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1950-51 Theatre Catalog, 9th Edition, Page 340 (318)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 340
Page 340

1950-51 Theatre Catalog, 9th Edition, Page 340

Choosing the Proper Type and Size Arc

Recent Developments in the Science of Projection

Are Analysed With an Eye to Specific Applications

The requisites for good theatre projection were initially outlined in an article entitled "What Type and Size Arc Should I Install in My Theatre V, which appeared on pp. 219-222 of the 1942 Edition of THEATRE CATALOG. During the intervening years, there has been much technological progress in the field of theatre projection equipment. New developments and advances in projector carbons, lamps, and optical systems offer new possibilities. Also, in recent years other forms of entertainment have encroached upon the motion picture theatre field and are making strong, and, in some instances, very successful bids for the entertainment dollar. This makes it more imperative than ever before that high screen lighting standards be reached and maintained. Attractive and comfortable theatre interiors are merely accessories. The real drawing power is the picture, properly projected. As an aid to better projection, a new and up-todate analysis considering present day equipment is herewith presented.


Brightness of Image

The brightness of the image on the motion picture screen is one of the most important factors in creating a vivid feeling of realism in the mind of the observer. Adequate screen illumination gives sharp definition to details of the picture which are lost when the screen light is dim. It also permits the use of a higher level of general theatre illumination, thus facilitating movement of the patrons within the theatre.

The recommended standard for indoor theatres states that the brightness in the center of a screen for viewing 35mm motion pictures shall be 9 to 14 footlamberts when the projector is running with no film in the gate. With a screen refiection factor of 75%, the corresponding intensities of light projected onto the screen will be 12 to 18.7 foot-candles in the center of the picture area. The total amount of light which is required to produce a given illumination depends upon the level of that illumination, the size of the screen and the distribution of light over the screen, that is the ratio of the side to center illumination. Fig. 1 shows this relationship over the range of screen sizes and distributions of interest in motion picture projection. This illustration also shows the range of illumination intensities necessary to produce recommended brightness with a 75% screen reflection factor.

Color Quality of Ligh'l'

Another factor of prime importance to the impression of realism created by the picture is the color quality of the light. Due in all probability to our adaptation to daylight, a light of the color


National Carbon Division Union Carbide and Carbon Corporation

BRIEF: Nearly every theatre owner . . . at one time or another . . . has been confronted with the choice of an arc that will give him the finest light obtainable on his particular screen . . . for he knows that the projection booth and its equipment constitute the true jihearW of his house . . . Luxurious appointments are all to the good . . . but in the final analysis it is the quality of

entertainment presented on the screen

that counts . . . Even an Academy Award winner will produce many patron complaints if poorly projected.

The selection of a suitable projection lamp and carbons has never been an easy one . . . but there are certain definite requirements which should be met . . . The following article aims to

point out what these general criteria are . . . and how they may be fulfilled with the vastly improved equipment now on the market . . . Discussed for the guidance of every theatre owner are such matters as: factors needed to create an impression of realism . . . various types of projection lamps . . . types and sizes of carbons . . . light-absorbing devices, etc.


quality of daylight appears "whiter" and more pleasing than some artificial light sources. For example, illumination from incandescent tungsten lamps appears an unattractive yellow in direct comparison with daylight. This reaction is noticeable with black and white pictures and resulted in a preference for snowawhite projection light even before

FIGURE l-Fool-Candles at center of screen with shutter of 50% transmission.

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color features were known. The advent of colored motion pictures has, of course, greatly increased the importance of color quality in projection lighting.

In order to obtain faithful reproduction ofrthe various colors during projection, the color of the projection light must be suitably adapted to the requirements of the film as finally processed for projection. All 35mm motion picture color processes have been designed for projection with a light of daylight quality which has an essentially even balance of intensity of radiation in the various component colors making up the white light. The widespread and growing prevalence of color features today, therefore, makes color quality of the light practically equal in importance to quantity of light when considering the type of projection lamp to be installed.


There have been available two general types of carbon arc projection lamps, those using low-intensity arcs and those with high-intensity arcs. However, due to their outstanding advantages, the high-intensity arcs have largely replaced low-intensity ones in existing theatres and are being used in all new installations. It should be only a matter of time until the conversion of existing installations to the high-intensity arc is completed.

High-Intensity Arc Advantages

The high-intensity arc employs rare earth materials in the core of the positive carbon or, in the case of the a.c. high-intensity arc, in the cores of both carbons. By operating the arc at the proper arc length and suitable current density, a light of daylight quality and of extremely high brightness is produced. As shown in Fig. 2, the brightest portion of the light source occurs in and near the crater of the positive electrode. It is readily possible to obtain a crater brightness in the range 350 to 1,200 candles per square millimeter and values over 2,000 c/mm2 have been experimentally obtained. By way of contrast, the low-intensity arc is limited to a brightness of approximately 175 c/mm'.

Furthermore, the color quality of the light produced by the high-intensity arc is excellent for motion picture projection, especially for the projection Of color pictures. Fig. 3 shows that the light is very evenly divided between the three principal divisions of visible colors. This assures a proper color balance for the projection of motion pictures with faithful reproduction of the colors. Contrasted with this even color balance, the low-intensity arc is deficient in violet and blue and has an excess amount of orange and red which gives a yellowish tint to the light.

1950-51 Theatre Catalog, 9th Edition, Page 340