Learning Event 2:
DESCRIBE WHAT MAKES UP THE GRAY SCALE OF A TELEVISION SYSTEM
1.
The object of a television system is to reproduce a picture or scenes
for viewing at some distant location.
Most pictures and scenes contain a
wide range of total values and colors, which are reproduced by a monochrome
television system as gradations of brightness from white through gray to
black.
These gradations of brightness constitute the gray scale of
television system and refer to the relationship between the brightness
variations in the original scene and those in the reproduced display.
2.
Faithful gray scale reproduction requires that the gray scale of the
original scene be reproduced in the same proportion, or ratio, in the
display. The actual light levels in the original scene do not need to be
reproduced, but the ratio of lightest light to darkest light should be
maintained in the displayed picture. This level is called contrast range.
3.
For instance, a typical television subject may be lighted so that its
highlight brightness is 1000-foot lamberts and its darkest shadows are 10-
foot lamberts. The contrast range in this case is 100 to 1.
4.
Video display equipment, however, is incapable of yielding light
levels as high as 1000-foot lamberts.
A video receiver is capable of as
much as 100-foot lamberts maximum and 1-foot lambert minimum. The contrast
range of the receiver would be the same as that of the original scene (100
to 1) and a faithful reproduction would be obtained.
a. Actually, a 100 to 1 contrast range is practically unobtainable,
being limited primarily by the transfer characteristic of the pickup tube,
which can provide a nominal contrast range of about 40 to 1.
b. However, excellent contrast can be obtained by matching the contrast
ranges of the scene and display, and the lighting of a particular scene to
the capabilities of the television system.
5.
When the contrast range for a televised scene and the reproduced
picture match, the transfer characteristic of such a system is said to be
linear or have a gamma of unity (1).
a. While it would seem that a linear transfer characteristic would
always be ideal, this is not necessarily the case; however, a television
system is generally designed to have an overall linear transfer
characteristic.
b. A nonlinear amplifier (gamma corrector) in the camera control unit
is
generally
used
to
provide
control
over
the
system
transfer
characteristic.
2
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