Lighting Affects Color

The color you see for an object depends on the mix of light frequencies that reach your eye. That mix, in turn, depends on two things: the frequencies that the object absorbs, and the frequencies in the original light source. The figure on this page shows how this works with reflected light. If a surface doesn't absorb any colors, then all the colors are reflected, and you see white. If it absorbs all red and only red, then it reflects green and blue, so you see cyan, and so on.

Not so incidentally, note that the red, green, and blue we're talking about here--and as shown in the figure--is not what you normally think of as the colors red, green, and blue. Rather, they refer to three broad portions of the visible spectrum. These portions are defined by the three kinds of cones in the human eye. The cones are named for the color associated with the wavelengths of light they are best at absorbing, but each senses a broad swath of the spectrum, not just a particular frequency.

The key point for the moment is that before any frequencies of light can be reflected (or transmitted through a transparent filter, like colored glass), they have to begin first existing in the light source. If you start with a yellowish light--like a typical incandescent bulb--you start with a high proportion of photons in the red and green range compared to the number in the blue range. That's why an incandescent light bulb itself looks yellowish, and it's also why an incandescent bulb brings out the so-called warmer colors in objects--there's more red and green available in the light itself to bounce off the object and reach your eye.

If you start with a typical fluorescent bulb, on the other hand, there's much a much higher percentage of blue available to bounce off the object, and the additional blue brings out the so-called cooler colors in objects where the light bounces.

These differences in the light sources explain the common observation that incandescent light is more flattering to people than fluorescent light. Bringing out blue and other cool colors in a human face gives it an unhealthy-looking pallor. Bringing out the warm colors makes the face look healthy, if not radiant. Much the same comment applies to produce, which is why grocery stores avoid standard fluorescent bulbs near fruits and vegetables.

More generally, differences in light sources explain why colors change when you change the light source. Things actually have a different color under daylight than under incandescent light, and a different color still under fluorescent light. In fact, things will have different colors in daylight at different times of day. And objects that match in color under some lighting conditions won't match under others, a phenomenon known as metamerism.

Copyright © 2004 Ziff Davis Media Inc. All Rights Reserved. Originally appearing in ExtremeTech.