Chapter II Color Theory

Chapter II, Digital Color Theory: Lesson I What Is Color http://www.kodak.com/country/US/en/digital/dlc/book3/chapter2/digColorM1_1.shtml

                             Color enriches our lives as a natural, visual experience. Understanding color can help you use it more effectively. To see color, three essential elements must be present: light, an illuminated object, and an observer.

                             The colors we see are affected by the intensity of light and by its spectral content. At low levels of illumination, objects are less colorful. In bright daylight, we see more color, contrast, and saturation.

                             The color spectrum shows the range of wavelengths of energy that are visible to the human eye. Variation in wavelengths alters the colors we see. As Isaac Newton showed with his prisms, white light is a mixture of all the colors of the visible spectrum.

                             Actually, light sources we think of as "white" differ in spectral distribution. Skylight is a bluish white. Tungsten light bulbs are yellowish white. For critical color evaluation, special standardized light sources are used to avoid distorting colors.

                             To avoid distortions, standard viewing conditions employing special light sources are widely used to evaluate color proofs in the printing and publishing industry.

                             Light sources vary in color temperature. When you set up a computer monitor, you can adjust the "white point," which is its color temperature. If a television monitor will be used to display the images you create on the computer, you would choose a white point of 6500. For printed reproduction, a white point of 5000 will match standard viewing conditions.

                             Before light reaches our eyes, it is modified by colorants in the objects we see. Colorants used in reproducing images are pigments and dyes.

                             When light strikes an object, wavelengths may be reflected, absorbed or transmitted. Colorants can be mixed to control the wavelengths and colors we see.

                             Densitometers measure the amount of light reflected or transmitted. They are useful quality control tools for consistent color reproduction.

                             Light waves that reach the eye stimulate a visual process so complex it's not yet fully understood. Within the retina, cones respond to color hues and brightness. Rods sense only brightness. Three types of cones respond to wavelengths in ways that produce all the colors we see.

                             The human eye is an excellent judge of color in side-by-side comparisons. We can see differences that are difficult to measure especially among lighter colors.

                             The eye comes equipped with an automatic color balance feature called "chromatic adaptation." It adjusts to overall color shifts, like those produced by different light sources.

                             We can demonstrate chromatic adaptation right before your eyes. Watch the yellow towel. Let's cover the towel with a cyan filter and see what happens.

                             The filter has a rough edge so you can see it. When covered by cyan, the towel appears green, as you might expect. Now, lets cover the entire scene with that same cyan filter.

                             Now, the towel looks yellow again! The eye tends to cancel out overall color shifts using white as the strongest reference point.

                             Color perception is also influenced by tones and colors surrounding an image. The color patches on the left and right are the same.

                             Observers may also suffer from deficiencies in color vision. Six to eight percent of men and a few women have some degree of color deficiency.

                             Generally, we human observers outperform any instrument in judging color ... because we see color in context through the unique psycho-physical process called perception.

                             Color can be defined by three properties: hue, saturation, and lightness or brightness. When we call an object "red," we are referring to its hue. Hue is determined by the dominant wavelength

                             The saturation of a color ranges from neutral to brilliant. The circle on the right is a more vivid red than the circle on the left although both have the same hue.

                             Lightness or brightness refers to the amount of light the color reflects or transmits.

                             Color ordering systems, such as the Munsell System, use the three properties of color to identify unique colors. Notice that colors are distributed in three dimensions.

                             We commonly see colors arrayed in two dimensions. This is a useful, but incomplete representation. Colors actually occupy a three-dimensional space.

                             Lightness is the third dimension that is not shown in color wheels often used in image editing software.

                             To measure and predict the appearance of a particular color, we need a way to link perception to numbers and formulas. Scientific color values were established earlier this century by the CIE group. CIE models for defining color space all rely on the same basic numbers.

                             CIE values were determined by testing human observers. These perception-based values can be used to measure and compare colors produced by different methods of reproduction. The CIE chromaticity diagram is one way to plot the colors the human eye can see.

                             A color can be specified by its colorimetric values. A colorimeter is an instrument that measures color using numbers derived from CIE values.

                             A spectrophotometer is another instrument for measuring color. It samples wavelengths across the color spectrum.

                             Measuring color allows us to compare the color gamut, or range of colors produced by different methods. We find that color transparency film produces a wide range of colors including some a monitor cannot display.

                             Digital color printers and printing presses have different color gamuts. They can never capture all the colors in an original transparency, but they can simulate the appearance very successfully if color reproduction is understood and controlled. Color management software, described later, helps transform color from one gamut to another.

Let's review what we have just learned Let's review what we have just learned. Select the correct space that color represents by naming the appropriate side of the image.

Let's review what we have just learned Let's review what we have just learned. Choose which of the following is not an example of chromatic adaptation by naming the appropriate side of the image.

This process is also used to check for color blindness. Look at the Squares below, what numbers or letters do you see?

How about this one, different types of color blindness will see different numbers, or no numbers at all

Can you see this? It’s called a “Magic Eye” drawing.