Service manual

STP 11-25R13-SM-TG
S - 9
eye do not have time to respond to an image on the retina. The time threshold is also dependent upon
the size, brightness, and color of the object.
e. The above four factors are all-important factors taken into consideration in the development of
the color system for television. The foregoing has shown how the eye can see. Let us now discuss the
light sources, which we must have so the eye uses its seeing facilities.
S-7. Light Sources.
To see we must have a source of light, just as in the process of hearing we must have a source of
sound before we can hear. When we speak of light, we usually think of light coming from the sun or the
light, which is emitted from some artificial lighting source such as electrical lighting. This type of light is
referred to as direct light.
a. Another type of light is indirect or reflected light, given off by an object when direct light strikes
it. Direct light falling upon an object is either absorbed or reflected. If all the light is reflected, the object
appears white. If direct light is entirely absorbed, the object appears black. The larger the amount of light
that is reflected by an object, the brighter the object will appear to the eye.
b. In addition, the more direct the light source, the brighter the object will become. This can be
demonstrated by casting a shadow upon a portion of an object and noting the difference in brightness of
the two areas. The portion without a shadow will appear brighter.
S-8. The Visible Light Spectrum.
Light is one of many forms of radiant energy. Any energy that travels by wave motion is considered
radiant energy. Classified in this group along with light are X-rays and radio waves.
a. As shown in Figure S-6, light which is useful to the eye occupies only a small portion of the
radiant energy spectrum. Sound is located at the lower end of the spectrum; cosmic rays are at the
upper end. Light falls just beyond the middle of the spectrum. Along the top of the spectrum illustrated in
Figure S-5 is the frequency scale, and along the bottom is the angstrom unit scale.
b. Wavelengths in the region of light may be designated in micrometers (1 micrometer = 10 cm).
These units are also shown along the bottom of the spectrum in the illustration. Visible light is made up
of that portion of the spectrum between 380 and 780 nanometers, Figure S-7. (Formerly the term micron
was used for micrometer and millimicron for nanometer). The usable portion of the light spectrum for
television has been designated as falling between 400 and 700 nanometers as shown in Figure S-8.