Service manual

STP 11-25R13-SM-TG
S - 8
Section III. FUNDAMENTALS OF COLOR I
S-4. Introduction.
To understand color TV systems, a basic background in the fundamentals of color is essential. The
entire concept of color television is based upon how our eyes react to light. From studies conducted on
how we see, it was learned that the color signals would occupy a certain amount of space in the
frequency bandwidth for color television. The process of specifying and determining colors had to be
accomplished before the color television system could be developed. This is a science in itself and is
known as colorimetry.
S-5. Colorimetry.
The science and practice of determining and specifying colors are referred to as colorimetry. The
science of colorimetry has been especially important to the color engineers who contributed to the design
of the present color television system. Since the system had to be designed to reproduce things as they
are seen in nature, the characteristics of light, vision, and color had to be taken into consideration.
a. Colorimetry is a very complex subject. It is not necessary to be an expert colorimetrist in order
to understand the makeup of the color picture signal and the way in which it is used in the color receiver.
However, a better understanding of the color television system is attained if some of the most important
fundamentals of colorimetry are known. The principles of color as applied to television are slightly
different from those, which many of us have been taught in connection with other types of color
reproduction.
b. The properties of light and vision must be understood before studying the principles of color.
Light is the basis of color; the eye must be able to convey picture sensation to the brain. But the eye has
certain limits.
S-6. Limits of Vision.
Four factors: intensity threshold, contrast, visual angle, and time threshold chiefly determine the
limits of vision.
a. Intensity threshold
is the lowest brightness level that can stimulate the eye. It is very much
dependent upon the recent exposure of the eye to light. When a person enters a darkened room, the eye
takes a long time to reach its maximum sensitivity. The required time, which is usually about an hour,
differs among individuals. When a person returns to a lighted area, the time it takes for the eye to reach
its maximum sensitivity is very short, actually just a matter of minutes.
b. Contrast
represents a difference in the degree of brightness. The limit of vision with respect to
contrast is the least in brightness difference that can be perceived.
c. Visual angle
. As an object smaller or is placed farther from the eye, the light rays from the
extremities of the object to the eye become smaller. This is referred to as the visual angle.
(1) In order for the eye to respond, the visual angle must be such that the image covers a different
area on the retina. If this area were decreased, a point at which the eye could no longer see the object
would be reached.
(2) This principle is used in eye tests. The minimum visual angle is dependent upon the contrast
and brightness of the image. For example, an object having sharp contrast could be distinguished at a
narrow visual angle while the same size object having a lower contrast might not be visible.
d. Time Threshold
. There is a minimum time during which a stimulus must act in order to be
effective. This is called the time threshold. If the exposure interval is too short, the rods and cones of the