Richardson's handbook of projection (1927)

176 HANDBOOK OF PROJECTION FOR With the foregoing in mind it readily will be seen that the necessary enlargement of the crater image at the spot will depend on the number of amperes we are using, since the diameter of the crater increases in proportion to the number of amperes used. (See Page 394.) The distance from crater to apex of curved surface of collector lens, and from the apex of curved surface of converging lens to film, are known respectively as "distance X" and "distance Y." The points from which they start are shown in Fig. 42. The average projectionist who reduces his spot diameter by pulling his lamp back a greater distance from the face of the collector lens is more than likely to discredit the foregoing, because, while he has decreased his spot diameter, he has either not increased the screen illumination at all, or to no appreciable extent. The fact is, he has not acted intelligently. He should have installed a shorter focal length converging lens in order to maintain his arc-to-lens distance at minimum, at the same time setting up a condition which would cause a heavier convergence of the beam on the other side of the lens. Of course we realize that intelligent work in this direction is hampered by difficulty in securing lenses of necessary focal lengths, but much may be accomplished, nevertheless, and the whole matter forms an interesting and profitable field for experiment and study on the part of projectionists and engineers. The problem is to keep distance X the minimum possible, without setting up excessive lens breakage, at the same time maintaining a maximum distance and standard diameter spot. In this connection remember that: The spot diameter will always be as many times the diameter of the crater as distance X is contained into distance Y. H£N/SCl/S Q/CQMVCK Figure 42.