Moving Picture World (Jul 1916)

July 22, 1916 THE MOVING PICTURE WORLD Ml Uneven Illumination and Condenser Diameters. We are gradually accumulating a whole lot of rather disconnected dope on the optical system. Recently I submitted a problem to Brother Griffiths of Ansonia. It was a very difficult case, and he replied that it might be necessary to stop down the condenser dismater 3#cm Focus \ ft1 3 I* Si in order to avoid unevenness of illumination of the picture, since, under the condition, the light ray was bound to be of larger diameter than the objective lens at the point of back focus. I could not see where that applied, and told Brother John so, whereupon he comes forward with the following: Replying to your question as to why stopping down the condenser would make for evenness of illumination, with reference to the case in question, the rectangle of light reaching the objective would be much larger than the diameter of the objective. Now, this rectangle of light is not made up of a single, solid rectangle of light, but of an infinite number of smaller ones wuich are more or less incorrectly superimposed upon each other. The size of each individual rectangle may be found by drawing four lines from any point in the equivalent plane (i. e., the center of the combination of the condensers), and having them pass through each extreme corner of the machine aperture, and on the point of back focus of the objective. This will give the four corners of each individual rectangle, because from each point of the condensers there is a cone of rays which carry a full film image. Now, if we first consider a cone from the center of the condenser on the axis, we would get an evenly illuminationed image on the screen, though it would be very faint if we consider but one point of the condenser. The illumination would, however, be even, because the rectangle of light would only be a little larger than the aperture of the lens, and would not be obstructed in its passage through the objective lens. But if we take the cone of rays which is passing through a point of the condenser near its bottom edge, it will still pass through the aperture, but the rectangle of light, which carries a full film image, will reach the objective lens. We now have, at the screen, not only a faint, even image, but an additional part of an image, and this second part will appear brighter because it is receiving twice as many rays, being fed through two points instead of one. Let us take, for instance, one of the small individual rectangles that, coming from the edge of the condenser, would occupy one of the corners of the large rectangle, but one corner of the film image would reach the screen, though in the other three corners of the large rectangle there would be a similar small rectangle, each having a corner passing through the objective leus, but each corner passing through would be a different corner of the image, so that instead of getting four full film images, we would be getting four corners, consequently the corners of the screen would be better illuminated than its center, and we would have more points of the condensers feeding the corners than we have points feeding the center, and the result of the total will be a dark ghost in the center. If, on the other hand, the small individual rectangles from the extreme edges of the condenser are being obstructed, so that a little more than half of the film images are passing through the objective, then the center of the screen will be fully illuminated, and the corners less fully, which will result in a light ghost. It must be carefully remembered that there are as many individual small rectangles as there are points on the condenser, and that is literally numberless whereas we can only consider the effect of a few, and the only way we can be sure of even illumination is to reduce the size of the large rectangle, or light ray, so all of the small individual rectangles will reach the screen, whereupon its illumination will be as even as the illumination at the aperture is. From this viewpoint, stopping down the condenser makes for evenness of illumination, where the objective lens is of small diameter, because it reduces the size of the rectangle of light covering the objective lens. Referring to the sketch, in which the conditions you asked about are duplicated, you will probably see that if the whole rectangle of light is allowed to reach the objective, then the outside edges of the condensers will deliver a large number of film images to the objective, of which only one corner can reach the screen, and while they are giving but little illumination to the screen, what little they are giving is going to the corners. I have only shown three of these small rectangles just to set forth my idea. A, B and C are three vertical points of the condenser. To show them other than vertical would require solid geometry, and make a complicated drawing. You will see from this idea that it is possible to have both light and dark ghosts, even though your aperture does not cut into the ghost zone, and I think this is the kind of ghost which □ gets the experienced operator's goat, though they are not so common now as they used to be. (I mean the ghosts, not the goats.) If you will turn to page 597, Gage's "Optic Projection," you will find this idea corroborated where he says : "Increasing the diameter of the objective, or diaphragm, has the same effect as increasing the diameter of condenser." Note what he says about opening and closing the diaphragm of objective, and apply the same meaning to the diaphragm of the condenser. He tells the effect, and I think I have found the cause. It begins to look to me like we need a new form of condenser worse than we need anything else. I don't like to reduce the diameter of the condenser, and if we are going to avoid that, and keep the crater within 3y2 inches of the lens, where it ought to be, I believe we have got to get the condenser further back, and get a more nearly parallel light beam beyond the aperture. The other possibility for accomplishing the paralleling of the light beam is shown on P'age 1108, May 13th issue of the World. Still another plan is shown in the accompanying diagram of a telephoto condenser, of which Brother Ralph W. Martin, Los Angeles, writes : Herewith a triple condenser system is shown, with a diverging lens placed next the arc. If we now regard the action of the diverging lens alone, it is found that the real crater becomes focused as a virtual image at a position nearer to the lens, as shown in the drawing by the dashed lines, and also, the virtual image is smaller than the real crater. There TELEPHOTO CONDENSER fore, it is seen that the function of the diverging lens is, optically, to reduce the size of the crater ; also to move it closer to the condensers. The purpose of this action is quite obvious from the drawing, for if we now consider the diverging len§ as for the moment removed, then the real crater becomes focused by means of the meniscus-bi-convex set, with a 1%-inch spot at a position nearer to the condensers (as shown by the dotted lines) ; but by replacing the diverging lens, the real crater is replaced by the virtual crater, which now becomes focused at a position much farther from the condensers (as shown by the full lines), while also the reduced size of the crater enables the aperture spot to be maintained at its desired minimum size of iy2-inch diameter. The advantage of increasing the distance between condensers and aperture is already well known, but we again call attention to the fact that in the diagram the position of the arc and the size of the aperture spot are maintained at maximum efficiency, and also the condenser system has six curved surfaces which can be advantageously selected for reducing spherical aberration. This system is really a telephotographic lens, but placed and used in a reverse manner. Why not place a negative lens between the condenser and aperture, and thus get the effect of added distance between condenser and aperture, without its actuality? This suggestion is by Brother Griffith.