Address:
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NIST, 100 Bureau Drive
, Stop 8441 Gaithersburg, MD, 20899, United States
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Abstract:
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A radiometer-photometer system has been developed in a National Institute of Standards and Technology (NIST) and Gamma Scientific (GS) cooperation to satisfy the increased requirements in the dissemination of NIST radiometric and photometric scales. The reference responsivity scales have been extended from the silicon wavelength range to 2500 nm and in addition to spectral power responsivity calibrations, spectral irradiance and radiance responsivity calibrations are performed as well. The new generation radiometers and photometers can be utilized to propagate the extended NIST scales to field applications with a minimal increase in the measurement uncertainty. The new radiometer-photometer system has a versatile measuring head design. The front geometry of the heads can be simply modified to measure radiant power, irradiance, radiance, or the photometric equivalents, luminous flux, illuminance, and luminance. The different head constructions are made using the combination of apertures, diffusers, filters, filter combinations, and a variety of detectors depending on the head design for a given application. In addition to silicon photodiodes, nitride passivated silicon (for the ultraviolet), InGaAs, and extended-InGaAs photodiodes (for the near-infrared) have been applied. Detectors with large area and spatially uniform responsivity are used for power mode measurements where the detectors are underfilled with the incident radiation (beam). In addition to diffuser-input irradiance meters that can measure different source sizes, irradiance meters without diffusers have been developed to measure either "point sources" (where the detector is overfilled with the radiation from a small and distant source) or collimators. In order to perform lower measurement uncertainty, both the filters and the detectors can be temperature controlled independently from each other using thermoelectric (TE) coolers/heaters. Usually, detectors are cooled to increase their shunt resistance and filters are heated to avoid condensation. The pre-amplifiers, mounted in the back of the measuring heads, are matched to the detector electronic characteristics to optimize the signal gain, loop gain, and the closed-loop voltage-gain at the signal frequency. In DC measurements, usually the optimization is simplified for selecting (and/or cooling) the detectors for high shunt resistance. Only the optimization of these three gain characteristics made it possible to measure modulated (chopped) radiation with high sensitivity and low measurement uncertainty. Typically, the photocurrent-to-voltage conversion uncertainty is 0.02 % (k=2). The responsivity calibration results are usually reported at DC (zero Hz). To obtain exact decade ratios for the different signal gain selections at a given (other than DC) signal frequency, the frequency dependent gain (responsivity) curves are measured and then corrections are applied based on the measured data. Operational and performance evaluation tests have been generated for the radiometers/photometers to minimize errors and assure acceptable performance during fabrication.
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