International projectionist (Jan-Dec 1947)

METAL MIRRORS: NOTES ON MAINTENANCE The accompanying notes, intended to serve the needs of many inquirers, were compiled by Heyer-Shultz, Inc., pioneer workers in the field of metal reflective surfaces. ONCE a metal mirror is installed it does not have to be removed from the lamphouse for cleaning. It may be cleaned in place while hot or cold. As these mirrors do not pit, they are extremely easy to clean. Enclosed in each shipping case is one 2-ounce container of cleaner (Bon Ami). This particular cleaner is recommended because: 1. It is non-inflammable; 2, it is inexpensive and easy to obtain, and 3, it is a foolproof cleaner not affected by time or temperature. Before cleaning, dust the mirror lightly with a clean dry cloth to remove carbon dust and other particles of loose material from the face of the reflector. Sprinkle cleaner on a wet rag and rub into a paste, apply to surface and wait until dry; then remove with clean dry rag. If applied to a hot surface, the water will steam off instantly and no waiting will be necessary. Do not be afraid of applying wet cloths to the very hot surface, as it will not harm the all-metal reflector in any way. Scratching of No Consequence As the all-metal reflector is a firstsurface mirror, the thought comes to mind that in continual cleaning abrasive particles must find their way into the cleaning process and leave scratches. This is very true, but of no consideration. Scratching is noticeable to the eye on the highly polished finish; but in a test conducted to determine just how serious an efficiency loss is encountered, it was found that after scouring a new reflector with sand until the entire surface was covered with these marks, a loss of only 4% was effected, a condition that is definitely more extreme than that to be found in normal projection practice. It is of interest to note that rhodium (the reflecting surface of the all-metal mirror) has a Rockwell hardness of 90, whereas tool steel has a hardness of only 60. The specifications for each model allmetal reflector accompany each mirror, and it is important that they be set up in the proper working distance outlined for each model. To determine a perfect focus it is suggested that a pinhole (approximately .048" in diameter) be inserted at the aperture in a central position and a white card held just in front of the objective lens, where a picture of the reflector will appear. When perfect focus has been accomplished, a clear white field will appear on the card. If the focal distance is long, bluish areas will be evident; and if the reverse is true, yellowish and brown areas will appear. This test will enable the projectionist to set up his lamp without projecting a picture to the screen. Booklet Terms Application In the specifications booklet, "working distance" is the measurement from the first surface of the reflector at the cen. ter hole to the aperture. "Focus" is the j measurement from the crater floor of the ! positive carbon to the first surface of the reflector at the center hole. All the figures given for working dis tance and focus for simplified highintensity are computed from an 8-mm positive carbon operating at specified current density for this size carbon. The working distance may be increased approximately x/2 inch for each millimeter smaller positive carbon. These distances are all computed to cover standard projection lenses of average equivalent focus and speed. PROJECTIONISTS' SERVICE MANUAL $3.O0 1. Don't fool yourself, sound trouble could make this happen at your theatre. 2. And result in lost receipts that bring on box-office blues. 3. Regular checkups by RCA experts avert trouble— assure.high-quality sound. 4. Continual, high-class showings please patrons— bring them back. How to put an end to box-office blues When you buy RCA Service, you assure yourself of trouble-free operation, economical upkeep, and Academy-approved tone adjustments for your sound system. In addition to regular checkups by a skilled, well-equipped techni cian— you get fast service on parts — prompt help in an emergency. Unload your sound equipment problems on RCA Service specialists. For the cost of a few admissions per day, you may prevent box-office blues. RCA SERVICE COMPANY IMC. RADIO CORPORATION of AMERICA CAMDEN M.J. INTERNATIONAL PROJECTIONIST • October 1947 29