• Background
  • Instructions
  • Illustration
  • Quiz


Can we see in color at night when only one type of receptor, rods work? We answered that no, with the dicussion of the principle of univariance. But what about people we usually call color blind, dichromats. Dichromats have two classes of cones, so they have two types of receptors each maximally sensitive to a different wavelength.

In this activity, you will first try to make a match between a test and comparison patch of light, using only one wavelength like in the Color Matching in Monochromats-see if it is possible to make a match in this case.

Then you can try the circumstance of making matches when you can control the intensity of two different primaries in the comparison patch. You will be able to adjust the intensity of a blue and red primary separately. Now can you make a match? You might be able to in all circumstances. Ask yourself under what circumstances can you not make a match.



Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab


Below is a list of the ways that you can alter the model. The settings include the following:

Test and Comparison Patches: at the top are two squares. They represent the appearance of the two stimuli to the simulated eye. Your job is to make the two patches look the same in this simulated eye. The left-hand patch is the test stimulus, and the right is the comparison. You will adjust the comparison to make it look like the test. Add Second Primary: change to being able to adjust two primaries.

If only one primary
Relative Intensity: adjust the relative intensity of the comparison patch. The value is the relative physical intensity of the stimulus. If the value is 1.0, then the intensity of the comparison is the same as the test patch.
Change Wavelength of Comparison: select to allow yourself to adjust the wavelength of the comparison stimulus.
Wavelength: select the wavelength of the comparison to try a different wavelength.

If two primaries
Blue Primary Intensity: adjust the relative intensity of the blue primary in the comparison patch.
Red Primary Intensity: adjust the relative intensity of the red primary in the comparison patch.


Pressing this button restores the settings to their default values.