We describe a cryogenic radiometric calibration system suitable for high se
nsitivity, low background power bolometers, The system uses a variable temp
erature blackbody source capable of temperatures of 4 to 20 K, whose output
covers the sub-millimeter wavelength regime. The detector is an eight-chan
nel, filled-focal-plane array of superconducting transition edge sensor (TE
S) bolometers, The system is designed to test components of the optical sys
tem such as polarizers, filters and stray light suppressors as well as the
performance of the detector array elements, The bolometer response to incid
ent submillimeter radiation is measured and compared to the calculated outp
ut of the source seen by the bolometer.