SSE- and Noise-Optimized InGaAs Radiation Thermometer

For measurements of radiance temperatures in the range from 150°C to 1,000°C, low uncertainties in the temperature measurements can be achieved by using near-infrared InGaAs radiation thermometers. The design and construction of the NIST near-infrared radiation thermometer (NIRT) that is optimized for low size-of-source effect (SSE) and noise-equivalent temperatures are described. The NIRT utilizes a 50 mm diameter achromatic objective lens with low scatter that images a 4.5 mm diameter spot at a distance of 50 cm from the objective in an on-axis design. A Lyot stop is implemented in the design with the aperture stop placed after the field stop resulting in a collection f/12. A 3 mm diameter InGaAs detector is cooled to − 70°C using a four-stage thermoelectric cooler to obtain high-shunt resistance for linear, low-noise operation at high transimpedance amplifier gains. For thermal and structural stability, the optical components are placed on four, 15 mm diameter graphite-epoxy rods making the optical throughput stable. Optical ray tracing with a commercial program is used to determine the Strehl ratio and other imaging parameters. A possible approach for a detector-based temperature scale in this range which could result in 10 mK (k = 2) thermodynamic temperature uncertainties at the In-point is discussed.