PttOTOMETRIC equipment in use with integrating spheres has not undergone any radical changes since the sphere was first introduced by Ulbrieht. It usually consists of a bar photometer and accessories, including a set of standardized sectored disks and a Lummer-Brodhun photometer-head.
In I917, R. Von Voss, 1 in Germany, described a new type of photometer for use with spheres, but this does not appear to have come into use in this country. This laboratory has recently built apparatus designed along somewhat similar lines, but with several improvements which add to its convenience and usefulness.
The bar photometer used with the sphere usually has a candlepower scale attached to the comparison-lamp carriage, to read directly the candlepower of the lamp being photometered. In order to make this scale read correctly it is necessary to adjust the intensity of the comparison lamp by varying its voltage. This usually results in a color-difference between the lights compared. Another factor contributing to the color difference is selective absorption by the sphere paint. To overcome these color-differences it is necessary to prepare special color-filters, which is very difficult for most laboratories.
The apparatus built here is shown in the three illustrations. A suitable comparison lamp is enqlosed in a rectangular box painted inside with the same paint as that used in the sphere. In the end of the box toward the photometer-head is an opal glass window, 8 eros. square, before which moves vertically a metal slide with a V-shaped opening. This slide carries a plate with three photometric scales, via., 25 to 75, 5 ยฐ to I5o, and Ioo to 3oo candles. Inside of the box at the top and bottom of the window are two horizontal broad bars (Fig. ), the space between them * Communicated by the Director.