Jeans to the Wien regime,
3. estimate where the peak of the function should be located, and then
4. obtain the temperature from Wien's Displacement Law.

In practice, it is a little trickier since stars aren't perfect blackbodies. What is normally done is to measure the color in several filters, and then compare the measured colors to tabulated values of colors for stars with precisely-measured spectral types. You can then determine the star's spectral type based on only a few observations. It is usually easier to determine a spectral type this way because it takes less time to measure the brightness in four or five wide-band filters than it does to obtain a spectrum.

Certain photometric colors are also used as indicators of other intrinsic properties of stars. Perhaps the most important is the chemical composition of a star, specifically the amount of metals in the stellar atmosphere. Metals cause what is called line blanketing in stellar atmospheres -- metal atoms absorb blue light, and re-emit this light in red light. Thus, metals "blanket" the blue side of the spectrum. So what happens is that at a given effective temperature, a metal-rich star might appear redder than a metal-poor star with similar effective temperature.

You can also obtain the luminosity of a star with colors, though in a slightly more convoluted way. In the near-ultraviolet, there is a spectral feature called the Balmer decrement, caused by the strong absorption of ultraviolet light by hydrogen. The strength of this decrement is partly a function of the surface gravity of the star, which is a function of the mass of the star and its radius. A star with a lower surface gravity is likely to be an evolved, giant star, while one with higher surface gravity is likely closer to the main sequence. Since giant stars are more luminous, there is a difference between the spectra of giant stars and those of main sequence stars, even if they have the same temperature and chemical composition. You can use some photometric color indices to measure the strength of the Balmer decrement, and thus obtain the luminosity.

Ages of star clusters and galaxies

The color can also be used to measure the age of a star or a group of stars in star clusters and galaxies. When individual stars in a star cluster can be resolved, the color can be used to build a Hertzsprung-Russell diagram in the form of a color-magnitude diagram. The color-magnitude diagram is almost identical to the Hertzsprung-Russell diagram, but with color replacing the temperature, and magnitude replacing the absolute luminosity. The age of the cluster can be determined by fitting isochrones to the diagram.

If you can't resolve individual stars and assemble a color-magnitude diagram, then you can measure the color of a population by determining the surface brightness of the diffuse light in two different filters. This is commonly done when observing galaxies too far away to resolve individual stars. If you find that galaxy has a low color index (for example, (m_blue - m_red) = 0.1), then most of the light in that galaxy is coming from bluer stars, suggesting that the galaxy is still forming stars today. If the color index is higher, then most of the light is probably coming from older, redder stars (like old red giant stars). Often, different parts of a galaxy may have different colors. For example, the bulge of a spiral galaxy will have a redder color than the spiral arms because bulges are usually very old (and red), while spiral arms may contain lots of newborn, hot, blue stars.

Interstellar reddening and extinction

Our Milky Way galaxy, like most spiral galaxies, is filled with dust. This dust has the effect of reddening any light that passes through it -- blue light is preferentially scattered away by dust particles in space, while red light passes through relatively unimpeded. This can and does foul up our attempts to measure the intrinsic color of stars when there is dust in the interstellar medium between us and them. However, there's one way around this. Suppose we take a finely-detailed spectrum of a bright star in the region we're interested in, rather than relying on the colors. The shape of its blackbody curve will still be distorted, but we can use other things like the strength of the hydrogen absorption lines or other absorption and emission lines in the spectrum to determine the spectral type. We can then measure the difference between the colors we should see and the colors we actually measure to determine the amount of reddening toward the area we're observing. This difference is known as the color excess, given by

E(A - B) = (A - B)_observed - (A - B)_intrinsic

If you then make the (often unwise) assumption that the reddening is the same for all stars in that same general direction, then you can use the measured color excess to determine the intrinsic colors of other stars in the same area. However, this is not always a good idea, given that dust in our Milky Way is very patchy and uneven. It is better to take several spectra in the same region to see whether all stars have similar reddening in their spectra.

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Sources:
Increasingly dim memory.
Also, Radiative Processes in Astrophysics.

The Visible Spectrum

The human eye can perceive electromagnetic waves with wavelengths from approximately 390nm (violet) to 750nm (red). The visible wavelengths of light are collectively referred to as the visible spectrum, and constitue a small part of the total electromagnetic spectrum. From longest to shortest wavelength, the colors are: red, orange, yellow, green, blue, and violet. A chart of the visible spectrum is just a cross section of rainbow that labels the colors with what wavelength of light they produce (see http://www.photo.net/photo/edscott/vis00010.htm). Other animals can see different frequency ranges, but otherwise their visible spectrum is fundamentally no different from ours.

The Color Wheel

You can probably remember seeing a color wheel somewhere back in elementary school - that picture with the primary colors and intermediates you would get if you mixed them together? Technically, if yours had red, yellow, and blue (or magenta, yellow, and cyan) as the primary colors, it was demonstrating subtractive colors - the kind of colors you get by mixing paints together. Light is additive, but the concept is the same; for light, the primary colors are red, green, and blue, and the intermediates are brighter than the primaries because there is more total light present in them. Check out http://home.wanadoo.nl/paulschils/06.00.html for a nice picture of an additive color wheel.

The visible spectrum is laid out in a line, but the color wheel is circular. You can get a fairly good idea of their relationship if you imagine bending the visible spectrum into a circle. The color wheel is useful for observing multiple wavelengths of light simultaneously (what do I get if I mix red and blue?). The color wheel is not representative of the real world - only our perception of it. For many species, our color wheel would be meaningless.

Primary Colors

The primary colors are the basis for the color wheel. The whole point of the wheel is that all the other colors on the wheel are made up of varying amounts of just those three colors. However, the primary colors are entirely a product of our perception of light - they are meaningless outside of our eyes. Each primary color of light (red, green, and blue) corresponds to a type of cone in our eyes suited to observe that particular wavelength of light. Red cones are best at 564nm wavelengths, green cones at 534nm, and blue cones at 420nm. Upon seeing red light, the "red" cones are excited and send an appropriate signal to the brain. Yellow light (560nm) is between red and green in wavelength, and will excite both red and green cones - the brain interprets the combined red-green signal as yellow. Your brain cannot tell the difference between a pure single-frequency yellow light, and a combination green/red light that triggers the green and red cones in the same proportion and amplitude.

Computer monitors take advantage of this fact. If you look at a monitor with a magnifying glass, you will see tiny clusters of three dots; red, green, and blue. By adjusting the brightness of each dot, our eyes can be fooled into thinking they are seeing any color of the visible spectrum (and even a few that aren't in the spectrum - keep reading).

So if the primary colors are only a function of the cones in our eyes, could animals have different primary colors? Yes, and in fact they do. The red-sensitive cone is a fairly recent evolution in primates, and many mammals (including dogs and cats) do not have red sensing cones in their eyes. Bees, on the other hand, have extra primary colors, including one in the ultraviolet! The number of possible colors grows exponentially with the number of primary colors available because each color is perceived as a combination of the primaries.

What About Violet?

It is easy to see how mixing red (650nm) and green (535nm) light might make you see yellow (560nm) light, because yellow is somewhere in between red and green. A wavelength between red and green will trigger both the red and green cones, and can be mimicked by separately triggering both. Similarly, any frequency of light between green and blue can be recreated in our eyes by using a combination of pure green and blue light (remember that the light does not actually combine to form an intermediate frequency, it just looks that way to our eyes). But why would combining red light (700nm) with blue light (420nm) make our eyes perceive violet (390nm)?

Each of the three types of cones in our eyes is tuned to a particular wavelength of light (Red: 564nm, Green: 534nm, Blue: 420nm). These are the wavelengths of light each of these cones responds to most strongly, but they are not the only wavelengths they respond to. Green and blue cones have fairly centralized responses around their associated wavelength, but red cones actually respond faintly to wavelengths much shorter than normal red light. The red response declines steadily from 564nm to about 500nm, but then remains constant and even rises slightly below 450nm. This means that although blue light primarily triggers the blue cones, it also excites the red ones slightly.

When we see the blue light, we get a lot of response from the blue cones and a slight response from the red ones. Heading toward violet, the red response remains slight but steady, while the blue response falls off. In this way, the wavelengths below blue make our eyes see proportionately more red, even though the absolute value of red cone response is remaining the same! Green and blue cone response is much more restricted to their respective wavelengths; under violet light, green cones will respond very slightly, but not enough to be noticeable compared to the blue or red cone response.

It is impossible to see "pure" green or blue, because green and blue light will always trigger a slight red cone response. If the optical nerves were directly stimulated, it might be possible to see a color more green than is physically possible; under ordinary circumstances all green light will trigger some red cone response in addition to the main green cone one.

Now what if you have light composed of a lot of red with a little bit of blue - a color like magenta. This is not a color that can be achieved with a single wavelength of light! Take a look at the visible spectrum (http://www.photo.net/photo/edscott/vis00010.htm) - magenta isn't there. Any color between violet and red on the color wheel actually requires two wavelengths of light to create. Of course, white light (when all three types of cones are active) requires multiple wavelengths of light as well.

Also in the visible spectrum, you will notice that at shorter wavelengths than blue the colors turn more and more violet until we can't see them. But once you reach red, our vision just stays red - once our red cones are active and the blue and green cones are not, we have no way of distinguishing 675nm from 700nm red. The longer wavelengths will seem darker the further they get from the red cone's main frequency, but to us this is indistinguishable from just dim lighting.

References:

http://madsci.wustl.edu/posts/archives/mar2001/985572799.Ph.r.html

http://www.photo.net/photo/edscott/vis00010.htm

Col"or (?), n. [Written also colour.] [OF. color, colur, colour, F. couleur, L. color; prob. akin to celare to conceal (the color taken as that which covers). See Helmet.]

1.

A property depending on the relations of light to the eye, by which individual and specific differences in the hues and tints of objects are apprehended in vision; as, gay colors; sad colors, etc.

⇒ The sensation of color depends upon a peculiar function of the retina or optic nerve, in consequence of which rays of light produce different effects according to the length of their waves or undulations, waves of a certain length producing the sensation of red, shorter waves green, and those still shorter blue, etc. White, or ordinary, light consists of waves of various lengths so blended as to produce no effect of color, and the color of objects depends upon their power to absorb or reflect a greater or less proportion of the rays which fall upon them.

2.

Any hue distinguished from white or black.

3.

The hue or color characteristic of good health and spirits; ruddy complexion.

Give color to my pale cheek. Shak.

4.

That which is used to give color; a paint; a pigment; as, oil colors or water colors.

5.

That which covers or hides the real character of anything; semblance; excuse; disguise; appearance.

They had let down the boat into the sea, under color as though they would have cast anchors out of the foreship. Acts xxvii. 30.

That he should die is worthy policy; But yet we want a color for his death. Shak.

6.

Shade or variety of character; kind; species.

Boys and women are for the most part cattle of this color. Shak.

7.

A distinguishing badge, as a flag or similar symbol (usually in the plural); as, the colors or color of a ship or regiment; the colors of a race horse (that is, of the cap and jacket worn by the jockey).

In the United States each regiment of infantry and artillery has two colors, one national and one regimental. Farrow.

8. Law

An apparent right; as where the defendant in trespass gave to the plaintiff an appearance of title, by stating his title specially, thus removing the cause from the jury to the court.

Blackstone.

⇒ Color is express when it is asverred in the pleading, and implied when it is implied in the pleading.

Body color. See under Body. -- Color blindness, total or partial inability to distinguish or recognize colors. See Daltonism. -- Complementary color, one of two colors so related to each other that when blended together they produce white light; -- so called because each color makes up to the other what it lacks to make it white. Artificial or pigment colors, when mixed, produce effects differing from those of the primary colors, in consequence of partial absorption. -- Of color (as persons, races, etc.), not of the white race; -- commonly meaning, esp. in the United States, of negro blood, pure or mixed. -- Primary colors, those developed from the solar beam by the prism, viz., red, orange, yellow, green, blue, indigo, and violet, which are reduced by some authors to three, -- red, green, and violet-blue. These three are sometimes called fundamental colors. -- Subjective ∨ Accidental color, a false or spurious color seen in some instances, owing to the persistence of the luminous impression upon the retina, and a gradual change of its character, as where a wheel perfectly white, and with a circumference regulary subdiveded, is made to revolve rapidly over a dark object, the teeth, of the wheel appear to the eye of different shades of color varying with the rapidity of rotation. See Accidental colors, under Accidental.

 

© Webster 1913.

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Col"or (?), v. t. [imp. & p. p. Colored (?); p. pr. & vb. n. Coloring.] [F. colorer.]

1.

To change or alter the bue or tint of, by dyeing, staining, painting, etc.; to dye; to tinge; to aint; to stain.

The rays, to speak properly, are not colored; in them there is nothing else than a certain power and disposition to stir up a sensation of this or that color. Sir I. Newton.

2.

To change or alter, as if by dyeing or painting; to give a false appearance to; usually, to give a specious appearance to; to cause to appear attractive; to make plausible; to palliate or excuse; as, the facts were colored by his prejudices.

He colors the falsehood of Aeneas by an express command from Jupiter to forsake the queen. Dryden.

3.

To hide.

[Obs.]

That by his fellowship he color might Both his estate and love from skill of any wight. Spenser.

 

© Webster 1913.

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Col"or, v. i.

To acquire color; to turn red, especially in the face; to blush.

 

© Webster 1913.