After discussion of the advantages of physical densitometers and microphotometers over the older types using the eye method, the merits and disadvantages of the more important recent designs are summarized. The conclusion is reached that the thermoelectric densitometer is most accurate for direct-reading work, while the photoelectric cell is best adapted for the null method. The disadvantages of the thalofide cell and the selenium cell seem to outweigh their only advantage of large currents.
A simplified standard type of direct-reading densitometer is described, with vertical optical system, in which accuracy equal to the best claimed for the photoelectric null method, one-tenth of one per cent, was attained. The sensitivity was such that 1000 mm deflections on an easily read scale were obtained through a clear plate with a slit .05 mm wide, using only 20 watts in a lamp rated at over 70 watts. A single easily constructed thermocouple was used with a low resistance d’Arsonval galvanometer. Curves are given showing the gradual improvement of deflection steadiness, from variations of two per cent to almost complete steadiness, as various sources of disturbance were eliminated.
GEORGE R. HARRISON, "PRECISION DENSITOMETERS FOR PHOTOGRAPHIC PHOTOMETRY," J. Opt. Soc. Am. 10, 157-160 (1925)