Conventional Projection systems project a
beam
of light onto a slide. The light is then focused by a convex
lens
to form an
image
of the
slide
on a
projection
surface. The
image
thus created is an upside-down mirror
image
of the slide.
The
optical system
of
projection
systems includes a tungesten-halogen or discharge lamp, a
reflector
and a number of lenses, each of which has a different function. Heat shields and fans protect the slide, as well the condenser lenses and the lamp, from overheating.
The light of the
lamp
is collected by a
reflector
and converged by an array of condenser lenses until it is focused on the slide. As the
beam
projected on the
slide
must be no smaller than the
slide
itself, the diameter of the condenser lenses must be at least the same size as the slide.
The
beam
of light then passes through an objective
lens
which focuses the
image
of the
slide
onto the
projection
surface. Some
projection
systems use variable-focus lenses which enable control of the size of the image, while other systems enable easy replacement of the
lens
to facilitate different
projection
sizes.
The focal length of the condenser system is designed so as to ensure that the slide
image
falling on the objective
lens
fills it completely. If the slide
image
falling on the objective
lens
is too big, some light will be lost, causing reduced brightness of the projected image. If the
image
falling on the objective
lens
is too small, the converging power of the
lens
will not be fully utilized.
The focal length of the objective
lens
system will determine the angle of convergence of the
beam
of light and consequently the size of the projection. The projected
image
will be inverted in both directions (up/down and left/right). Slides are inserted into projectors upside down and flipped left to right so that the projected
image
will be correct.
Ideally, a
projector
should be placed perpendicular to the
projection
screen, directly in front of it. At this position there will be no noticeable distortion of the projected
image
. This, however, is not usually practical in theatre as the
projector
would have to be in the middle of the auditorium. Any deviation from the optimal position of the
projector
will cause distortions . Such distortions must be calculated and the
slide
corrected to take the distortion into account. Rear
projection
can be used where there is room behind the
projection
screen, which allows placing the
projector
centrally thus avoiding distortion . The use of mirrors can help to overcome situations where the
projection
throw is insufficient .
The
projector
is placed in front of the screen and centered, so that the center of the screen is on the optical path of the projector.
When a
stage
is not deep enough for rear projection, the
projector
must be hung on a pipe or placed in the wings, and the resulting distortion must be corrected on the slide.
Special
care must be taken when planning the lighting and the mise-en-scene, to avoid shadows of the actors being projected on the screen .
When the
projector
is placed above the screen, the
image
will be disorted to a trapeze shape (keystone).
In rear projection, the
projector
can be placed in its optimal position withput being visible.
When projected from a side position, the
image
is distorted (keystoned) and shadows of the actors are evident.
