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Color Mixing

This applet illustrates both additive and subtractive color mixing; additive mixing will be discussed first.  In the additive case beams of red, green, and blue light of roughly equal brightness are directed onto a perfectly reflective screen.  The red light contains wavelengths near the long-wavelength end of the visible spectrum, the blue light contains wavelengths near the short-wavelength end of the visible spectrum, and the green light contains wavelengths near the middle.

In regions where the beams overlap the colors are combined additively.  For example, in a region where both red and green light strike, the reflected light has both long and intermediate wavelengths, and the resulting color is yellow.  If all three beams overlap and are added, the light containing roughly equal amounts of all visible wavelengths is white.

One can also see from the diagram that if one begins with white light and then withdraws long-wavelength or red components, the remaining light containing short and intermediate wavelengths will have the hue called cyan (similar to turquoise). As explained below, with this applet one can also explore the effect of adding red, green, and blue lights of different intensities.

Additive color mixing is important in theatrical lighting, color television, and computer monitors.  The more lights that are added, the brighter is the net effect.

Subtractive color mixing, which is important for paints and inks, has the opposite behavior.  The more colors that are mixed together, the dimmer is the surface.  In general, when light is reflected from a surface some of the light is absorbed in the surface and only the remainder is reflected.  Colored inks and paints have the property that the amount of absorption depends on the wavelength of light.

If white light shines on a paint that preferentially absorbs long wavelengths and thus removes red from the light, the reflected light has predominantly short and intermediate wavelengths and is cyan in color.  Thus that paint is cyan-colored.  If white light shines on a pigment that absorbs short wavelengths and thus removes blue from the light, the reflected light has predominantly long and intermediate wavelengths.  The pigment is yellow in color.  If one has a mixture of cyan and yellow inks, only the green wavelengths remain unabsorbed and survive to be present in the reflected light.  If cyan, yellow, and magenta inks are mixed, more or less all of the wavelengths of white light are absorbed and the net result is no reflection, or black.

The basic inks in a color printer are cyan, magenta, and yellow.

Reference: Williamson and Cummins, “Light and Color in Nature and Art,” (Wiley, New York, 1983).

 

Operation

1. At the bottom left of the window is a slider that may be used to adjust the degree of overlap of the three colored disks.

In addition, a triplet of sliders on the right adjusts the characteristics of the three colored disks. Their function is somewhat different for additive and subtractive mixing, as explained below under RGB Sliders.

2. At the top of the window is a menu bar that controls the display.  Available menu choices and their effects are the following:

Menu Selection Action
Color Mixing > About Color Mixing Reveals the name of the author and the year the applet was written.
Type > Addition Sets up the display to show in a dark space the addition of beams of red, green, and blue light of equal or unequal brightness.  In regions of overlap, the colors are combined additively.
Type > Subtraction Sets up a display of subtractive color mixing.  Shown is white light bathing three disks of ink that absorb wavelengths in the red, green, and blue parts of the spectrum, respectively.  In regions of overlap, two or more colors are eliminated from the reflected light.

 

RGB Sliders

In additive mixing, the three sliders may be used to vary the intensity of each of the three beams of light to be mixed. The numerical scales on the right go from 0 to 255, the range used in java to specify color intensities.

In subtractive mixing, the sliders have a different function. Consider the cyan disk. It represents white light shining on a disk of ink that absorbs red wavelengths completely but reflects green and blue wavelengths without modification. The red slider changes the extent to which the red wavelengths are absorbed by the cyan disk. At the 255 end, the red wavelengths are completely absorbed. At the 0 end the red light is not absorbed at all, so the reflected light has the same spectral composition as the incident light. That is, it is white and disappears in the background of the ink-free white screen. Similarly, the green slider controls the extent to which the magenta disk absorbs green wavelengths, and the blue slider controls the extent to which the yellow disk absorbs blue wavelengths.