An edited article: Original by Annie Sethiwan, November 14, 2003
Visible light is a small portion of the electromagnetic spectrum with wavelengths from about 400 to 650 nanometers. The shorter wavelengths are perceived as blue/violet, the longer wavelengths are perceived as red. The continuous spectrum from red to violet (sunlight) is seen as white.
Different wavelengths travel with different velocities through a dispersive medium such as glass. Longer wavelengths travel more rapidly through glass and refract less than shorter wavelengths. A rainbow is formed as light is refracted in water droplets suspended in the air. The effect is similar to that of a prism.
The human eye perceives colors in two situations; when light is reflected from an object, and when light is viewed directly from a radiant source.
Two color systems are used when reproducing color: the additive system in direct light and the subtractive system in reflected light.
The additive color system that applies to the images that appear on television and computer monitors begins with the absence of light; black. Colors are then added to the black background to form new colors. The standard additive primary colors are red, green and blue. If these three primary colors are added in equal amounts, the combination is seen as white.
What a person sees is determined by the retina, which contains just layers of detectors called cones, sensitive to wavelength bands in the red, green, and blue. A full color TV image can be constructed with three-phosphor spots, because the eye samples the spectrum in only three places. If the eye/brain system were capable of the more detailed spectral analysis involved in hearing the difference between the sound of a violin and a trumpet, full color TV type image reproduction would be almost impossible.
The rules of additive color mixing are ...
The subtractive color system applies to the colored images printed in books, magazines and on signs. The background color is white; a reflection of all possible wavelengths (colors). Dyes applied to the white surface act as filters subtracting portions of the reflected spectrum, resulting in different colors.
In the subtractive color process cyan, magenta and yellow are the primary colors.
Cyan removes just the red
Yellow removes just the blue
Magenta removes just the green
Cyan and yellow together remove both the red and the blue leaving green..
The rules of subtractive color mixing are....
In magazine printing three transparent pigments [cyan, magenta and yellow] are used to filter the reflected wavelengths. When ideal dyes of these three colors are combined in the right proportions the result is an accurate reproduction of the original, but in practice, images often appear dirty and muddied. This defective result is not due to a flaw in the theory, but to the pigmentation of the inks used. ie. a true black cannot be produced from any combination of the three dyes. For this reason a fourth dye, black, is added into a subtractive printing process, which becomes CMYK rather than CMY.
| Note: artists call the three basic paints primary colors. The three paints are actually Yellow, Magenta and Cyan, secondary colors, but artists refer to them as Yellow, Red and Blue. They say that yellow and blue make green. Mixing the paint in the pots marked Y and B does make green, but the pots would be better labeled Yellow and Cyan (top right hand diagram) . |
See Primary and Secondary Colors