Exhibitor's Trade Review (Mar-May 1922)

1062 EXHIBITORS TRADE REVIEW Volume 11. Number 15 PRO J ECTION TALKS ON PROJECTION How Light Is Measured |^__^^^^^__ By Charles Simpson I When new light sources began to supersede the candle, it was necessary that the luminosity of these lights be expressed in terms that everyone understood, and as candles had been the means of illumination for generations and it was pretty well known approximately how much light was emitted by one, it was but natural that the luminous intensity of these new lights should be compared with intensity of the candle, so the candle-power has become the unit by which the luminosity of a lamp of any description is measured. While this unit answered the purpose for a while it was soon found that candles varied so in size, material and light giving qualities that it was not accurate, so it lead to the adoption of a standard size candle made of a specified material that is now known as the "International Candle" as the unit with which to measure or compare light. It is the amount of light that is given off by a spermaceti candle burning at the rate of two grains a minute, and any light, no matter what kind, is rated in candle-power according to its brilliancy in comparison with this source of light. By that is meant a forty candle-power electric bulb will give off the same amount of light that a spermaceti candle burning at the rate of eighty grains per minute would. The candle-power of a light is now measured by an instrument known as the photometer. There are several different types of photometers in use, each one working on different principles, and practically all of them are more or less complicated and require a great deal of skill to operate. The most simple one, called the bar photometer, consists of two lamp sockets placed a certain distance apart on a bar. Between these two sockets, a vertical paper screen with a translucent spot in the center of it, is mounted on this bar so that it can be moved back and forth. In one socket is placed an electric bulb which has been tested and its exact candle-power is known, and in the other socket, the bulb that is to be tested or measured. The lights are then turned on and this screen is moved back and forth between the two lights until a position is reached where the spot will disappear. In order that both sides of the screen may be seen simultaneously, mirrors are mounted obliquely behind it, for when the spot can no longer be seen on the screen, the illumination on both sides is the same. A reading is then taken from a scale that is marked off on the bar, and from this X'eading the candle-power of the bulb being measured is computed. The above is simply a rough description of how the candlepower of incandescent bulbs is measured. Screen illumination, however, in which projectionists are more interested, is not measured in this way, but by a foot candle meter, as the unit of measurement of the intensity of light on a screen is a foot candle and not candle-power, although the latter is a factor. The foot candle is more closely associated with our everyday use of light than the candle-power is as a unit of measurement, for that is really the unit by which we judge whether a screen has sufficient light on it or not, as candle-power alone gives no indication of the quantity of light. A foot candle is the illumi nation produced by a source of one candle-power on a perpendicular plane one foot away. As an illustration, hold a piece of paper of any size one foot from a lighted candle, and the amount of light that is thrown on the paper is one foot candle. The use of this term also brings in another one which is little understood, the lumen. This expresses the amount of light flux that is required to illuminate a surface of one squarp foot to an average intensity of one foot candle. In other words,, the lumen is the unit by which the total amount of light emitted from any source is measured. These two are the terms by which screen illumination is measured. Until the foot candle meter was perfected, it was more or less difficult to measure screen illumination, as photometers were expensive and considerable practice was necessary in order to read them accurately. This instrument, however, measures intensities of from 0.5 to 25 foot candles quickly and with a fair degree of accuracy. It is simple, consisting only of a lamp in a box that illuminates the under side of a photometric screen to a much higher intensity at one end of it than at the other. It has a scale on the outside of this screen above which areclosely spaced translucent dots, these dots serving the same purpose as the translucent spot in the screen of the bar photometer. This scale is accurately graduated with a lamp in thebox that burns at a voltage regulated by a voltmeter and rheostat so that it does not vary any to that at which the instrument was originally graduated. The method most commonly used in measuring screen illumination with a foot candle meter is to divide the screen into sixteen or more equal squares and then place the instrument in. the center of each one of these squares. The light in fallingacross the scale will make the spots brighter at one end than at the other, and at the point where the spots are neitherbrighter nor darker than the white paper scale, the illuminations from within and without the box are the same, and a reading of the scale is taken. The readings from all squares are then added up and divided by the area of the screen in square feet, and the result is the average intensity in foot candles at which the screen is illuminated. Troubles in England Operators in England seem to have plenty to contend with asthe following, taken from "Bioscope," indicates. At a special Operators' Meeting the Chair was taken by Councillor Hugh Roberts, supported by Mrs. Rouse, Messrs. Callaghan, Collins, Garner, Hartfield, Kelly, H. Kemp, McCarthy and Wright.. After introductory remarks by the Chairman, Mr. Muir opened his speech by regretting that there was such a difference of opinion within the ranks of operators, and that instead of being a united body they had split themselves into five different camps. There were not only the E. T. U. and the N. A. T. E., but alsc the A. E. U., the G. W. U. and the Dockers' Union, each oneprofessing to be catering for operators. Until the operators, defined their own policy and became united no progress would be made. They must secure an industrial organization. He dealt briefly with the commencement of the organizing of the operators. The first attempt was made in 1907 by the N. A. C. 0., a branch of the N. A. T. E. The E. T. U. made no attempt in pre-War Why the Brenkert Brenopticon "Model F" is the most useful equipment ihat has been offered to theatres in Itite years. It dissolves projection of lantern slides and stage effects or simultaneously projects both Skillful Design— Rigid Conslruclion—Powerful in Projection BRENKERT LIGHT PROJECTION CO., Inc., Detroit, Mich. Blue Seal Film Mending Shears $3.50 BliHE SR/VL, PR01>rCT.S CO.. Inc. Mfr», of Motio^i Pictvrc A cce.isories 2.") Liifaj-ette Ave., Brooklyn, N. Y. CircuUirs on Request Ten Thousand Dollars Reward will be paid to any person who can prove that B. F. Porter did not sell and install Simplex Machines in the Capitol Theatre at 51st Street and Broadway, New York City. B. F. PORTER, 729 7th Ave., New York