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834 HANDBOOK OF PROJECTION FOR
time converge it so as to form the "spot" at the aperture of the projector mechanism.
The other type uses a mirror which reflects the light back, in the form of a parallel beam, the full diameter of the reflector surface, to a single plano-convex condenser lens of the same effective diameter as the mirror. This lens converges the beam to form the spot at the projector mechanism aperture. The system is illustrated in Fig. 315B in which A is the Tight source, located approximately three inches from the surface of the mirror. The mirror "picks up" an angle of light of approximately 120 degrees, which, as has been said, is all the light which it is possible to utilize efficiently in such a system, and probably very close to ninety per cent of the total light given off by the light source. It will therefore be seen that the system employed is very efficient in light collection.
The condenser, as is seen, receives the parallel beam on its piano surface. The lens is quite thin and the curvature slight, hence both reflection and absorption is reduced to the minimum. If the lens be kept clean the combined loss of reflection and absorption should not exceed ten per cent.
The condenser forms an excellent, sharply defined spot, and the resultant screen illumination is very evenly distributed. In Fig. 315B, B is of course the mirror and C the condenser.
There is considerable argument between the manufacturers of the two types of lamp as regards the light losses due to size of spot, but since the parrabolic and elliptical mirror has displaced the spherical mirror in reflector of lamps, the performance of the two with relation to the smallness and sharpness of spot is essentially the same. However I shall give you the setup of one non-condenser reflector arc lamp, made by a large projector manufacturing company, and you may size up its possibilities for yourselves, always remembering that I have not shown the aberration.
Figure 315C is the non-condenser line-up. In this instance the mirror itself reflects and converges the light beam to the spot at the aperture. The distance mirror to aperture is long, which is necessary in this type of lamp. It is what might be called the "straight reflector arc lamp, and has shown some astonishing results in screen illumination.
POWERLITE REFLECTOR ARC LAMP. GENERAL DESCRIPTION. — This equipment consists essentially, exclusive of the motor and relay control, of three principal units, viz: a positive carbon holder, a negative carbon holder, and a