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utilized that all of the light is spread over a surface of one square foot, that surface will be lighted to an average intensity of one foot-candle. This relation is an important one for once the number of square feet to be lighted and the intensity of illumination which it is desired to provide are known, it is a simple matter to find how many lumens must be supplied to the surface. If, for example, it is desired to illuminate a surface of lOO square feet to an average intensity of five footcandles, 500 lumens must be utilized.
Photometry
Photometry is a specialized branch of the science of illumination which in itself may be made the subject of an extended study. The man doing field work in illumination has no need for an intimate knowledge of all the details that enter into this branch of the art. In the following discussion, photometers are treated in a broad, general way and for more detailed description of the instruments, the reader is referred to standard works on the subject of illumination and photometry.
Fig. 7 Essential Parts of Horizontal Photometer
A sketch of the most simple type of photometer is given in Fig. 7. The essential part of this photometer is a vertical paper screen between the lamps to be compared, at the center of which is a grease spot. When the illumination on one side of the screen is greater than that on the other, the spot will on this side, appear darker and on the other side lighter than the surrounding paper. By sliding the screen back and forth on the bar, a position can be found where the outlines of the spot will vanish and the spot itself will disappear. When this condition obtains, the illuminations on both sides of the screen are the same.
In order that both sides of the screen may be seen simultaneously, mirrors are mounted obliquely behind the screen. In Fig. 8, Case A, it will be noted that the spot as viewed in the left-hand mirror is darker than its surroundings and as viewed in the right-hand mirror is lighter than its surroundings, which indicates that the left-hand side of the screen is illuminated to a higher intensity than the right. In Case B of the same figure, it will be noted that the conditions are reversed; therefore, in this case the illumination on the right side of the screen is greater than that on the left. Somewhere between these two positions is a position at which the spot will cease to be visible, as shown in Case C of Fig. 8.
From what has been said above, the intensities of illumination on both sides of the screen in Case C are equal. Now, since we have a re