Why do colors illuminated with moonlight lack color?

Asked by: Amanda Bond

Answer

Short answer is that they do not lack color. It is just that there is insufficient light for your eyes to perceive color. The fact that objects illuminated with moonlight seem to lack color has very little to do with moonlight (other than the weakness of its intensity) and has a lot to do with the way that your eyes perceive color. The human eye is a very complex device which is still _way_ more complex than the most advanced electronic camera. The retina, where the image is formed contains two types of light-sensitive cells: Cones and rods. The rods are all of the same type, and all they can do is detect the intensity of the light. They have nothing to do with color perception. The cones, on the other hand, come in three kinds, each sensitive to a different wavelength of light (essentially a different color). The catch is this: While the rods can only sense intensity and are unable to detect colors, they can operate at very, very low intensities of light. If I recall correctly, they can even react to very few photons, such as 5-10. The cones, while able to tell colors apart, need quite high intensities of light to actually detect anything, and are essentially "blind" when light intensity falls beneath a certain threshold. So, what we are really talking about here is the perception of lack of colors when there is very little light. This is because your cones can not operate, and all your brain receives is signals from the rods. Hence, the black and white vision... As a side note, lack of one (or more) of the four types of "detector cells" in the eye causes various visions defects. Lack of one of the cone types results in color-blindness. Such people can see some colors, but they can not tell apart some colors. To figure what happens, cut one of the RGB lines on a color monitor. If two kinds of cones are missing, we have total color blindness, they can tell apart no colors (like a green monitor, for instance.) Having very few or no rods gives rise to a different phenomenon: Such people can function normally under well-illuminated conditions, but they can not see at all when light intensity falls under a certain threshold -- their eyes just won't "adjust".
Answered by: Yasar Safkan, Ph.D., Software Engineer, Noktalar A.S., Istanbul, Turkey