International projectionist (Jan 1959-Dec 1960)

TABLE V. Lumen Outputs of Condenser-Type Lamps. POS.-NEG. TRIM ARC AMPS. POWER VOLTS CONDENSER "SPEED" LIGHT AND IEAT OUTPUT Without heat fltr. With dichroic fltr. LUMENS CENTER APERTURE 0 F> LUMENS CENTER APERTURE 0 F 13.6 mm* 7/16" 13.6 mm* 1/2" 13.6 mm 1/2" 13.6 mm**-|"*** 13.6 mm**-i"*** 125 150 160 170 180 68 7^ 77 70 7^ f/2.0 f/2.0 f/2.0 f/2.0 f/2.0 15,000 19,500 20,500 20,700 2^,800 2100 2700 2900 3000 3300 13,500 17,600 18,500 18,600 22,300 1100? l!f00? 1 500? J 1 600? 1 700? * Regular unplated high-intensity projector positives. ** "Hitex" positives. *** "Orotip Heavy Duty" negatives. All other negatives regular "Orotip". (All carbons by National Carbon Co.) F/1.7 coated lenses and 0.600" x 0.825" apertures used. the shutter factor corresponding to the angular width of the shutter blades used. The shutter factor is 2 for 90° blades; others may be found in Table II (2-blade shutters only). Lamp lumen rating = Lumens in Table I x Shutter Factor in Table II. To save you the trouble of further multiplications, the lamp-lumen tables (Tables III-VI) give the lumen ratings with silvered mirrors alone, silvered mirrors with heat filters, and the new "cold" mirrors. As an illustration, suppose a medium-size drive-in has a mildly directional screen 60 feet in width (lowgain aluminum of 1.2 center gain). How many lumens are needed for 4 foot-lamberts of brightness? What lamp rating is required if 90° shutters are used on the projectors? Which specific lamps, carbon trims, and currents are suitable? Table I gives 9000 lumens as the value for 4 foot-lamberts on a 60-foot 120% screen. Lamp rating required = 9000 x 2 (shutter factor found in Table II) = 18,000 rated lumens. Consult Tables IV and V for lamps and trims giving 18,000-19,000 lumens. The most powerful lamps available (Table VI) are rated at 46,000 lumens at their highest current and with Ultrex carbons. These lamps are capable of furnishing 10 foot-lamberts on a 60-foot screen of 120% reflectivity! Lumen Ratings of Arcs The actual number of screen lumens obtained from any lamp depends upon such factors as mirror reflectance and heat-filter transmittance as well as upon such mechanism characteristics as shutter efficiency, aperture transmission, and the optical speed of the lens. As stated previously, to simplify matters the screen-lumen tables pub 's lished by the manufacturers of lamps and carbons are based upon the values obtained with standard apertures and //1.7-1.8 coated lenses, and without shutter, heat filters, or port glass. Tables III-VI, presented herewith, are based in the main on the averaged data published by National Carbon Co.,** but in addition to the lumen values obtained with silvered glass reflectors, the values with heat filters (dichroic, or interference-type) and with "cold" mirrors are listed. The lumen values for mirror , lamps burning 13.6-mm carbons (Table IV) were obtained from independent sources, and the blown-arc data in Table VI were supplied by Strong Electric Corp. The Perennial Heat Problem The center-aperture temperatures for all lamps and carbon trims are given in the tables in Fahrenheit de ** National Projector Carbon Bulletin No. 4. grees, and were obtained from the radiant-energy flux data furnished by a number of sources. (Note that aperture heat cannot be correlated with the lumen values, as the heat at the center of the aperture depends upon positive-crater factors, distribution of the radiant flux, and the type of mirror or condensing lenses.) The temperature data for 13.6-mm mirror lamps and for "blown" arclamps are somewhat uncertain.*** Radiant heat absorbed by the film is a problem that has become increasingly serious as the power of arclamps has increased. Fortunately, such new developments as effective infrared-absorbing heat filters and "cold" mirrors have more than counteracted the effects of radiant heat on the film. These devices, together with watercooled gates, have done very much to minimize film buckling and print damage. The relative heat-reducing efficiencies of the various types of mirrors and heat filters are worth careful consideration. The glass mirror having a coating of silver on its rear surface has been a standard item for many years ; and the radiation reflected from such a mirror includes 90% of the light (visible radiation) falling upon it together with much of the invisible infrared emitted by the HI carbon arc. Bolometric measurement reveals that about half of the heat generated (Continued on page 24) ***To convert to watts per sq. mm, divide the Fahrenheit temperature by 3000. TABLE VI. Lumen Outputs of "Blown"-Arc, Cold-Mirror Lamps. POS.-NEG. TRIM ARC POWER AMPS. VOLTS MIRROR DIAM. (In.) LIGHT AND ESTIMATED HEAT WITH COLD MIRROR (No heat fltr needed) LUMENS CENTER APERTURE 0 F. 10 mm** 7/16"* 10 mm** 7/16"* 10 mm*** 7/16"* 10 mm*** 7/16"* 130 71 138 76 155 7h 160 76 21 21 21 21 36,lfOO Vl,000 ^5,000 ^,000 1900? 2100? 2300? 2300+? * Solid graphite-rod negative. ** "Hitex" positive. National Carbon Co.) Screen-light side-to-center distribution 80%. F/1.7 — 1.8 0.825" apertures. * "Ultrex" positive. (Carbons by lenses and 0.600" x INTERNATIONAL PROJECTIONIST JUNE 1959