60 L E T T E R S T O T H E E D I T O R

On the Standard Source for Low Level Photometry KARL S. WEAVER

Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York November 16, 1949

IN the October issue of the J. Opt. Soc. Am.,1 Mr. J. Terrien states, in effect, that it is a "pity" that I did not use the

melting point of platinum, 2042°K, instead of 2360°K as a stand­ard for the specification of the low level photometric scale in my recent paper on Low Level Photometry.2 Mr. Terrien bases his assertion on a resolution, No. 2, adopted at the 11th session of the ICI in Paris, June, 1948.

As stated in my paper, the luminosity tables incorporating the 2360°K standard source were accepted by a majority of the American authorities consulted. Opinion was not unanimous, but, in view of the facts that time was limited and our British co­workers were using 2360°K, it was agreed that this value should be accepted. I t thus became the joint recommendation of the British Standards Institution and the American Standards Association.

In view of the fact that my report covered work done in 1942, it could hardly be expected to conform to a resolution adopted in 1948. This resolution was first brought to my attention in a letter from Mr. M. Leblanc dated June 29, 1949, with which he transmitted a copy of the communication of Mr. Terrien dealing with this question.

To the best of my knowledge, resolution 2 has not been circu­lated in this country, published in any of the regular journals, or adequately circulated for discussion and criticism within the four-month period provided for dissenting votes. I have seen no evidence that the question of low level photometry was considered at all when resolution 2 was adopted.

Suggestions for a standard light source for low level photometry have ranged all the way from the equal energy spectrum to the melting point of platinum, including 2360°K, 2848°K, ICI Illuminant C, monochromatic red, and monochromatic green. Since there has been such a variety of opinions on the choice of a standard source for low level photometry, and since the reasons given for the selection of 2042°K. for this purpose are in question, the final choice must be considered undecided.

I do not wish to urge final adoption of 2360°K as a standard light source to be used in fixing the luminance scale, but it seems to me that a color temperature of 2042°K would be inconveniently low. The fact that the standard melting point now used in re­producing the candela is 2042°K is without significance.

Aside from the merits of resolution 2 in the present application, there is evidently need of more prompt publication of the findings of the international standardizing committees in the leading journals with supporting material giving the reasons for the adoption of important resolutions, as well as the reasons for non-acceptance by the dissenters.

1 J. Terrien, J. Opt. Soc. Am. 39, 888 (1949). 2 K. S. Weaver, J. Opt. Soc. Am. 39, 278 (1949).