CRYOGENIC RADIOMETER :PERFORMANCE AND CHARACTERIZATION AS A PRIMARY STANDARD AT INMETRO
Author(s): M. S. Lima , A. P. Cunha , H. P. Grieneisen  and I. B. Couceiro 
Affiliation(s): National Institute of Metrology, Standardization and Industrial Quality - Inmetro  Radiometry and Photometry Laboratory,  Legal Metrology Directory,  Optical -
Topic: 2. Development of Optical Measurement Systems
This paper presents the progress of characterization and performance of a high accuaracy cryogenic radiometer. This primary standard for optical radiant power enables highly accurate determination of absolute detector responsivity. Up to the present we have carried out the measurements with the He-Ne ( 632.8nm ) and the Ar-ion laser ( 514.5 and 488 nm). The laser beams have been prepared with a spatial filter to obtain a smooth gaussian beam. The beam power was stabilized by means of an EO-modulator to be better than one part in 105. Performance tests of the system for uncertainty estimates considered among other parameters of influence, window transmission and reflectance losses from the Brewster window, reflection losses from the radiation trap, stray light and optical-electrical equivalence tests. The combined standard uncertainty was determined to be in the 0,02% range. Good agreement was found between the measured value of the spectral responsivity and the reported value by LNM-Cnam for a standard trap detector. Continued work is in progress with the construction and calibration of trap detectors to be used as secondary transfer standards and the extension of the calibration range to towards longer and shorter wavelenghts.