Introduction
This activity demonstrates color constancy, or the tendency for the colors of objects to appear relatively unchanged despite substantial changes in lighting conditions. In this case, there are 18 color swatches consisting of red, orange, yellow, green, blue, purple, cyan, magenta, black, brown, beige, and several shades of gray. The user can apply different transparent filters to the scene that either cover the entire display of 18 swatches or just cover the swatches themselves with no bits of the color filters visible between the swatches.
Description of Activity
If the user clicks “Blue Lighting,” blue light covers the entire region where the color swatches are displayed. Despite the blue lighting across the screen, the color swatches don’t appear to change color much.
However, if the user clicks “Blue Rectangles,” the same blue light is shown, but this time it is localized just to cover the swatches themselves. (A small, transparent blue rectangle is place exactly on top of each colored swatch so that it perfectly fits with no extra showing.) In this case, the swatches appear to change color! For instance, the yellow patches continue to look yellow when the blue lighting covers the entire area but appear rather greenish when the blue rectangles just cover the yellow swatches and nothing else. The gray patches in the bottom row continue to look gray when the blue lighting across the entire are is switched on but appear to be bluish gray when the blue rectangles are turned on.
If the user tries the same procedure with the yellow, green, and red lighting, they should become convinced that this phenomenon is not some artifact of blue light. Why do the swatches seem to stay the same color when the lighting changes are spread over the entire area but not when the lighting changes are confined to the color swatches themselves?
What’s Going On Here?
Human color perception is pretty robust to changes in lighting – this is the phenomenon of color constancy. When perceiving the color of a surface, the visual system discounts the illuminant. That is, it determines what variations in the color of a surface are due to the lighting source and subtracts that color away from the object to get a more accurate perception of what color the surface really is, not how it appears.
When the color rectangles are overlaid exactly on top of the color swatches, there is no evidence that this is a change of illumination because the lighting doesn’t also cover the regions in between the color swatches. So the visual system doesn’t know that this is a trick of lighting and assumes that the change in the color appearance of the swatches is a genuine change to their color, not an accident of lighting. Therefore, the visual system does not discount the illuminant, causing the color swatches to appear to change colors.
Overall, human color constancy is not perfect, but it does a good job of allowing one to figure out the color of an object in the world. You are not distracted by the local color on the retina because the system discounts the illuminant…even if it does not totally eliminate it.