Color constancy refers to the observation that we see the same color despite changes in the composition of the wavelengths of light that is striking that object. Thus, a green mug appears to be the same color regardless of whether the light illuminating it is natural sunlight, a fluorescent lightbulb, or an incandescent lightbulb. This is true even though the object is now reflecting different absolute amounts of light at different wavelengths under each illumination condition. Color constancy serves an important perceptual function—the properties of objects seldom change as a function of changes in the source of illumination. Thus, a system that sees an object as a constant color across such changes leads to accurate perception. Interestingly, the distribution of wavelengths in sunlight changes across the day. Evening light has more long wavelength light than light earlier in the day. Although we might enjoy the colors of twilight, we do not normally see objects as changing colors, though we are aware of general changes in illumination when we attend to them. We can see this in Figure 6.25. The statues of the presidents appear to be the same color despite the change from peak sunlight to twilight.
In this illustration, you can simulate changing the illumination in a scene. Since the rest of the world does not change in illumination, you will clearly see the change in color by changing the illumination in the photograph. So you will be able experience the degree of color constancy we have by experiencing its breakdown.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
On the Illustration tab, you can adjust the parameters and start a simulation of dark adaptation.
Below is a list of the ways that you can alter the illustration. The settings include the following:
Illumination Type: preset standard types of illumination. Press the
desired button: Incandescent, Fluorescent, Noon, or Twilight, to see the image illuminated with
that type of light.
Illumination Control: use these sliders to create any illumination source that is possible on your monitor. The colored square at the end of the red primary slider shows the color of the current illumination.
How Display: Single refers to showing only one image which has all your current settings. Compare will put the original photopic color on the left and the image in the current eye state on the right.
Use Your Own Image: allows you to upload your own image to simulate the Purkinje Shift.
Pressing this button restores the settings to their default values.