| TV cameras used to use iconoscope and image orthicon tubes
to photograph people and events. In these tubes the video signal is created
by an electron beam bombarding a photosensitive plate. The plate emits
secondary electrons in varying numbers as the beam hits successive points
on its photosensitive surface.
This principle was also used in another television camera tube of simple construction. This tube was known as a monoscope (also called a phasmajector or a monotron), and was built into a simple camera which produced a fixed image used for testing equipment in the television station. |
The patterns obtained from a monoscope are shown on this web site. The patterns were printed on an aluminum plate with ordinary printer's ink. The carbon in the ink and the aluminum have different secondary emission
characteristics. The aluminum gives off about twice as many secondary electrons
as the carbon when hit by an electron beam.
The printed plate is mounted in the tube where an electron beam scans the printed half-tone pattern, causing greater secondary emissions from the aluminum surfaces (the white portions of the picture) than the black printed portions. The varying secondary electrons are collected |
on an electrode which consisted of a conductive coating
on the wall of the monoscope tube. The video signal developed across a
load resistor connected between the coating and the ground.
The chief advantage of a monoscope camera was that it offered a standard
reproducible image for comparing the performance of equipment over a period
of time. No source of lighting was required, so there were no variations
in illumination and an image of constant contrast was obtained.
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