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Comparison measurements of low level infrared radiation at 1064 nm wavelength of the modified Rj-7000 series radiometer (hereafter termed MRj), custom manufactured by Laser Precision Corporation, were performed with the NBS P-3 low level pulsed laser radiometer constructed at the National Bureau of Standards, Boulder, Colorado. A brief description of these instruments and measurement data are reported in this paper. This is considered of interest for those engaged in the design of radiometers or associated with low level monochromatic performance measurements used with laser ranging and designating systems and test sets. Due to time constraints, interest was focused on the practical lower limit signal-to-noise measurement capability of the MRj radiometer. Data obtained indicate that this radiometer, which is essentially the conventional model specially fitted with an aperture wheel, a reflex mirror, and telescope assembly featuring a 10 cm diameter collecting lens, has a practical lower limit measurement capability of approximately 10 fJ/cm2. This is essentially a few orders of magnitude greater in sensitivity than the conventional, unmodified version, and is in agreement with the theoretical gain obtained using a lens of this size. These results were obtained by comparing the MRj measurements with those utilizing a conventional Laser Precision radiometer without the telescope assembly, in the region of 10-12 J/cm2 and the NBS P-3 in the range of 10-13 J/cm2 to 10-16 J/cm2. A nominal 1064 nm wavelength laser simulator LED was placed at the focal point of a 12.7 cm diameter lens of 60 cm focal length to provide a collimated source of approximately 15 ns FWHM pulse length with a PRR of 30 Hz. It was also noted that the LED output power decreased with increasing pulse repetition rate which was correlated with NBS P-3 radiometer observations between 30 Hz and 125 kHz.
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