Splitting the White Light and the Selection of Primaries

We can see around us an extremely large number of colors. To deal separately with each of them, with each shade, each hue, each saturation and brightness, would require an immensely complex and elaborate technical system. However, in Chapter 2 it was shown that white light, which surrounds us, can be obtained by an additive matching of three adequately selected monochromatic colors, the three primaries. By varying the proportions of these three primaries, it is possible to obtain all colors of the visible spectrum. Therefore, the reverse statement must be true—we can split into three primaries the light reflected from the objects and beings around us and represent that multicolored world by different proportions of these primaries.

The selection of the primaries is a delicate operation. On the one hand, it is necessary to select them in such a way as to ensure that the triangle they create is as large as possible, thus providing as wide a gamut of colors as possible. On the other hand it is necessary to ensure that it is possible to produce at a reasonable cost materials (phosphors), which would radiate with a sufficient efficiency the monochromatic lights selected as primaries. In that respect, a statistical study was conducted that showed that over 90% of colors encountered in our surroundings lie within a space that is smaller than the one occupied by the chromaticity dia- gram (Figure 4.1). In other words, it was possible to choose shades of red, green, and blue to define a “color triangle” that would adequately reproduce the multi- colored world around us by offering most colors that we encountered every day.

4.1 Splitting the White Light and the Selection of Primaries 35 Green Blue Red R G B

Figure 4.1 The space of real colors and the selected primaries.

The selection of the precise shades of red, green, and blue was dictated not only by the availability of phosphors with a good brightness but of those that could be manufactured at a reasonable cost.

In the early 1950s the U.S. Federal Communications Commission (FCC) defined precisely the coordinates of these three colors using the standardized white light, known as Illuminant C, as a reference white (see Chapter 2). That set of values became known as FCC primaries. Following that, major developments in the man- ufacture of phosphors offered a much wider choice and allowed the selection of a different set of primaries, which would allow the enlargement of the color trian- gle. The European Broadcasting Union (EBU) therefore proposed the adoption of a different set of values whose reference white was another standardized white light, the Illuminant D65. Due to a lack of international standardization, or rather,

due to its failure, both sets of primaries are now used in parallel in different parts of the world, causing some confusion.

In order to obtain a white color, the selected FCC primaries have to be matched in the following proportions:

0.30 red+ 0.59 green + 0.11 blue = white

Matching of just two of the three primaries produces complementary colors: red+ green = yellow

green+ blue = cyan blue+ red = purple

It is important to stress again that this “color matching” represents the additive process of direct mixing of colored lights and not the subtractive process of mixing colored pigments.

While the primaries are at the corners of the color triangle, the complemen- tary colors are located at the middle of each side. The matching of the three complementary colors in the following proportions will lead again to white:

0.70 cyan+ 04.1 purple + 0.89 yellow = white

Although we could make white in this way, if we used the complementary colors as corner points, we would define a different color triangle of smaller dimensions encompassing a white light but a smaller number of natural colors. Therefore, red, green, and blue are selected as primaries because the color triangle that has them as corner-points encompasses the largest number of natural colors.

Starting from a selected set of primaries, it is possible to base a television system on the following operations:

• separation of the incoming optical picture into three primaries • conversion of these lights into three electric signals

• transmission of these three signals

• their reconversion, at the receiving end, into three primary light signals • reconstruction, through additive matching, of the optical picture

This chain is in principle similar to the processes of the eye-brain system of human vision:

• The incoming light is split within the eye into three pieces of primary neural information.

• These are then relayed to the brain where the multicolored picture is reconstructed.

The hypothetical system represented in Figure 4.2 intended to show how color television pictures are generated, transmitted, and displayed. Three black- and-white cameras are pointed at the same scene. In front of each camera is a filter—red, green, or blue. By allowing the passage of only one primary color per camera, these filters split the incoming light into three primaries. Consequently each camera will receive a filtered light (red, green, or blue) and will each gen- erate one electric signal that will represent the red, green, or blue contents of the optical picture.

Assume we convey these three signals via three separate transmission paths to three black-and-white television screens, or CRTs, having a sufficiently powerful light output to be able to project the displayed picture on a screen. If in front of each of these three CRTs we install the same type of filter that we used in front of the cameras, we shall obtain three superimposed pictures on the screen—one