Comparison of monochromator-based and laser-based cryogenic radiometry

We present results of measurements obtained with a new monochromator-based cryogenic radiometer facility at NMi/VSL. General design considerations of the facility and associated transfer detectors are discussed. At present, Si detectors can be calibrated between 330 nm and 1200 nm at power levels ranging from 5 mu W to 80 mu W. A vacuum trap detector of the reflective type has been constructed, which accepts an f/8 optical beam in conjunction with spot sizes up to 5 mm in diameter. To check the performance of the system, the trap detector has been calibrated directly against the cryogenic radiometer using lasers at 488.0 nm, 543.4 nm and 632.8 nm and subsequently using the monochromator at the same wavelengths. The agreement is better than 0.02%.