Measure men ts of t he spectral transmi ttance and i nternal transmit tan ce of fi ve optical glasses fOT the wavelength region 200 to 1,000 millim icro ns are reported .
Shortly after the introduction of cOlUmercially available photoeLectric spectrophotometers, the National Bureau of Standards began issuing calibrated disks for checking the photometric scale of spectrophotometers. At the time these standards were selected no precise information was available regarding their permanence. The present paper presents data showing that the glass s tandards kept in t he files for 16 years with only occasionaL use have not changed. Data are also given showing the nature of the changes occurring in glas es similar to the s tandards when exposed under gLass to south skylight (including sunlight) for period up to 3 years.
Shortly after the introduction of cOlUmercially available photoeLectric spectrophotometers, the National Bureau of Standards began issuing calibrated disks for checking the photometric scale of spectrophotometers. At the time these standards were selected no precise information was available regarding their permanence. The present paper presents data showing that the glass s tandards kept in t he files for 16 years with only occasionaL use have not changed. Data are also given showing the nature of the changes occurring in glas es similar to the s tandards when exposed under gLass to south skylight (including sunlight) for period up to 3 years.
In 1934, Gibson, Walker, and Brown developed sets of four colored glass filters to serve as working standards of spectral transmittance for checking the reliability of spectrophotometers. Several sets of these glasses were measured carefully and reserved and designated as future reference standards. Duplicate standards evaluated by comparison with the reference standards are available by purchase to the public. The current set of reference standards was established in the years 1945 to 1947, and one of these reference standards (selenium-red) was recalibrated in 1952. This paper reports a recalibration, made in 1961 and 1962, of all four glasses (selenium-red, carbon-yellow, copper-green, cobalt-blue) on three spectrophotometers (Cary 14, Beckman DU, König-Martens). Except for the cobalt-blue standard, the values of spectral transmittance found differ from those previously assigned by amounts differing at some wavelengths by as much as or slightly more than the uncertainties estimated for the present values, though not by amounts exceeding the combined uncertainties of the present and previous determinations. The indicated changes for these three standards are fairly regular, however, and support the view that the selenium-red and carbon-yellow standards are changing chiefly by formation of a reflectance-reducing film on the surfaces. The indicated rate of upward drift is slow, and suggests that it takes about 10 years for the drift to exceed the assigned uncertainity.
The scope of activities of the National Bureau of Standards is suggested in the following listing of the divisions and sections engaged in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief description of the activities, and of the resultant reports and publications, appears on the inside of the back cover of this report.
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