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Projection Lenses — Ray ton
105
jection system has been reduced to its lowest terms, the source of Kght, the condenser, the fihn gate and the objective. The sohd lines limit the pencil of light which, originating in the central point of the light source, is converged by the condensing lens into an image in the plane of the objective. The dotted lines intersecting in the center of the film gate at the point P' measure the angular aperture of the illuminating system only under the condition that the source be large enough to fill the dotted cone extending to the left from the condenser to the point P which is the image formed b}^ the condenser of the point P\ The illustration shows the source just large enough to
CONDENSEP
FILM qATE
OBJECTIVE
LiCiHT SOURCE
Fig. 3 — Schematic Motion Picture Projection System.
FRONT
rr
BACK
+■
FILM
Fig. 4 — Projection Objective with Short Back Focus.
meet this condition. If the light source were smaller than shown in this sketch it would be impossible for the condenser to appear filled with hght when viewed from the point P' . This follows from the fact that no ray of light leaving any point of the light source other than the extreme margin can pass through the margin of the condenser and also through P' . The figure also shows that the angular aperture of the objective is larger than that of the illuminating system since the dotted lines do not reach to the margin of the lens. No light, therefore, reaches the marginal zone and the excess diameter is useless in so far as the center of the field, at least, is concerned. For the same light source the angular aperture of the illuminating system can be increased or decreased b\^ altering any one of several dimensions of the system but consideration of these points would lead