no filter |
The picture at right is un-retouched, taken with sunlight, from a window, a 100 watt light bulb and the camera's flash. To recreate this photo using tricolor theory, three separate photos were taken from a stationary camera: one through a red filter, another through a green filter and the third through a blue filter. This filtration was done with (very expensive) pieces of colored glass, not by separating the channels in Photoshop.
red filter | + | 
green filter | + | 
blue filter |
R + B + G filters |
Now these separate images were combined using Photoshop's channels into a single image, seen at right. (An error caused by not matching the transmittance of the filter to the luminosity of the color lead to darker blue hues.) The (synthetic) colors in this image are within the colorspace defined by colors of the three filters.
As 'discovered' by the brightness matching experiment, my own eyes have attenuated response in the middle of the spectrum, in an area corresponding to the M-cone's sensitivity, giving me only two color receptors. What color space will result in an image by mixing only two filters? According to tricolor theory, two colors can only mix to make a small subset of colors - those that lie 'between' the two primaries. But experiments of Edward Land and Johann Goethe showed that dichromatic color (that is, color derived from two primary colors instead of three) could result in a large color space.
Mixing red and green light makes (orange) yellow light, so mixing the images made using the red and green filters in equal proportions (with Photoshop) result in a yellowish image, shown below on the left:
R + G filters | ?
= | 
yellow filter |
On the right is an image taken with a yellow filter. The color of these two images are very similar. The tricolor image above is made by mixing red, green and blue in nearly equal portions. Can a yellow filtered image replace the mixture of the red and green filters?
yellow filter
blue filter |
|
Y + B filters |
Here (at right) is a mixture of the images made with the yellow and the blue filters. (The yellow and blue images are shown at left so you don't have to scroll up.)
Although there are obvious color shifts (remember, blue is dimmed by a filter factor error), this image includes a wide range of colors including reds and greens not included in either of the parent images!
Here are the three 'full color' images:
no filter | ?
= |
R + B + G filters | ?
= |
Y + B filters |
Although the three images are not identical, it appears that a larger range of color can be produced by mixing only two primary colors than tricolor mixing theory would predict.
Here are the transmission spectra of the four filters used in this experiment:
From top to bottom, they are
Red - B+W #090 (Wratten #25)
Yellow - B+W #023 (Wratten #15)
Green - Tiffen #61 (Wratten #61)
Blue - Tiffen #47 (Wratten #47)
