Motion Picture News (Nov - Dec 1926)

1978 M o t i o ii Picture N e w s X> Projection itics.Ekct'icity^Prdctical Ideas <J advief Inquiries and Comments Reflector Arc Systems [ME and again Ave have been called upon to express our opinion concerning the merits of various types of reflecting arc systems and have consistently refused to meet the rei on the score that it was hardly our place to serve in the role of judge — at least, as far as competitive equipment is concerned. It is well within the power of every theatre manager to determine for himself the truth of the claims put forth by manufacturers of such equipment by simply forcing two or three competitors to give a practical demonstration of their equipment in the theatre at the same time. No small number of demonstrations have, in truth, already been made and the respective claims have either been upheld or knocked into a cocked hat, thereby leaving no doubt in the minds of the observers as to whether or not one equipment was better (as far as concerned results) than another. For Those Who Seek Advice There is always a certain number of individuals, however, who are unable to form judgments of their own and must needs iel\ upon the word of some outside party. Principally for the enlightenment of this ip, we are giving a re-hash of the entire reflector arc situation in the hope that it may serve to clear up some of the dovibt. Indeed, we believe there is no need for our being called into such arguments in view of the fact that a complete analysis of the various reflector are systems was given in the form of a paper by Lander Stark before the Society of Motion Picture Engineers at Roscoe, N.Y., October, L925. The results of this investigation are now available to all in the Transactions of the Society, and anyone wishing detailed information concerning the several systems is advised to study this paper. We will, however, touch lightly upon the they are given in the paper, in or der to imparl some information to those individuals having an immediate need for them. A reading of Mr. Stark's paper leaves us with the impression that the elimination pherical aberration is the determining factor in the -election of a system Eoi mirror arc projection and he pie cut-, his data 0 lead up to the choice of a in which has been entirely corrected pherical aberration Ml j terns are, therefore, compared on the basis of -plnrical alienation and I It-, expressed in absolute values of screen illumiiven. The Spherical Mirror He starts off by showing the spherical under-correction of a commonly used type of spherical reflector having two parallel spherical curves for zones up to 60 degrees of the axis. By spherical under-correction, we mean that the rays of light from the edge of the reflector cross the optical axis nearer to the reflector than do the rays of light from the zones of the reflector nearer the axis. A mirror 8% in. in diameter was used to allow for the representation of a zone 60 degrees from the axis. The working distance, or distance of the arc to the reflector is 4 in., and the distance from the reflector to the aperture is 24 in., there being an axial magnification (of the crater) of 6 to 1. At an angular aperture (of the mirror) of 60 degrees from the axis, there is an amount of spherical under-correction equal to about 250 millimeters, or 10 in. That is, the rays from the edge of the mirror cross the axis at about 10 in. in front of the aperture. This accounts for the extremely hazy and diffused spot on the cooling plate when spherical reflectors of such large angular aperture are used. The spherical under-correction in this reflector is so great that the marginal portions of the mirror are in reality of no value whatever— even when used with the highest speed projection lens — a 5 in. Series II, working at a speed of f/2.0. If a screen be placed a few inches ahead of the projection lens, and the image of the reflector, as formed by the projection lens, is observed, it will be seen that a portion of the reflector equivalent to about 6% in. diameter is illuminated and being used when the source employed is very small — a five-ampere arc, for instance. This means that a spherical reflector of 6% in. diameter, having a working distance of 4 in., and an axial magnification of 6 in., will deliver to the screen nearly as much light as will the same reflector 8 in. in diameter when small light sources are used. As a matter of fact, this same reflector is supplied to the motion picture trade in a diameter of 6% in. In actual practice the 8 in. diameter mirror will deliver somewhat more light to screen than the 6% in. type for the reason that the marginal zones of the reflector will also usefully image portions of the crater off the axis when sources as large as, say, a 25-ampere arc are used, whereas we have been considering on the imaging of a point on the axi-. Parabolic Reflector and Condenser In the case of a parabolic minor in eombination with a condenser, each having a diameter of 8 in., the working distance (source-mirror distance) being about 3a/2 in., the focal length of the unit is such that the axial magnification is 6 to 1. The spherical under-correction in this case is a little more than 3 in. for the 60 degree zone. This error is due only to the condenser, as the parabolic mirror is free from spherical aberration for an object in its focal point. It is possible, of course, to correct this unit further by employing a parabolic condenser in place of the planoconvex type. The general performance of the system would not, however, be improved sufficiently to warrant the introduction of an aspheric condenser. With this unit, the spot on the cooling plate is fairly sharp, not diffused as in the ease of the spherical reflector. The screen illumination is much more uniform and much greater (in intensity) than with the spherical reflector. If a screen be placed a few inches in front of the projection lens, it will be found that the whole of the reflector is brightly illuminated and functioning usefully, so that in spite of the 3 in. of spherical under-correction, light from the edge portions of the reflector still passes through the film aperture into the projection lens, utilizing an angular aperture of 60 degrees from the axis. Mangin Mirror and Condenser A Mangin mirror is a special form of spherical mirror in which the spherical aberration has been reduced to a minimum by the proper choice of two spherical curves. It is really a combination of spherical reflector and concave lens which neutralizes the spherical aberration of the reflector. It was designed as a substitute for the parabolic mirror. The diameter of both the Mangin mirror and condenser was 8 in., the working distance of the reflector being about 3% in. The focal length of the unit was such that the axial magnification was 6 to 1. "With this combination there was a maximum over-correction of something more than one inch in the 60 degree zone, and a maximum of under-correction of nearly two inches in the 40 degree zone. This system, which is partially over-corrected and partially under-corrected, is peculiar to the use of this type of mirror with a condenser. Tn brief, it represents a better condition for correction than the combination of the parabolic mirror with the condenser. The spot on the cooling plate was just a little sharper and the screen illumination jusl a little more uniform than it is with the parabolic reflector and condenser. The intensity of the screen illumination, although higher than thai obtainable with an ordinary spherical reflector, will be less than that of the parabolic mirror with condenser due to the loss of light by absorption in the double passage through such a thick reflector. In order to prevent heavy breakage it is necessary to make this mirror of best resisting glass. (To be con eluded)