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Table 2— Effect of Mirror on Screen
[llumination
LAMP
CONDENSER MIRROR
LUMENS
Biplane No.
2I-IO
Corning — 2^" x 8" None
467
'
2I-IO 2I-IO 2I-IO
" ~2}4" X 8" M. E. 8" Piano-Convex sH & y}^ None
" 5K & 7K M. E. 8"
584 Gain by mirror 24.6%
544
712 Gain by mirror 30.8%
c
2I-II
Corning — 2}^" x 8" None
556
i
2I-II 2I-II
" —2}4" X 8" M. E. 8" Piano-Convex 6}4 & 7K None
681 Gain by mirror 22.5% 575
'
2I-II
" 6y2 & 7K M. E. 8"
686 Gain by mirror 19.3%
Average Gain by mirror 24.5%
The mirror in each case was a Macbeth-Evans spherical mirror, having an 8'' diameter and 5 5-8'' radius of the reflecting surface. The gain due to the use of the mirror in the tests ranges from 30.8% to 19.3% and gives an average gain of 24.5%. All of these tests were made with biplane filament lamps. Two different lamps were used and two different combinations of plano-convex condensers. A further test was made using biplane lamp No. 21-10 in order to determine how much the current supplied to the lamp could be reduced in order to give the same illumination as was originally obtained with this lamp without the mirror. This test showed that changing the current from 28 amperes on a lamp used without a mirror and giving a screen illumination of 544 lumens to 27 amperes on the same lamp with a mirror, gave a screen illumination of 564 lumens. This change in current in the lamp is equivalent to a change of about .06 in spherical watts per candle. However, there is a gain due to the use of the mirror which is not accounted for by the change in current and the resulting change in temperature of the lamp filament. This gain is due to the light from the reflected image of the filament which tends to give a more uniform light source so that the illumination on the screen is more uniform and of higher intensity than can be obtained from the filament alone.
CONDENSER SYSTEMS AND OBJECTIVE LENSES
A number of different condenser systems have been tried out in the development of apparatus for the use of incandescent lamps in motion picture projection. The two which have given the best results are the corrugated type of condenser and the plano-convex condenser systems. Each of these condenser systems possess some advantages and also some disadvantages in comparison with the other, and it is a matter of choice as to what points should be sacrificed in making a decision as to the type of condenser. One advantage of using the plano-convex condenser system is that the working space between the lamp house and the head of the motion picture machine is great enough to permit of easy manipulation of the framing lever or to make any adjustment to the film which may be necessary. There is also another advantage in that the angle at which the light rays converge is more acute and therefore there is less loss in picking up the cone of light beyond the aperture plate and collecting this cone in the objective lens. This is especially true where a fairly long focus objective
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