International projectionist (Jan-Dec 1945)

according to their "F number," as F-4, F-3.5, etc. The F number of a lens is obtained by dividing the focal length (or the E.F., in the case of a compound lens) by the diameter. A projection condenser lens might be rated that way. If the focal length were 6 inches, and the diameter 4 inches, the F number would be F-1.5. Mirrors Spherical' and parabolic mirrors are used to some extent in projection, but the favored form is the elliptical mirror when a point source of light such as a projection arc is located at the primary focal point. Thus, either a mirror or a converging (condenser) lens may be used in a lamphouse. Both are used; in some cases, as in projection with incandescent light sources, both mirror and condenser may be used in the same lamphouse. Projectionists have no practical problem in ordering replacement mirrors or replacement condensers for lamphouses, since the manufacturer of the lamphouse specifies all particulars, and the projectionist need only order a new mirror or condenser according to specifications. Projection lenses are an entirely different matter, because the choice of a lens depends on the distance to the screen and the size of screen image desired. Since this is a matter of individual requirements, the projectionist must either know how to calculate the E.F. of the lens he needs, or write the facts about his theatre to a dealer or manufacturer, who will make the calculation. But the practical formula is as simple as Ohm's Law. and there is no reason why the projectionist can't do his own figuring on this point. Ordering Lenses To determine the E.F. of a lens needed for a given throw and screen image width, a sufficiently accurate formula is given by E.F. = T/W, where T is the throw or distance from lens to screen and W is the width of the screen image. Thus, if the throw were 90 feet and the screen image width 15 feet, the E.F. of the lens required would be 90/15. or 6 — that is. 6 inches equivalent focal length. This simplified formula is not completely accurate, but it is widely used, and is as easily converted into two corresponding formulas as Ohm's Law: thus. T = E.F. X W and W = T/E.F. Greater accuracy is obtained by using a very slightly more complicated version, 0.825 X T in which E.F. = — . This ap 0.825 + W plies only in the case of 35-mm projection, the figure 0.825 being the width of the standard projector aperture in inches. In using this formula T and W should be expressed in inches, not feet. This formula can also be written in three forms, the other two being: W = 0.825 X (T — E.F.) E.F. X (0.825 + W) and T = . E.F. 0.825 For 16-mm projection, substitute the figure 0.380 wherever 0.825 appears in the above expressions, that being the width in inches of the standard 16-mm sound projection aperture. Projectionists are expected to know these formulas and to be able to do the very simple arithmetic they involve; in many jurisdictions this is required in licensing examinations. For practical purposes, however, charts are available, and even easier to use than the formulas. A chart for 35-mm projection was printed in I. P. for March 1944, page 22. A similar chart for 16-mm projection will appear in an early issue. Optical Alignment Let us refer again to Figure 3 and the heavy horizontal lines representing the optical axis of each lens there shown. When several optical elements are combined into a system, they also have an optical axis, and must be aligned accordingly. In projection work the light source, the mirror or condenser, the film aperture and image, the projection lens and the screen constitute one optical system, having a single optical axis on which every part must be squarely aligned. Instructions for securing correct optical alignment with different types of equipment are supplied by lamphouse manufacturers and by projector manufacturers. These instructions should be followed, and the apparatus checked from time to time to make certain alignment and centering of the several optical elements has not been disturbed by vibration or other factors. It is always unwise to rely on short-cuts or penny-saving makeshifts in anything that has to do with the optics of projection. If the optical system needs attention or adjustment, sufficient time should be allowed to make the adjustment. A poor optical system puts a poor picture on the screen. Also it tends to waste light, and projection light is expensive. Defective optical relations with respect to the sound track produce poor sound, and the most expensive amplifiers and speakers can't do anything to improve. The optics at the photocell must be corrected. It is very unwise practice to try to "get by" with cheap lenses of poor quality, or in poor condition. Nothing is saved; the resulting light loss alone will soon add up to more than the cost of the best lenses. The advantages of using coated lenses have been repeatedly set forth in these pages. The actual dollars and cents economy, not to mention the improvement in screen results, much more than pays for the cost of such lenses. New 6AK5 Tube Interests Electronic Engineers Electronic engineers are showing considerable interest in the 6AK5 receiving type of electronic tube which was designed by the Bell Telephone Laboratories and first manufactured by the Western Electric Company and its subcontractors, the latter company points out. This miniature pentode stems from the 384-A and 386-A tubes designed by Bell for broad-band, high frequency amplification essential to coaxial cable systems, and has played a vital role in military equipment. Its performance is said to promise an even bigger role in the FM and television equipment of the future. It is pointed out that at the beginning of the war, when circuit engineers of the Bell Laboratories were searching for a small tube to use with high frequency circuits for the armed forces their associates in electronic development offered them an adaptation of a tube which was developed for broad-band carrier systems. This was the 6AK5. and it was found the tube fitted the vital requirements of high transconductance, low capacitances, low noise, and high input resistance. Use of this tube rapidly spread to other equipments until the demand was such that several other companies also were asked to manufacture it. The 6AK5 is described as a pentode tube with indirectly heated cathode and has the suppressor grid tied internally to the cathode. It is capable of providing better than twice the signal-to-noise ratio that can be realized with any tube previously available for use as an intermediate-frequency amplifier at frequencies of the order of 50 megacycles. However, the 6AK5 is capable of operating satisfactorily at frequencies up to approximately 350 megacycles. Also, its lower power consumption and smaller physical size proved particularly advantageous to the armed services and, likewise, will help to conserve chassis space for post-war equipment. The important characteristics of high transconductance. low noise, and high input resistance of the 6AK5 at the higher frequencies are fundamentally dependent upon the spacing between the cathode and the grid which is in the order of 0.0035-inch. The low capacitances of the tube are said to be another example of this precision technique of using the smallest of elements and eliminating unnecessary metal parts in the tube. Minimum lead inductance has been obtained by mounting the structure close to the 7 pin-button stem. FRANKLIN T. WOODWARD DIES Franklin T. Woodward, general patent attorney for the Western Electric Company from 1937 until his recent appointment as consulting patent attorney, died on Sept. 17 following a heart attack in his town residence at the Holley Hotel, N. Y. C. Mr. Woodward, who was in his 63d year, had been in failing health for some time. 10 INTERNATIONAL PROJECTIONIST