The National Institute of Standards and Technology (NIST) supplies calibrated photodiode standards and special tests of photodetectors for absolute spectral response from 200 nm to 1800 nm. (This service will soon to be expanded to 20 mu]m in the infrared.) The scale of absolute spectral response is based solely on detector measurements traceable to the NIST High Accuracy Cryogenic Radiometer (HACR). Silicon photodiode light-trapping detectors are used to transfer optical power measurements from the HACR to monochromator-based facilities where routine measurements are performed. The transfer also involves modeling the quantum efficiency (QE) of the silicon photodiode light-trapping detectors. A description of current NIST 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.