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The NIST LOCR is based on a commercially available cryogenic radiometer [10], which relies on a servo control system to maintain a constant temperature during laser heating of the radiometer cavity.
The responsivities of DUT1 and DUT2 were determined by direct substitution of the LOCR using the calibration system shown in Fig.
Four correction factors were used: the LOCR window transmittance (TW), the LOCR receiver absorptance (AR), the relative aperture transmittance (TA), and the LOCR electrical calibration (kL).
The ECPR is considered a secondary standard traceable to the LOCR. In addition to being used to quantify the optical power absorbed by the ECPR, the electrical substitution provides thermal compensation for the pyroelectric response.
A direct comparison of HACR and LOCR, with one radiometer next to the other measuring the same source of optical radiation, was not possible due to instrument size and location.
There are relevant similarities and differences in the HACR and LOCR instrumentation and measurement protocols.
LOCR is designed with the cavity accepting a horizontal optical path, eliminating the steering mirror and the limitation on the transfer detector size.
Both the LOCR and the isoperibol calorimeters contain cylindrical cavities that absorb laser radiation.