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REFLECTOR PROJECTION LENS
CONDENSER LENS
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A
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LAMP SLIDE OR FILM
Fig. 2
DIASCOPIC PROJECTION Diascopic simply means "direct" projection, such as in an optical lantern. The light is projected in a straight line from the lamp
to the screen.
The Lamp and Reflector
Projection light may be provided either by the carbon arc or by means of a special filament lamp. The former is chiefly used in 35-mm. cinemas and the latter in 16-mm. and other narrow-gauge projectors. The carbon arc is considered to be the best light for projection, in that its light is of high intensity and fairly concentrated. It is not, however, a very safe or practical form of lighting for amateur use. Manufacturers have therefore devoted considerable research to the betterment of the filament lamp, and the ones in use to-day compare very favourably with the carbon arc.
A number of modern 16-mm. machines are now fitted with a very efficient 1,000-watt lamp, and some have been used in standard cinemas without the patrons being aware of the fact.
These lamps are mostly prefocussed, being automatically aligned and centred for projection when inserted in the projector.
Many projectors have an open top to the lamphouse and where the lamp is inserted in an upward direction it may be advisable to use a lamp fitted with a metal cap or blackened with paint at the end, as this will prevent the upward flow of light from the lamphouse, and which in a small hall may considerably spoil your picture. A possible drawback, however, is that the method sometimes tends to increase the heat within the lamp and thus helps to shorten its life. On the other hand, extra heat means extra light, but without the use of extra current.
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