The digital camera you take photographs with and the computer monitor you’re currently looking at both create colors by combining various amounts of pure red, pure green and pure blue. These are called the additive primaries of tri-chromatic vision. We make our digital photography gear work this way because it is how our eyes work. In the central part of the retina we have three types of color-receptive cells (called “cone cells”): One sensitive to red, one sensitive to green and one sensitive to blue.
Visible light is electromagnetic radiation between the wavelengths of 400 nanometers (nm) for violet and 700 nm for red light. Rather than try to work out the wavelength (and corresponding color) directly our eyes just compare how any given light source stimulates each kind of cell. A beam of light of 580 nm wavelength will trigger the Red cells a lot, the green cells moderately and the blue cells not at all. “Ah-ha!” says our retina, “this is yellow!”.
Below is a link to a pop-up window with a small Shockwave application that will help you understand how it works (Shockwave is another Adobe plug-in like Flash — if you don’t have it in your browser already you’ll get a brief message prompting you for an automatic download and install.) The red, green and blue curves show the approximate sensitivity of the three kinds of cells in our eyes. At the top is the spectrum of colors we can see and at bottom is a scale of the wavelengths of light visible to us. Drag the vertical arrow at the left side of the box to any point on the spectrum and you’ll see the color at top, the wavelength at bottom and the approximate response of each kind of cell at the points where the vertical line meets the sensitivity curves.
Click the image below to launch the Shockwave pop-up
NOTE: Due to a bug in the latest version of Apple Safari, this Shockwave app sometimes gets a “wrong version” error (even though it is the correct version) and will not run. If you’re a Safari user you’ll have to try Firefox, Opera or even (ugh) Internet Explorer. (And please report the bug to Apple).
Human color vision is actually a little more complex than this but this is a pretty close approximation of how cameras and other digital imaging devices operate.