Figure 14-2.The scanning process.
around the banner is gray. When light hits the banner, it
reflects from the three different shades in different
amounts. The white background reflects the most light,
the gray reflects less, and the word NAVY reflects very
little light. From this you can see that a scene made up
of different shades or colors reflects different amounts
of light. The television camera takes these various levels
of light reflection and changes them into electrical
impulses of varying strength.
A television camera is optically similar to a movie
camera, except it does not use film. Instead, light
reflections from the scene are focused by a lens and pass
through the face of the photoelectric transducer (also
called a pickup tube) of the camera. The pickup tube
does the job of film in a camera. Its surface is coated
with thousands of tiny globules of silver mixed with
other chemical elements. This coating is photosensitive,
which means it gives off electrons when exposed to
light. Light from the scene covers the entire surface of
the pickup tube and electrons are forced off its rear
surface. The number of electrons forced off any part of
the pickup tube is determined by the amount of light that
strikes this part.
In figure 14-1, the Navy banner causes the pickup
tube to give off electrons corresponding to the amount
of light reflected from various parts of the banner.
At the present time, there is no practical method for
transmitting a complete video picture instantaneously as
a whole unit. Therefore, in television, the picture is
broken into tiny units called elements, which are
transmitted individually in sequence. The elements are
so small that the human eye cannot distinguish one from
the other in the complete picture. The process of
registering all the elements of a video picture in
sequence is called scanning. During the scanning
process, the television camera encodes the elements;
then the television receiver is used to decode them in
the proper order to recreate the original image (fig.
In this section, we examine the following electronic
characteristics of a television camera:
l Operating light level
l Video noise
Operating Light Level
You need a certain amount of light in order for the
pickup tube of the camera to perform its function.
Although there are several ways to measure light, the
footcandle is one of the more common units of
Whatever term is used, make sure your light-
measuring device is in the same language as the manual
for your camera. For example, if the manual calls for a
minimum of 100 footcandles of light, you will need a
light meter that reads in footcandles.
Some cameras have a way to give you more light
when you need it. The dB gain switch (fig. 14-3), usually
located on the back of the camera, has two positions
6dB and 12dB. For every 6dB of gain, the camera output