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 D = 3.3. The other densitometer, called fiber densitometer, measures densities up to D = 6. For both densitometers, the expanded ISO uncertainty of density measurement (coverage factor k = 2) is 0.003 for densities below 1.5 and increases to 0.006 at density 6 for the fiber densitometer with spectrally neutral samples.