Moving Picture World (Jan-Jun 1910)

THE MOVING PICTURE WORLD 459 THE OPTICS OF THE MOVING PICTURE By Thomas Bedding. CHAPTER II. The position of the light point in relation to the con Functions of the Condenser. dens^s is ^ty f°u"d by ar s!mPle experiment. It is, roughlv, at the back focus of the condensers. It comes Let us endeavor to tell the reader in as simple language *? *isi t,he P°in} of liSh} should be approximately in as possible what are the precise functions of the condens the back focus of *;he condensers in order that a perfect ers in a moving picture projection machine. To begin ™ne of ra>'s "^ be, transmitted through the picture and with, last week we saw that light is of necessity a point, ™en converged on the projection lens If light and conthat is to say, the theoretical condition to strive for in denser ar,e to,° "ear each other, dark shadows will be the projection is an electric arc, infinitely small but in S<f" °n tbe sht*\ If th^ arerto° far back the margin finitely luminous. That is obvious. If we were working ?• , plct,ure wlU, have colored fringes. _ The correct posiwith a large sheet of flame, as in the old days when -10" 1 condensers isat a pon* in the axis of the oil lamps were used for projection purposes, then most llght whe^ the ra-vs lllu™nate the picture aperture and of the light would of necessity be lost; moreover it is converSe the rays into the projection lens That point not easily controlled bv the lens end of matters. [s approximately, as we _ have said, at a distance equal Even though our light be the theoretical point it is equai to ihe back f°cus °" a Pair of condensers, ly susceptible of loss, that is to say, it has to be brought T{he foa of single Plano convex condensers are usually under control in order to be effective in forming, or as ollows helping to form, the picture on the screen. Suppose we Diameter. y Focus. followed the path of the rays as they leave the luminous 4 inches 5V2 or 6V2 inches point. Most of them go straight on to the picture and 43^ inches kj/2 or 6J/2 inches pass through it. But others diverge, pass outside the ^y2 inches <y2 or 6V2 inches picture, and are lost or scattered, taking no part in form 5 inches 6 inches ing or projecting the picture on the screen. Then in ^/2 inches 8 inches order to prevent the escape of these rays we use lenses 6 inches 10 inches to condense (or compress) them ; hence the name "con &/, inches 10 inches denser." These lenses or condensers gather up the rays y inches 12 inches as they lea^e the point of light in the crater of the elec 8 inches 12 inches trie arc, pass those rays through the picture and then q inches 14 inches carry them on to the projection lens, which in its turn 10 inches 15 inches casts the enlarged picture on the sheet. 12 inches 18 inches The condensers in general use are usually two in 14 inches 21 inches number, each consisting of a piano convex glass, placed in a round metal cell so that the flat sides of the glasses When two single condensers are combined, the focus are turned outward. The condensers should be of col of the combination is half that of the single elements: orless glass, free from surface markings or internal flaws, thus the focus of a pair of 6-inch piano convex conair bubbles, striae or other defects. These defects, of densers would be 3 inches, measured from the center course, interfere with the passage of the light rays of the combination. A sheet of around glass between through them, and so there is a loss of light or the image the condensers or between the latter and the light aids is degraded. the better diffusion of the light. What decides the size of these condensers? Roughly it is this. Their diameter should exceed the diagonal Mounting Condensers. of the picture to be projected The diagonal of a single pairs of condensers, mounted or unmounted, can be moving picture is roughly 1% inches, so that the diam b ht at the followi ices . eter of the condensers should exceed that measurement in order that the whole of the image is included within Pair Per the circumference of the cone of light which the con of Lenses Single Lens densers have to form. Diameter in inches. Focus in inches, mounted, unmounted. What would happen, supposing condensers were not 4 5^01-6^ $4.00 $1.25 used? Merely that the picture on the screen would only 41^ ri/2 or 6 1/2 6.00 1.50 show that part of it which was actually in front of the c 51^ 7 r0 : 7c light. The intensity of this light is governed by the 3 8 12.00 3.00 amount of it which the condensers collect and converge 8 12 32.00 7.50 into the projecting lens. Substitute for the condensers o 14 40.00 10.00 in a projection machine a sheet of ground glass, the picture would be fairly well illuminated but the glass could The condensers should be set loosely in the cell with not collect sufficient light from the luminous point, so the convex or round sides facing each other, but not that practically none of it would be converged into the touching. Very often a sheet of finely ground glass if projection lens. placed between the lenses will overcome any slight flaws The condensers in a projection system serve, it will be which might be in them. This also tends to equalize the seen, a double function. They collect the light rays by illumination. Of course, in theory, the condensers should preventing them, from scattering and then they condense be of "optical glass" in order that the utmost purity of focalize them on to the projection lens. Indeed, con should be obtained, but then the cost is so prohibitive that densers are actually lenses of definite foci and practically it is not to be thought of. Still the purer, clearer and form the images. We do not say that you could take more brilliant are the condensers the more efficiently will photographs with them, because they are not corrected they perform the double office which we have described, for the purpose, but they have a capacity of bringing namely, that of collecting the light rays and then of light rays to a sharp focus. condensing or focusing them upon the picture aperture.