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LENSES AND MIRRORS
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( 141 ) From all this it is evident the mirror is a far more efficient collector <A light than the 4.25 inch Mac diameter 2-lens plano-convex condenser. This is by no means its only advantage. Most of the reflector type
lamps n.se no condenser at all, the mirror reflecting the collected light directly hack and converging it into the "spot" at the cooling plate-aperture. It eliminates the very heavy light loss due to reflection from the four glass surfaces encountered in 2-lens condensers, in addition to the loss sustained in the absorption of light by the glass itself.
The loss on account of reflection will remain high if the lens .surface is not kept perfectly clean.
As against this high efficiency in light collection and delivery at the projector aperture, the lens in the reflector type lamp, using a thin condenser, wastes (mostly by reflection) about J 2 percent of the rays, and the mirror itself wastes ( again largely by reflection at the glass surface) about 8 percent; the remainder is sent forward to the cooling plate and aperture.
(142) The mirror offers less reflection and loses less light through absorption but it creates a very hot spot at the aperture, there being no condenser to absorb any portion of the heat rays. Whenever a condenser is used, thin and of wide diameter, the aperture heat is somewhat reduced because the lens absorbs and radiates about IS percent of it.
(143) When the rotating shutter was located on the screen side of the projection lens it in no wise' reduced the spot heat produced by reflector type equipment. But
Fig. 46. — Showing mirror
diameter 7.5 inches parabolic
curve.