448 abstracts: photometry 



sten standards, using Lummer-Brodhun contrast photometers in step- 

 ping from corresponding groups of 4 wpc carbon standards. Although 

 the agreement between the two laboratories was very satisfactory and 

 subsequent measurements of the new standards at the Bureau checked 

 the original values, it was realized, in view of the small number of 

 observers in each laboratory, that if other groups of observers had 

 made the measurements or if some other photometric method had 

 been used the results might have been different. 



Therefore, in order to obtain information as to the agreement which 

 might be reasonably expected among different groups of experienced 

 observers working by the same and by different methods, the Bureau 

 invited the Nela Research Laboratory, the Electrical Testing Labora- 

 tories, and the Physical Laboratory of the United Gas Improvement 

 Company to cooperate in an intercomparison of photometric measure- 

 ments of blue glass screens and tungsten lamps involving color differ- 

 ences such as were encountered in the establishment of the new stand- 

 ards. The first two laboratories, like the Bureau, used Lummer- 

 Brodhun contrast photometers, while the third used a special flicker 

 photometer, and in no laboratory were the screens and lamps measured 

 at the same time. 



The results of the intercomparison show that each observer, regard- 

 less of the kind of photometer used, maintained a fairly definite cri- 

 terion with respect to the mean of his laboratory, and that each labo- 

 ratory likewise consistently maintained its relation to the mean of all, 

 as judged from the measurements on the screens and those made on 

 the lamps some months afterward. 



Considering the difficulties involved in the measurements, the differ- 

 ent characteristics of observers, and the wide difference in illumina- 

 tion employed, the agreement among the laboratories was remarkably 

 good. It is true, however, that although the differences are small 

 they are not negligible in precision photometry. It is evident, there- 

 fore, that measurements to establish standards involving color differ- 

 ence should be left as much as possible to the standardizing laboratory, 

 where the observers must be carefully selected and a considerable 

 number employed, and the kinds of instruments and other conditions 

 definitely fixed. 



An examination of the Bureau's observers who took part in this 

 intercomparison and who were included in a group of 114 observers 

 tested at the Bureau by Crittenden and Richtmyer, using a special 



