The standard slide
projector
used in audio-visual presentations, lecture rooms, and similar application, is also widely used in theatre. Standard AV projectors are not very powerful and a special
projection
screen is required in order to achieve a bright projection. The lighting must also be carefully planned so as not to spill on the
projection
surface.
AV
slide
projectors are often used in theatre to project documentary material, as well as text, such as surtitles for opera. Several AV projectors can be combined to create a unified composite
image
or a multi-image
pattern
in the fashion of audio-visual presentations.
The AV projector, which uses a low-voltage tungsten-halogen
lamp
(typically 250w/24v), projects standard 24x36 mm celluloid slides in glass
slide
holders. The standard AV projector, the Kodak Carousel, has a circular
slide
tray containing 80 slides. Usually at least two projectors will be aimed to form an overlapping
pattern
on the
projection
screen. A dissolve unit can be used to
cross-fade
between the two projectors, so that one
image
dissolve into the next . The dissolve unit, which regulates the rate of the cross-fade, can be controlled by a dedicated manual control, by electronic signals recorded on a sound tape (in synch with sound - a popular control method of AV presentations), or by digital signals of a computer. A wide variety of
lens
systems, including
zoom
lenses, is available, allowing flexibility in choosing the position and
throw
of the projector.
An improvement on the standard
AV projector
is the semi professional slide
projector
where a powerful discharge
lamp
produces a much brighter projection. Slides are made of plate glass or another transparent material such as celluloid which carries the visual material to be projected. Most projectors use heat shields and forced air cooling systems to protect the
slide
from the excessive heat produced by the light source. Standard AV slides come in a size of 36 x 24 mm.
Preparing the slides can be a wearying process as the visual material must be photographed and the
slide
transparency developed before a
projection
can be tried. This can make experimenting with the visual material arduous in case where a
slide
must be distorted in order to compensate for an off-axis
projection
angle. Certain computer programs enable generation of the slides directly from the computer, a much faster and more flexible process. Scanning can be used when the visual material was not generated by computer.
