brightness; room light in excess of this
(a) The brightness of the Schmidt system is
quantity will reduce the necessary 10 to 1
limited by the maximum brightness of the
contrast range) and utilize front screen
CRT, less the losses that take place within
projection techniques.
Rear screen
the light transmission path .
projection would further reduce the picture
(b) Manufacturers claim satisfactory pictures
light available, due to the transmission
varying in width of 6 feet to sizes
loss of light through the projection screen.
employed in motion picture theaters for
(h) In order to permit maximum available
large audience viewing.
Equipment
light to reach the projection screen, the
brochures general rate the light output
optical system must be kept surgically
from these projectors in foot candles per
clean. Television projectors require high
square foot, based on a screen from 6 to
voltage supplies that attract dust particles
8 feet wide.
from the air. Since optical paths are in
(c) The high-light brightness reproduced on
the immediate location of these supplies,
the screen will be in the magnitude of 5-
constant cleaning becomes a necessity.
foot candles per square foot.. This high-
Numerous
innovations
have
been
light.
brightness will represent the
fabricated to eliminate this problem, but
brightest area within a projected picture
current solutions still leave much to be
therefore, other shades of gray will be
desired.
represented by lesser amounts of light.
(i) Projection screens are available with
The range for contrast reproduction from
effective gain factors; these normally are
black to white, will be in the order of 10 to
rated on front projection techniques. The
1.
screen surface collects and reflects
(d) In order to obtain maximum light output
focused light toward the audience.
from these projectors the electron beam
(2) Light amplifier projectors.. The lighter cost units
current within the CRT normally is
which utilize a light amplifier technique,
increased to its upper operating limit.
sometimes referred to as controlled layer
Brightness (light output) is dependent
projectors, or light valves, have application
solely upon the quantity of electron beam
where the requirement is of such magnitude that
current and the residing time of the
properly trained personnel can be provided to
electron beam on a phosphor area.
perform operations and the necessary routine
(e) Operational history of these units includes
maintenance.
such occurrences as CRT imploding. This
(a) The projected light obtainable from a light
has occurred when the vertical and
valve is derived from a xenon arc lamp.
horizontal protection circuits in the
This light is controlled by passing it
deflection
system
have
failed
through a layer of oil modulated by
simultaneously. The high beam current
electrons from an electron gun similar to
required to develop maximum CRT light
the one used in a conventional cathode
output can penetrate the glass face plate
ray tube. The incoming video signal is
of the CRT. A common cause for CRT
applied to the electron gun, hence the
replacement is phosphor burning as the
modulation is impressed by electrostatic
result of particle deflection failures.
action upon the surface of the film of oil.
(f) The brightness or light intensity of a
It is the disturbance of the oil surface that
projected television picture from a CRT
permits the xenon source light to be
source is relatively low in brightness when
reflected through a proper combination of
compared with other types of projection
lens, onto a projection screen. This is an
equipment that utilize incandescent light
oversimplification
of
the
operating
sources.
principle of controlled layer projections,
(g) Operational experience has shown it is
but will suffice for the purpose of this
necessary to reduce the normal ambient
Technical Bulletin.
room lighting to a negligible level (less
than 10 percent of projected high-light
TAGO 2A
4
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