When two lights with c1(λ) and c2(λ) are combined, the resulting light has c(λ) given by
c(λ) = c1(λ) + c2(λ) (2.7)
Since the lights add in (2.7) this is called an additive color system. By adding light sources with difterent wavelengths many different colors can be generated. For example the lighted screen of a color television tube is covered with small glowing phosphor dots arranged in groups of three. Each of these groups contains one red, one green, and one blue dot. These three colors are used because by proper combination they can produce a wider range of colors than any other combination of three colors; they are the primary colors of the additive color system. Colors of monochromatic lights change gradually, and it is difficult to pinpoint the specific wavelength corresponding to red (R), green (G), and blue (B). The C.l.E. has chosen λ = 700 nm for red, λ= 546.1 nm for green, and λ = 435.8 nm for blue.
The three primary colors of the additive color system are shown in Figure 1.5. In the additive color system, a mixture of equal amounts of blue and green produces cyan. A mixture of equal amounts of red and blue produces magenta, and a mixture of equal amounts of red and green produces yellow. The three colors yellow (Y), cvan (C), and magenta (M) are called the secondary colors of the additive color system. When roughly equal amounts of all three colors R, G, and B are combined, the result is white. When roughly equal amounts of R, G, and B components are used in a color TV monitor, therefore, the result is a black-and-white image. By combining different amounts of the R, G, and B components other colors can be obtained. A mixture of a red light and a weak green light with no blue light, for example, produces a brown light.
Nature often generates color by filtering out or subtracting some wavelengths and reflecting others. This process of wavelength subtraction is accomplished by molecules called pigments, which absorb particular parts of the spectrum. For example. when sunlight, which consists of many different wavelengths, hits a red apple, the billions of pigment molecules on the surface of the apple absorb all the wavelengths except those corresponding to red. As a result, the reflected light has a c(λ) which is perceived as red. The pigments subtract out certain wavelengths, and a mixture of two different types of pigments will result in a reflected light whose wavelengths are further reduced. This is called a subtractive color system. When two inks of different colors are combined to produce another color on paper, the subtractive color system applies.
The three primary colors of a subtractive color system are yellow (Y), cyan (C), and magenta (M), which are the secondary colors of the additive color system. The three colors are shown in Figure 1.6. By mixing the proper amounts of these colors (pigments), a wide range of colors can be generated. A mixture of yellow and cyan produces green. A mixture of yellow and magenta produces red. A mixture of cyan and magenta produces blue. Thus, the three colors, red, green, and blue, the primary colors of the additive color system, are the secondary colors of the subtractive color system. When all three primary colors Y, C, and M are combined, the result is black; the pigments absorb all the visible wavelengths.
It is important to note that the subtractive color system is fundamentally different from the additive color system. In the additive color system, as we add colors (lights) with different wavelengths, the resulting light consists of more wavelengths. We begin with black, corresponding to no light. As we then go from the primary colors (RGB) to the secondary colors (YCM) and then to white, we increase the spread of the waveleneths in the resulting light. In a subtractive color system, we begin with white, corresponding to no piements. As we go from the primary colors (YCM) to the secondary colors (RGB) and then to black. We decrease the spread of the wavelengths in the resulting reflected light.
In an additive color system, we can think of red, green, and blue light as the result of passing white light through three different handpass filters. Mixing two colors can he viewed as passing white light through a filter, which is a parallel combination of the two corresponding bandpass filters. In a subtractive color system, we can think of yellow, cvan, and magenta as the result of passing white light through three different handstop filters. Mixing two colors can he viewed as passing white light through a cascade of the two corresponding handstop filters.