The National Institute of Standards and Technology supplies calibrated photodiode standards and special tests of photodetectors for absolute spectral responsivity from 200 nm to 1800 nm. (This service will soon be expanded to 20 m in the infrared.) The scale of absolute spectral responsivity is based solely on detector measurements traceable to the High Accuracy Cryogenic Radiometer maintained by the National Institute of Standards and Technology. Silicon photodiode light-trapping detectors are used to transfer the optical power unit from this cryogenic radiometer to monochromator-based facilities where routine measurements are performed. The transfer also involves modeling the quantum efficiency of the silicon photodiode light-trapping detectors. A description of current measurement services is given along with the procedures, equipment, and techniques used to perform these calibrations. Detailed estimates and procedures for determining uncertainties of the reported values are also presented."