the color television system is attained if 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
principals of color.
Light is the basis of color and the eye must be able to
convey picture sensation to the brain. But the eye has certain visual limits.
3. Limits of vision. The limits of vision are chiefly determined by four factors;
intensity threshold, contrast, visual angle, and time threshold.
a. Intensity threshold. This 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 it takes the eye a long time to reach 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 maximum is very short, actually just a matter of minutes.
b. Contrast.
This represents a difference in the degree of brightness and in
the intensity between black and white elements of the reproduced picture.
The
range of contrast should be great to produce a strong picture, with bright white
and dark black for the extreme intensity values.
c. Visual angle.
As an object is made smaller or is placed further from the
eye, the angle formed by the light rays from the extremities of the object to the
eye becomes smaller. This angle is referred to as the visual angle. In order for
the eye to respond, the visual angle must be such that the image covers a definite
area on the retina. If this area is decreased, a point at which the eye could no
longer see the object is reached.
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 with sharp contrast can be distinguished at a narrow visual
angle, while the same size object with 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 eye do not have time to respond to
the image on the retina.
The time threshold is also dependent upon the size,
brightness, and color of the object. These four factors are all important factors
that were taken into consideration in development of the color system for
television. Now, light sources must be discussed.
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