Shortly after the introduction of commercially 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. Recent measurements show that the glass standards kept in the files for 16 years with only occasional use have not changed. Data have also been obtained showing the nature of the changes occurring in glasses similar to the standards when exposed under glass to south skylight (including sunlight) for periods up to nearly three years.
KASSON S. GIBSON and MARION A. BELKNAP, "The Permanence of Glass Standards of Spectral Transmittance," J. Opt. Soc. Am. 40, 435-436 (1950)