No wholly adequate standards have existed until now for checking the accuracy of the photometric scale of modern, high-precision, recording spectrophotometers. A sample of glass has been found which has the characteristics required of such a standard. A densitometer was developed for two commercial spectrophotometers to calibrate these standards. By using this densitometer the absorptivity of the standards was measured with a coefficient of variation approaching 0.1%.
WALTER SLAVIN, "Photometric Standard for Ultraviolet-Visible Spectrophotometers," J. Opt. Soc. Am. 52, 1399-1401 (1962)