White light is a mixture of many colors of light combined in such a way that the eye responds to the mixture as if the light source were the sun as it shines on a clear day at noon. Some authorities on color prefer to use the "average daylight" value of sunlight instead of the "noon daylight" as the standard of comparison for white light, but since these two spectra of colored light are nearly the same, we will not make that fine a distinction.
When light passes through a prism the net effect is to bend shorter wavelength light more than that of longer wavelength. This separates the various wavelengths into distinctly identifiable bands of color in a process called dispersion.
One way to define color is to say that it is an imbalance in the visible spectrum of light reaching the eye. Imbalance here means a deviation from the average visible spectrum of energy as produced by the noon day sun. If the visual signals received by the eye are like those from the noon day sun we say that we are observing "white light". If the visual signals differs we are seeing some "color". Thus if there is a predominance of green wavelengths but very little reds and blues we would be seeing a "green color".
A colored light source is one that radiates more visible energy at certain wavelengths than at others. A transparent colored object transmits certain wavelengths more than others, thus filtering out some of the wavelengths originally present. Translucent colored objects reflect light in the same sort of selective manner. The bottom line in each process is that in each case a colored light source or object presents an imbalance in spectral energy to the eye. For this to be true some change in the "normal" spectrum energy distribution of white light must be made by the source or object under observation.
For objects that do not produce light themselves there are two fundamental ways that light can be modified. If the object is transparent it can show selective transmission of certain wavelengths. A green filter for example allows only green light to pass through thus selectively presenting only green light to the eye. An object that will allow no light at all to pass through will generally reflect light and if this process is also selective, only certain wavelengths will be reflected to the eye while other wavelengths are absorbed. The reflected light causes the eye to perceive a color if only some wavelengths are present so the process is called selective reflection. Note that both processes could be called selective absorption.
It should be noted that not only does an object absorb some of the wavelengths of light that strike its surface but it may absorb some of the energy at all wavelengths so that the transmitted or reflected light may be reduced in intensity. This does not affect the color of the light but will affect the "brightness" of the light as seen by the eye.
We must remember that all light waves are modified in some way by their interaction with any physical object. The color that we see is determined by the mix and energy of the spectrum of light which remains intact after these interactions to reach our eyes. The color is determined by the selective transmission of reflection while the brightness is a measure of the total absorption of all wavelengths after the interaction.
Colorimetry, the science of measuring and designating colors, is important if a certain color is to be properly identified or duplicated. The standardization of colors is achieved by the use of color systems, of which there are several. Before we study these color systems in detail, we must first understand the relationships between the primary colors of light and the primary colors of pigments.
When the primary colors of light are considered,we are guided in our choice of these primary colors by the operation of the human eye. The color receptors in the eye are divided into three groups. Each group is sensitive to one of the three colors; red, green, and blue. When the proper combination of these three colors is incident upon the eye we say that we are seeing "white light". These three colors are, therefore, considered to be the primary colors of light. Early experiments by James Clark Maxwell and Hermann von Helmholtz established the additive nature of red, green, and blue colored lights to form white light. Because the primary colors of light must be added together to produce white light, these three colors are often referred to as the additive primaries.
If any two of the primary colors of light are added together, one of the secondary colors of light is formed. The various combinations of the primary colors of light are often shown on color charts or color wheels. The following table lists these combinations for you along with the secondary color formed when the two primaries are added.
| Primary Combination | Secondary Colors of Light | |
|---|---|---|
| green + red | = | yellow |
| red + blue | = | magenta |
| green + blue | = | cyan |
| (also remember) | ||
| green + red + blue | = | white |
Complementary colors are any two colors that, when added together, produce white. In the case of light, the secondary colors combined respectively with the corresponding primary color not involved in the additive construction of that secondary color, will give white light. For this reason the following combinations are considered to be complementary colors of light.
| Complementary Color Pairs | Result of Addition of Complementary Colors | |
|---|---|---|
| yellow + blue | = | white |
| cyan + red | = | white |
| magenta + green | = | white |
The Primary colors of pigments are defined as the colors that are transmitted through a filter such that one of the primary colors as seen by the eye is completely absorbed and the other two primary colors of light are transmitted. The primary colors of pigments are yellow, magenta and cyan.
To understand this a little better, consider a white light source that is covered with a yellow filter. The yellow filter does not allow the blue color in the light beam to pass but it does allow the red and green colors to be transmitted. Using a yellow filter we have thus subtracted the color blue from the light while allowing the red and green to pass through. The resulting color that is registered by the eye is yellow. The colors yellow, magenta and cyan are, therefore, often called the subtractive primaries. The combination of all three subtractive primary color filters over the white light source will block all colors so that no light passes.
Combining the three subtractive primaries produces "black", the absence of all color. You may have discovered this "experimentally" as a child when you mixed several water colors together and came up with black (or at least muddy brown).
Light reflected from a painted or dyed surface behaves in the same way as the colored filters described above. The analysis of color filters covering a white light source can be considered analogous to the selective absorption process of a painted yellow wall that absorbs any blue light falling on its surface while reflecting both red and green light. This presents a visual impression of yellow to the eye.
Secondary colors of pigment are green, red, and blue. The complementary color combinations for color pigments are color pairs that when combined produce black. Example: Orange and blue combine to form black.
The many variations on basic hues can be achieved by adding black and/or white to the spectral color pigments. The results of such mixtures are summarized in the color triangle. When white is added to a pure color, various tints are formed. If black is added to a pure color, a series of shades are produced. Both black and white added to a pure color gives a variety of tones. Adding both black and white is equivalent to first mixing the black and white pigments to produce various grays. These grays can then be combined with a pure color to produce tones. The color triangle is the basis for several of the color systems in use today.
Dick Piccard revised this file (http://oak.cats.ohiou.edu/~piccard/phys203/color.html) on September 16, 2003.
Comments and suggestions are welcome by E-mail to "piccard@ohio.edu"