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Further Measurements of Illumination in Motion-Picture
Projection
By W. E. Story, Jr.
SYXOPSIS — The luctJwd previously described has been improved a)id applied to tJie iiieasurement of the total illumination obtained on the screen z^'ith different source sizes and different condenser systems compared to the nia.vimum obtainable zvith the given source brilliancy and given objective diameters. A comparison of the photoelectric measure)}ients zcitJi tJie measurements obtained using an optical photometer for condensers of different absorption and sources of tzi'o slightly different brilliancies shozi'S the ratio of the photoelectric cell readings to the photometer readings to be independent of color thru the range considered. Uniformity of screen illumination is also taken up in a general zcay by the application of direct observation of the screoi to the automatically drazi'ii cu}'ves registering total ilhiminatio}i.
Since the work described in the last transactions of this Society, new photoelectric cells have been made that are better adapted to the measurements required. Some minor changes have also been made in the rest of the apparatus.
It has been found advisable to substitute a condensing lens for the ground glass immediately beyond the dummy objective. This coritienser focuses the film aperture in the window of the cell, thus delivering all the light (barring certain constant losses of reflection and absorption) that falls on the objective to the cell instead of but a small part as did the opal glass. This change has resulted in a reasonably large galvanometer deflection, even when using an opal source as small as two-tenths of an inch square. The new arrangement offers the great advantage of a source the size of which can be changed at will, without change of any other characteristic.
To find the ratio of deflection to light received by the cell, an arrangement was used somewhat different from that previously employed. The condenser was removed from the optical bench as before, but this time the aperture plate was left in the system and covered with a piece of opal glass. If the opal glass is a perfect dift'user, the intensity of the illumination at any point beyond it will be inversely proportional to the square of the distance from the source of illumination to the opal glass, and accordingly by varying the distances and obtaining corresponding galvanometer deflections, the ratio of cell illumination to galvanometer deflection can be tested.
Figure 1 shows a plate made in this way, using a 2^-inch and 1^-inch objective with potentials of 67, 130 and 320 volts on the cell. Unfortunately, the curve made with the 2^-inch objective and 67 volts overlaps the 1^-inch objective — 320volt curve. Where the deflection is very small, a slight variation of the galvanometer zero or error of measurement has, of course, a great effect on the product of the deflection by the square of the distance. The constancy of this product, however, was well within any accuracy we could use in this work. Below is given a table show
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