This article describes the design and performance of two densitometers used for high-accuracy measurements of visual diffuse optical transmission densities. The two densitometers were developed and constructed at the Physikalisch-Technische Bundesanstalt (PTB), the German National Institute
of Metrology. Both devices comply with the ISO standards 5-2 and 5-3 and are primarily used for the calibration of standard step tablets. One of these densitometers, the PTB's inverse square law densitometer, is the national standard reference densitometer capable of measuring with high
accuracy optical densities up to
E. Buhr, D. Hoeschen, D. Bergmann, "The Measurement of Diffuse Optical Densities. Part II: The German Standard Reference Densitometers" in Journal of Imaging Science and Technology, 1999, pp 382 - 387, https://doi.org/10.2352/J.ImagingSci.Technol.1999.43.4.art00012