• Background
  • Instructions
  • Illustration
  • Quiz


Can we see in color at night when only one type of recepotor, rods work? How about a person during the day who has only one type of cone? The answer is no, but it has to do with the principle of univariance.

The principle of univariance means that any single cone system is color blind in the sense that different combinations of wavelength and intensity can result in the same response from the cone system. This implies that color vision depends critically on the comparative inputs of the different cone systems. The same is true of rods at nighttime.

The issue is all to do with making matches. Go back to the concept of metameric matches. Well, if I can take any wavelength of light and make it look just like any other wavelength of light but just change the intensity, can I truly say I have color vision?

In this activity, you will try to match one patch of light against another in a simulated eye with one receptor by just changing intensity.


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 Patch: 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. 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.


Pressing this button restores the settings to their default values and allows you to choose new wavelengths for the test and comparison stimuli.