Color Spaces, Device Characterization, and Color Management in Photoshop 7
What Is Color & How We See It
It's night and we can barely see; then the sun slowly comes up, and things begin to become recognizable. The light from the sun is allowing us to see more and more things. In that early morning light, things seem very warm and yellow. Then as the sun gets higher in the sky, that warm yellow appearance goes away and we get a whiter light. That white, midday light is made up of many wavelengths of light. Light is actually waves of excited electronic particles and those waves come in different wavelengths. When those light waves hit a surface, each different type of surface absorbs some of the wavelengths of light, and other wavelengths are reflected back toward us if we are looking at that surface. Now instead of the white light that comes from the sun, we are only seeing part of that light reflected back from a surface. The part or wavelengths that are reflected back to us determine the color of that surface.
Figure 14.1 The wavelengths of light and how you see them. The white line is an approximation of daylight wavelengths.
Our eyes have sensors called rods and cones. The rods sense brightness or light intensity, but it is the cones that actually detect color, and there are three different types of cones each sensitive to a different wavelength of light. One type of cone is more sensitive to red light, one is more sensitive to green, and the third is more sensitive to blue.
Figure 14.2 Seeing color by light reflecting back off a surface while the other colors are absorbed.
An Image on Paper
When we look at an image printed on paper, the color we see depends on the color of the incoming light that is illuminating the paper; that incoming light supplies all or most of the wavelengths of light that we could possibly see although there might be several different types of light illuminating the paper that increases the possibilities. The color and surface texture of the paper itself will subtract some of the wavelengths from that incoming light source and give the paper a certain color. The inks or other types of color that are painted on that paper will subtract further wavelengths from that original light and reflect back different colors that are the remaining nonsubtracted wavelengths. The angle that you view the paper might also influence how much light is reflected back. When you are considering how a particular image may look on a certain printer, the digital values in the original image, the type of inks that you use, combined with the type of paper that you print on as well as the way the printer puts the ink on the paper, all work together to create a specific range of colors you can see in that one situation.
An Image on a Computer Monitor
Color on a computer monitor comes from particle energizers, a type of light source, behind the monitor's glass that hits the coating on the inside of the monitor glass and produces different colors and light intensities depending on the numerical values that are driving the different colored light particle energizers. There is also light hitting the monitor from the outside due to other light sources within the room, and this light will have some effect on the color and brightness that you see from the internal monitor particle energizer. The way you see color on a computer monitor is quite different from the way you see color on a printed piece of paper. It is difficult to exactly match the brightness, color, and contrast characteristics of these two mediums. We will show you how to get as close as possible to a match using calibration.
Slide on a Light Table
When you see color by looking at a slide on a light table, the color you see there depends on the color of the light source behind the slide, the colors in the emulsion of the slide material, and also on the amount and intensity of the other light sources in the room.