Page:Journal of the Optical Society of America, volume 30, number 12.pdf/23

 filters isolating spectral regions centering at 501, 651, and 703 m&mu; for a white surface. The data obtained in this way, while meager with respect to wave-length, had the advantage of yielding values applying to 45°-normal illuminating-viewing conditions, and of being free from errors caused by fading during measurements. Polarization errors were eliminated by proper orientation of the Martens photometer, the plane of the sample, and the direction of illumination. Measurements were made by P. Rudnick and K. S. Gibson.

45° illumination, normal reflection, photographic method

Inasmuch as zinc oxide was understood to be an important constituent of many of the pigments of the Munsell samples it seemed desirable to obtain data from 380 to 420 mμ, rather than to estimate over this range solely on the basis of the values at 405 mμ, since the reflectance of zinc oxide was known to vary strongly between 405 and 365 m&mu; (5). Accordingly, measurements were made photographically on most of the samples, using the Hilger sector-photometer equipment (6) adapted to reflection measurements with incandescent lamps as source. All of the photographic measurements were made by H. J. McNicholas.

All of the data obtained by the above methods, to quote the 1926 report, “were plotted as a function of wave-length. A separate curve sheet was used for each sample. Values of $$R_x/R_0$$ (apparent reflectance of the sample relative to that of MgO) less than 0.10 were also magnified by 10 in plotting....