Page:Journal of the Optical Society of America, volume 33, number 7.pdf/22

 TABLE III. Effect of backing on colors of Munsell samples. Values are computed from the spectrophotometric curves shown in Fig. 10.

shown the curves obtained on four Munsell samples, each sample being run first with white backing and then with black backing. The spectral reflections of the backings used for Fig. 10 are shown in the figure. It will be noted that the effect of backing becomes appreciable at wave-lengths greater than 550 my, approximately, if the values of apparent reflectance are greater than 0.60 or 0.65. (The slight separation of the curves for PBP 8/2 between 480 and 600 millimicrons is not considered significant. It is probably caused by non-uniformity of the sample. Differences of this magnitude can be obtained when a sample is re-run with the same backing if the sample and backing have been removed and reinserted between runs.)

The effects of such spectrophotometric differences on the computed values of Y, x, and y are shown in Table III. Since these samples probably illustrate the maximum effects to be expected from the two backings it is apparent that the differences in color caused by measurement with white or with black backing are mostly unimportant. The use of calibration curves on each record sheet—those enabling corrections of wave-length errors, 100 percent and zero curve deviations, and aging of the MgO comparison surface, as used at the National Bureau of Standards—enables spectrophotometric data to be obtained with much less care and worry regarding certain details of operation than if these calibration curves were omitted. Omission of the curves makes it necessary for the operator to take great care, for example, in the insertion of the graph sheet in the instrument, in continually checking the wave-length calibration of the instrument and in controlling or watching the graph paper for expansion or shrinkage with change of humidity. A new MgO comparison surface must be prepared each day and the question of reproducibility of such surfaces thus enters. The possibility of erratic differences in results between the two investigations is thus present but since different actual samples were measured no further conclusions can be reached regarding the erratic differences between the Glenn-Killian and the National Bureau of Standards data.

With respect to differences between values of X, Y, Z, x, y, and z resulting from differences in computational procedure—30 selected ordinates as against weighted ordinates at every 10 my—it has been shown (21) that such differences are small for samples such as those considered here, much less than some of the differences shown. Only small and unimportant errors are therefore to be expected from this difference in computational procedure.

Detailed comparison of the values of x and y obtained by Glenn and Killian with those obtained at the National Bureau of Standards may be made by inspection of Figs. 2 to 8 or by study of the published data. Only two additional points will be noted here.

1. Certain consistent differences in the respective chromaticities are apparent when the (x, y)-data for certain groups of samples having the same hue designations (Figs. 2 to 8) are replotted in a single graph regardless of value level. This is particularly noticeable for the 10GY, GY, P, 10RP, and R samples. However, although the maximum (x, y) difference between the Glenn-Killian and the National Bureau of Standards data is Δx=0.0143 and Δy=0.0156, inspection of Figs. 2 to 8 shows that in the great