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1947-48 Theatre Catalog, 6th Edition, Page 588 (570)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 588
Page 588

1947-48 Theatre Catalog, 6th Edition, Page 588

The Carbon Arc for 16-mm. H-I Projection

Improved Light Intensity With Carbon Arcs

Yields Increased Screen and Audience Size

High-intensity carbon arcs for 16-mm. projection were first introduced in 1938. In the intervening years, 16-min. projection has grown so extensively and now promises such wide application, that high intensity light sources for this media are more than ever considered a necessity.

Before the war most professional 16mm. film was educational or industrial in nature. The vast number of teaching and overseas entertainment films printed on 16-min. for the Armed Forces during the war has popularized and stimulated the adoption of this narrow-gauge film for educational, commercial, training, sales, industrial, and small-theatre feature use. The latter development is particularly true of the European continent, Latin America, and other countries where lower cost of operation, shortage of the larger 35-mm. equipment, and lower transportation costs for the smaller film have infiuenced large numbers of theatres to employ 16-mm. features exclusively.

Whether a rapid increase in 16-mm. feature houses does or does not take place in this country because of the many installations and the high investment in 35-mm. equipment already made by American theatre operators, it is evident that high school and college auditoriums, factories, hotel ballrooms, churches, and small-town portable shows are destined to use 16-min. projectors extensively. Sixty thousand 16-mm. projectors are now available to school and adult educational groups alone, according to one estimate, and it is anticipated that 300,000 halls and 274,000 churches will gradually be equipped to show 16mm. films.


Regardless of whether 16-mm. showings are for theatrical or non-theatrical purposes, it is certain that larger screens and increased brightness will he demanded for economic operation and better audience enjoyment. This result can best be achieved by the use of carbon-arc trims to increase the light supply.

The most powerful incandescent lamp generally available for 16-min. projection is the 1,000-watt, 10-hour lamp,


National Carbon Company. Inc.

throwing 275 lumens of light on the screen through an f/1.6 untreated lens and shutter of 60 per cent transmission. National Carbon Companyls Pearlex carbon-arc trim delivers four times as much light, or 1,100 lumens, to the screen under the same conditionsl; and a new 6-mm. experimental carbon trim for 16mm. projection described recently2 will deliver two and a half times as much light as the Pearlex trim, or 2,750 lumens, to the screen. This is equivalent to ten times as much light on the screen as is being delivered by the incandescent lamp.

The desirability of this increased volume of light with respect to greater screen size and audience size is at once apparent.


The SMPE Committee on Non-Theatrical Equipment has recommended3 certain procedures and conditions to be observed in the presentation of 16-mm. motion picture film to provide a picture that can be viewed to good advantage by everyone present. Among the recommendations made are the following:

(1) Distance of farthest spectator from screen should not exceed six times the width of screen image.

(2) Distance of nearest spectator from screen should not be less than twice the width of screen image.

(3) Viewing angle of no spectator should be greater than 30 degrees.

(4) Optimum screen brightness, 10 foot-lamberts measured with shutter running but without film.

(5) Limits of screen brightness, not more than 20 foot-lamberts or less than 5 foot-lamberts, measured as above.

(6) Color-temperature of the light de 1KALB, W. C. Carbon arc projection of film. Jour. Soc. M. P. Eng. 41: 05.

ZZAVESKY, R. J., and W. W. LOZIER. Increased light for projection of 16-11'1111. film with carbon arcs. Jour. S00. M. l'. Eng. 48: 448. 1947.

3Report of the committee on non-theati)'i)callqig;iiipment. Jour. Soc. M. ii. Eng. 37:

TABLE l-Possible screen widths and approximate theatre seating capacity, using a Pearlex and an experimental carbon trim to produce intensities of 5 and 10



10 (LL.

Carbon 'l'riln

1100 11.3'


Experimental 2750 i 17.9'

Screen Width

Approximate Seating

1011].. I 5 ft.l..

5 ft.L. p 16' 400 800 25.2' 1000 l 2000

livered to the screen to be in the range from 3,000 to 4,700 degrees Kelvin.

(7) The use of matte type of screen tfin all cases where a projector of adequate illuminating power can be obtained."

It will be noted from the above that 10 foot-lamberts is the optimum value of screen brightness recommended. This is identical with the preferred value for viewing 35-mm. film as specified by ASA standard Z-22.39-1944.

The 1,000-watt incandescent lamp, delivering 275 lumens to the screen through a 2-inch, f/1.6 lens and shutter of 60 per cent transmission, will cover at an 80 per cent side to center distribution ratio only a 5.7 foot wide matte surface screen (of 75 per cent refiectivity) at this optimum 10 foot-lamberts brightness.

The Pearlex carbon arc trim provides an image 11.3 feet in width at the optimum recommended level of brightness, and fills a 16-foot screen at the minimum limit of 5 foot-lamberts brightness. The illumination provided by the new 6-mm. experimental trim is adequate to light a 17.9-foot screen to the preferred 10 foot-lamberts value.

Seating capacities based upon the use of 20-inch seats, 32 inches back to back, with a limit of 14 seats between aisles as prescribed by the laws of some states, show that presentation of 16-mm. film with Pearlex trim can be made before approximately 400 seated spectators at optimum screen brightness, and before an audience of 800 at minimum limit of brightness. With the experimental 6-mm. trim, seating capacities could be raised to approximately 1,000 at optimum 10 foot-lamberts brightness, or 2,000 at minimum limit of brightness. (See Table 1.)

Another consequence of the higher light output of the carbon arc is in the type of screen that can be employed with it. The almost universal practice with incandescent lamps is to use directional screens that throw the image back within a narrow angle. This has a limiting effect on audience size because the screen does not permit wide viewing angles. A fiat screen, which will permit much wider angles of vision (up to the full 30 degrees cited above) and larger audiences, can be used with carbon arcs.


Another advantage offered by the carbon arc is its decided economy of operation. The Pearlex carbon-arc trim is designed for operation on a direct-current through a rectifier from a llO-volt, single phase, a.c. supply, with a current demand of less than 15 amperesewhich is available from the ordinary convenience outlet. The carbons operate at 30 amperes do. with 28 volts across the arc.

1947-48 Theatre Catalog, 6th Edition, Page 588