In the lesson, you saw how light can be characterized by its spectral distribution. For example, you saw a graph that illustrated the percentage of light transmitted by a yellow filter as a function of the wavelength of the light. That diagram is called a transmission curve or a spectral energy distribution curve (SED). If you know the spectral distribution of filters you can predict what the spectral distribution of a combination of them will be. You can see some of these SEDs are like by going to the Rosco web link:
Link to Rosco filters. Click on products in the left hand column. Then select color filters.

Some filters act as "turn-on" filters. These are the red, orange, and yellow filters. As you look at their SED going from violet to red at some point the filter "turns on" and begin to transmit light. When two or more of these are overlapped, the result is to reduce the transmission of light and to shift the effective turn-on point to longer wavelengths as seen with the amber filter on the video. The reverse of this is the dilution of yellow food coloring in which the turn on shift to lower and lower wavelengths as the dilution continues.

One important determination of the color of a material is the size of the particles that make it up. Remember how dark blue the crystal of copper sulfate looked, yet the small grains were lighter. Then, when the grains were ground into a fine powder they looked even lighter or more pastel. The reason is that as the grains get smaller, less light is absorbed passing through so each grain contributes to only a tiny amount of coloring. In addition, because there are more surfaces to reflect the light, the result is a more unsaturated or pastel color. Perhaps the ultimate is in the white pigment used in paints, titanium dioxide. TiO₂ is clear, but when ground into a fine powder it looks white. Snow looks white for the same reasons.

In the last lesson we saw that color has three attributes: hue, saturation, and lightness. In the video with this lesson we saw a visualization of a color space or volume in which lightness was depicted along a vertical axis, saturation was shown as distance from that axis, and hue was seen as a position about the axis.

The color volume was defined by a three dimensional coordinate system. The coordinate system is cylindrical. The vertical symmetry axis is the lightness labeled L*. Perpendicular to the lightness axis is plane containing a circular coordinate system. The hue changes as the angle changes in that plane and the saturation increases with radial distance from the L* axis. Rectangular coordinate axes can be drawn on the plane containing hue and saturation. These are labeled a* and b*. The hues go from red along the +a* direction to yellow along the +b* direction to green along the -a* direction and finally to blue along the -b* direction. Any color can be uniquely described in terms of the three coordinates of the L*a*b* system.

Another quantitative diagram to describe colors is the CIE chromaticity diagram. This system was defined in terms of a "standard observer" that matched three primary colors of light to get the same sensation of any particular specified color. These primaries have complicated spectra and overlap each other. (They were shown in the video as the x, y, and z with a line over the letters.) If filters are made to match the characteristics of these primaries we can pass incoming light through each of the filters to get three special primaries called X, Y, and Z. These values are also known as the tristimulus values. The CIE chromaticity diagram is a plot of the relative values of X and Y. It it the horseshoe shaped diagram with a straight line at the bottom that is the line of purples. Refer to the color figures in the video or in your text. The colors that your eye can see can all be found within the CIE diagram. The colors that can be generated by television or in printing, etc., are found in smaller regions within the chromaticity diagram as seen in the video lesson.
The tristimulus values can be determined with a spectrophotometer and the filters just described. Hand-held meters that do this are readily available on the market. The L*, a*, b* values are computed directly from the tristimulus values. The instruments that compute the tristimulus values also readout in L*, a*, and b*.


The effects of combining paints are much more complicated than that of combining lights or color filters. The reason lies in the complexity of paints themselves. When combined they act neither as simple additive nor subtractive colors. In general, the behavior of paints includes both additive and subtractive behaviors. Consequently, prediction of the results of adding two paints together is very imprecise. What you find in paint stores where different amounts of pigments are added to make a desired color is a result of prior measurements and recipes.