Perception: Color & Luminosity -- Part V

Primary Colors

Article and Photography by Ron Bigelow

www.ronbigelow.com

Before we can look at how digital cameras, computers, monitors, and printers manage color, we need to take a look at primary colors.

Two Types of Primary Colors

In the last article, it was shown that the human eye has three types of cones that are used to determine color. The red cones are sensitive to red light. The green cones are sensitive to green light. Lastly, the blue cones are sensitive to blue light. Thus, the human eye detects wavelengths of light that are centered on these three colors. From these colors, all of the colors visible to humans can be derived.

Thus, for humans at least, all of the colors that we see can be created by mixing different quantities of three colors. A set of colors from which all colors can be created are called primary colors. Now, it turns out that there are actually two sets of primary colors: the RGB and CMY primaries. Two sets of primary colors are needed because color is created in two different ways.

RGB Primary Colors -- Additive Color

Figure 1: RGB Additive Colors

Based on what has been covered so far, it will probably come as no surprise that the first set of primary colors is red, green, and blue. After all, these are the colors around which the three types of cones are centered. These primary colors are called the additive colors because all of the colors visible to humans can be created by adding together different amounts of light of these three colors. Figure 1 demonstrates the additive nature of these colors. Adding red and green produces yellow. Adding red and blue produces magenta. Adding green and blue produces cyan. If all three colors are added, white is produced. The area surrounding the three circles shows that when all three colors are missing, black results.

Figure 2: Mixture of Additive Colors

Other colors can be produced by mixing different amounts of the three additive colors, as shown in Figure 2. In this figure, a large amount of red is mixed with a smaller amount of green and an even smaller amount of blue to produce the color at the bottom of the figure.

The additive colors are used when color is created by adding light. For instance, a CRT monitor creates color by exciting phosphors that emit light. Three different phosphors are used, each of which emits a different color of light. One phosphor emits red light, the other green light, and the other blue light. The final image on the monitor screen is created by blending the different colors of light produced by the phosphors. Thus, CRT monitors use light of the RGB primary colors to create an image.

CMY Primary Colors -- Subtractive Color

Figure 3: CMY Subtractive Colors

The second set of primary colors is cyan, magenta, and yellow. This set might seem a bit odd. After all, the human eye does not have cyan, magenta, and yellow cones. So, why would there be a primary set of these colors? Actually, it makes perfect sense. These colors are used when color is created by reflecting light off a surface. When light strikes the subtractive colors, they create color by subtracting color from the light (it subtracts the light by absorbing it). Cyan subtracts red from the light while reflecting green and blue. Magenta subtracts green while reflecting red and blue. Yellow subtracts blue while reflecting red and green.

The important thing is that the subtractive colors get us back to the red, green, and blue colors to which the cones in the human eye are attuned. This is demonstrated in Figure 3. Adding cyan and magenta produces blue by subtracting red and green. Adding cyan and yellow produces green by subtracting red and blue. Adding magenta and yellow produces red by subtracting green and blue. If all three colors are added, black is produced because all of the colors are subtracted. The area surrounding the three circles shows that when all three colors are missing, white results because no light is subtracted.

Figure 4: Mixture of Subtractive Colors

Other colors can be produced by mixing different amounts of the three subtractive colors as shown in Figure 4. In this figure, large amounts of cyan and magenta are mixed with a smaller amount of yellow to produce the color at the bottom of the figure.

As mentioned, the subtractive colors are used when color is created by reflecting light off an object. Prints are one example of the use of the subtractive primary colors. Prints create color when light reflects off the ink on the print. Thus, the ink subtracts light to create the colors in the print. Since light is being subtracted to create color, many printers use the subtractive primary colors in their ink set.

What About CMYk

No doubt, many of you have seen the subtractive primary colors listed as CMYk. The k stands for black. Theoretically, if equal amounts of pure cyan, magenta, and yellow are mixed together, black is produced. The problem is that the manufacturers of the ink that is used in the printers can not produce inks that are perfectly pure. In other words, the cyan ink is not pure cyan. It has some other colors mixed in with the cyan. The magenta and yellow inks are not pure either. When the three less than pure inks are mixed together, they do not produce a pure black. To resolve this problem, the manufacturers add black to the CMY ink set to create a CMYk ink set that produces better blacks.

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Perception -- Part IV