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212
RICHARDSON'S BLUEBOOK OF PROJECTION
(141) From all this it is evident the mirror is a far more efficient collector of light than the 4.25 inch free diameter 2-lens plano-convex condenser. This is by no means its only advantage. Most of the reflector type
lamps use no condenser at all. the mirror reflecting the collected light directly back 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.
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 12 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.
The loss on account of reflection will increase if the
lens surface is not kept perfectly clean.
(142) The mirror 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 a considerable portion 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. 92. — Showing mirror
diameter 7.5 inches parabolic
curve.