Exhibitors Herald World (Oct-Dec 1929)

November 23, 1929 EXHIBITORS HERALD-WORLD 43 Measuring Light Intensity By JAMES L. McCOY Of Weslinghouse Lamp Company EVER since the beginning of photography, the judging of light value has been one of the great problems, to the photographer or camera man. In many cases in the past as now, the person responsible for taking of pictures is required to judge light values using his naked eye as an indicating means. His accuracy or ability to do this is the product of his appreciation and his experience as received through this natural indicator. The unassisted eye at its best is considered unstable with a possible error of 100 per cent or more from day to day when used as a light intensity measuring instrument. It is unreasonable to expect that two or more men would come very close in a simultaneous check. In order to assist the eye in measuring light and to obtain consistent results, several types of photometers have been developed. In most cases, these instruments are rather awkward to use because they require a comparative slow process of matching of light intensities, to obtain a reading. Therefore, they might be classed as laboratory equipment, from a studio standpoint rather than practical and portable direct reading indicators. A As a natural result, there has been a need for a studio type of photometer that could be worked, set up and read quickly without making adjustments. For this reason, the Westinghouse Lamp Company and the Westinghouse Electric and Manufacturing Company have designed this new tool, to place in the hands of the photographer or camera man to assist him in his work. This new indicating electrical eye should have the same value to the camera man as the slide rule to the engineer. It is a quick reading light yard stick as simple as a volt meter to read. As this is a very new development, we are not sure just what the possibilities or limitations of this device might be, but it is felt that it has a wide application in the motion picture and other photographic industries. This meter is self contained and carried as a complete unit in one case. The light sensitive pick-up is a photoelectric cell covered with a shield. A window is cut in one side to admit the light to be measured. The photo electric cell unit is connected to the meter by a six foot double conductor cord making it possible to move the cell unit around within that radius without moving the case. The instrument contains a commercial portable micro ammeter calibrated directly in foot candles. The smallest size "B" batteries are wired and mounted into the case. As very little energy is required, the battery life will be about its shelf life. The photo-electric cell is generally known as a light sensitive tube having somewhat the appearance of a radio tube. The tube has two distinct parts, an anode and cathode. The cathode is coated light sensitive materials giving the tube its characteristics. Many other commercial uses have been found for the photo cell. It is used as a smoke detector, to sound an alarm in case of fire, to count the product of quantity production as the cars are counted as they pass through the Holland Tunnel, by the interrupted beam of light. The cell plays an important role in several different schemes of motion picture sound reproduction. (The _ resistance of the cell will vary with intensity of the light striking it.) There are a number of different types of cells and their characteristics vary materially with the elements used. They can be made of different *Presented before the Society of Motion Picture Engineers in Toronto, October 10, 1929. materials to respond to different wave lengths of light. Dr. Rentschler of our research department has constructed a photo electric cell that will respond only to ultra violet region of the spectrum. This cell is now being used in an ultra violet recording device, making | it possible to obtain a quantitive reading in ,■ ultra violet units. The cell used in our light intensity meter is a special cell having a very broad response, covering the visible spectrum. This response is such that it will fit in very well for light measurements where the combination of Mazda light and panchromatic film is used. A spectroscopic study of the response of the panchromatic film when plotted against the light of the Mazda lamp shows that the results are somewhat near a straight line making it possible for the meter to give an integration that will come close to the photographic results obtained with given light values. The use of this meter is suggested to be of considerable value when colored pictures are taken because of the integration of the values of different wave lengths of light through the visible spectrum. The same cell can also be used for the measurement of north sky daylight giving approximately the same values for the same intensity from a photographic standpoint. The photo cell as used is directional in its pickup making it possible to study the light from different angles. The pickup can be made non-directional if so desired. A The sensitivity is such that a range of full scale readings can be made from 100-foot candles to 3,000-foot candles. We believe the range of 400 to 500-foot candles, as read on a full scale of 1,000-foot candles will fit the studio requirements the best, but this is a matter to be controlled by the professional studio man. In order to provide a simple and effective means of calibrating the meter, without returning it to the factory, we plant to mount a low voltage lamp in the meter case to check the cell for calibration. At a definite spacing a calibration check can be easily made. The lamp will be a six volt type to be supplied with energy from a six volt external battery. M. W. Palmer of the Paramount Famous Lasky Corporation has been doing some work with this photometer during the past few months, to determine the light values on motion picture sets. He has informed me that while he was checking the light reflected from an actor's face, the light intensity meter picked^ up the change in illumination caused by this man lighting a cigarette. This incident may illustrate how sensitive this device is to change in light intensity. A We believe this device has an application in connection with the film printing machines. There are a number of different settings of the printing lamp required to have the film printed without glaring or too dark results. To control this condition, the different settings must be made bv the operator. In order to try the light_ intensity meter for this apolication, a snecial mask for the photo cell has been made to fit into the aperture of the printer. Mr. La Grand of the Paramount Famous Lasky Corporation has been making tests to determine the meter's value in this field. The meter has also been suggested as a means to analyze the light on a motion picture screen. As we feel this is a step in the right direction and there is a commercial field for this meter, we should like very much to have the comments of the interested engineers. THE Ultratone Super Duo-Dynamic Speaker combines the tonal excellence of an air column horn with the practically unlimited volume of an exceptionally powerful dynamic unit. 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