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822 HANDBOOK OF PROJECTION FOR
small angle falls upon the walls of the lamphouse and is wasted.
On the other hand the reflector arc lamps employ a mirror having a curved surface. This operates to reduce the loss through reflection, as compared to the loss before mentioned due to the heavy angle at which the incident light meets the flat surface of the outer zones of the collector lens of the piano convex condenser. However, since the light is reflected back, there is an additional loss of about three per cent due to reflection, and another loss of about 4 per cent as it passes back through the outer surface of the mirror.
The total loss due to these various things, however, certainly will not equal the loss inherent in the collector lens alone, where either a piano convex condenser or Cinephor is used.
In figure 310A we have the diagrammatic representation oi the 7.5 inch diameter parabolic mirror used in one type of reflector arc lamp which employs a condenser. No attempt has been made to draw a parabolic curve. Our only intent in this, and the other drawings of mirrors, is to show the arc distance and angle of light picked up by the mirror. We do not assert that the figures given as to angles of light are precise, but they are nevertheless correct within a very small margin either way.
In figure 310A you will observe that the arc distance from the vertex (center) of the mirror is 3.3 inches, and the angle of light picked up is 120 degrees.
In figures 311 A and 312A we see the diagrammatic representation of two of the mirrors used by non-condenser types of reflector arc lamp. These mirrors are spherical, hence have spherical aberration, as has been noted. The line-up for the eight inch mirror is : Arc distance from vertex of mirror 4.5 inches. Distance vertex of mirror to aperture 22 inches. This mirror picks up an angle of light equal to 100' degrees. The smaller diameter mirror has an arc distance of only 3.75 inches, hence, although of much less diameter, it picks up an angle of light equal to 95 degrees, or only five degrees less than the eight inch mirror. We mention this as showing the relation to arc distance and mirror diameter.
It must, however, be remembered that either for a parallel or a converging mirror beam, the amount of spherical aberration is greater with short than with long focal length mirrors, except for mirrors of not more than six inches diameter. This is true because of the fact that up to six inches of the diameter all mirrors used for projection work are essentially spherical.