Two-color infrared thermometer for low-temperature measurement using a hollow glass optical fiber

A low-temperature fiber optic two-color infrared thermometer has been developed. Radiation from a target is collected via a single 700 micrometer-bore hollow glass optical fiber coated with a metallic/dielectric layer on the inner surface, simultaneously split into two paths and modulated by a gold-coated reflective chopper, and focused onto two thermoelectrically cooled mid-infrared HgCdZnTe photoconductors by 128.8 mm-radius gold-coated spherical mirrors. The photoconductors have spectral bandpasses of 2 - 6 micrometer and 2 - 12 micrometer, respectively. The modulated detector signals are recovered using lock-in amplification. The two signals are calibrated using a blackbody (emissivity equal to 1) of known temperature, and exponential fits are applied to the two resulting voltage versus temperature curves. Using the two calibration equations, a computer algorithm calculates the temperature and emissivity of a target in real time, taking into account reflection of the background radiation field from the target surface.