International projectionist (Jan-Dec 1945)

Projection Angle Rule By J. G. JACKSON MEMBER, L. U. 348, VANCOUVER, CANADA A GREAT deal has been written about projection angle and its calculation, and after giving the subject considerable thought the writer devised a simple and accurate rule that can be made by any projectionist. Specifications are given in Figure 1, and the drawings are self-explanatory. The degrees on the scale are calculated from an ordinary school protractor. To use the rule, point the stick towards the screen from a point as near the height of the lens as possible. Look through the -BACK. SISUT MfTHOD OF SIGHTING ■TO 5CR.FE-N FIGURE 2 peep-hole in the back sight and move the front sliding sight so that its edges are in line with the top and bottom of the screen. When this is done the center fine of the rule will point directly to the center of the screen; that is, it will follow the "axis of projection" as shown in Figure 2. The wire arrow will, by gravity, point directly to the value of angle A in Figure 3. By carefully turning the rule slightly to the left the arrow will rest against the scale, and the friction will cause it to remain in place so that a reading can be taken. Definition of Angle Projection angle, in the true sense of the word, is the angle between the optical or projection axis and a normal angle drawn at right angles to the center of the screen. In other words, it is the angle 10 FIGURE 3 of incidence of a ray following the projection axis. In Figure 3 we note a rectangle showing the projection axis as the diagonal terminating at the center of the screen; the normal axis to the screen is indicated by a heavy line. The projection angle is B, which by the laws of simple geometry is equal to angle A. Therefore, if we measure angle A we have the projection angle. This holds true provided the screen is not tilted, for if the screen is tilted we must calculate the angle of tilt and deduct it from the value of angle A. In Figure 4, the screen is titled with the angle of tilt indicated by angle X. Here the projection angle is angle B minus angle X, or since angle A equals angle B the projection angle is angle A minus angle X. To measure angle X, remove the front sliding sight from the rule and place the rule against the frame of the screen as FOLD ON DOTTED LINES FIGURE 1 shown in Figure 5. The arrow will indicate the value of the angle of tilt. For example, if the value of angle A was found to be 18° and angle X was 6°, then the projection angle would be 18 minus 6, or 12°. This rule may also be used to measure the angle of a street grade or the pitch of a roof by removing the front sliding sight and putting a nail through the end of the stick so that it (as well as the back sight) is in line with the center of the stick. It then will only be necessary to line up the two sights with the slope of the street or roof, and the arrow will give a direct reading in degrees. SCEBE-N FR.AME -E.ULE INDICATE-5 ANGLEOF TILT FIGURE 5 FIGURE 4 BELL & HOWELL EMPLOYES ARE OFFERED OPTICAL COURSE Bell & Howell has opened a School of Optical Manufacturing, which is meeting in the company's modern optical shop at Lincolnwood, 111., it is announced. Only 25 employes at a time are instructed in order to insure thorough understanding between students and instructors. The course is based on an effective program of actual shop periods and seminars, with more than 100 applications on file. One of the more appealing features of the course is that upon completion of it each pupil will have made his own Galilean telescope from beginning to end, working to the close tolerances characteristic of high grade optical manufacture. The course, believed to be one of the finest and most complete of its kind given during the war emergency, consists of 16 weeks, with meetings being held twice a week. INTERNATIONAL PROJECTIONIST