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SOME PHASES OF THE OPTICAL SYSTEM OF THE
PROJECTOR
By F. H, Richardson
For the past twelve years we have been engaged in the study of the practical optics as applied to the motion picture projector. Through observation, we discovered that the projection lens of our projector did not pick up the entire light beam, whereas others that had lenses of equal diameter did pick up the entire light beam.
Some projectionists and theorists claimed that the beam beyond the aperture plate converged, while others, like myself, claimed that the beam diverged. We were both right; the difference being that the principal image of the arc crater was in the first case somewhere in or near the center of the projection lens, whereas, in all of my experience, the principal focus was at the aperture plate.
In order to obtain some data on this question, we employed John Griffith, Ansonia, Conn., to assist and to him the industry owes much for the very excellent work he has done in giving to it the first really workable data existent for enabling the projectionist to handle his projector optical train intelligently.
Briefly stated, the experiments which we undertook were chiefly to demonstrate that the beam from the condenser diverged between the aperture plate and projection lens when the arc crater was imaged at the aperture and this divergence was in proportion to the diameter of the condenser and inversely proportional to its distance from the aperture.
In Fig. I this theory is set forth diagrammatically. The whole condenser ray is indicated by the upper and lower converging dotted lines. Line B, drawn from the upper edge of the condenser to the lower edge of the aperture opening, and line A drawn from the lower edge of the condenser to the upper edge of the aperture opening, if continued beyond the aperture, marks the confines and shows the shape of the ray between the aperture and projection lens. Granting Fig. i to correctly illustrate what actually does take place, it is then readily understood that, condenser diameter remaining constant, the further the condenser is located away from the aperture the less will be the divergence of the beam between the aperture and the projection lens; conversely, the closer the lens to the aperture the greater will be the divergence, the exact effect of increased condenser distance being illustrated in Fig. 2.
In Fig. 2 the upper drawing illustrates the conventional method of showing the optical train of the projector. Insofar as it applies to arc light projection in practice, it is entirely wrong. The center drawing shows the effect were the condenser located g% inches from the aperture, with point of greatest concentration at the aperture. At 2 inches B. F. the projection lens would have to be 1 . 75" in diameter to accommodate the entire ray. At 5. 5'' B. F. it would have to be 3 . 75" in diameter. The lower drawing shows the effect with the condenser at 15'' from the aperture, while the
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