VNIIOFI Spectroradiometer Based on a Circular Variable Filter for the Spectral Range from 2.5 μm up to 14 μm

The VNIIOFI Spectroradiometer (VSR) has been developed for the Medium Background Facility which is intended for calibrations and comparisons of low‐temperature sources in the temperature range from −60 °C up to +80 °C. The VSR is designed to measure the radiance temperature of the blackbody sources in the spectral range from 2.5 μm up to 14 μm in a cryogenic vacuum environment (liquid nitrogen cooled shroud). The VSR is based on the Circular Variable Filter (CVF) assembly with a slit aperture and an InSb‐CdHgTe two color sandwich detector with an integral Stirling cooler. The Circular Variable Filter covers the 2.5 μm to 14.1 μm region in three segments. The width of the slit equal to 1.2 mm provides a spectral resolution from 80 nm to 360 nm for the spectral range from 2.5 μm to 14.1 μm accordingly. The Circular Variable Filter assembly is rotated by a stepper motor. The Cassegrain telescope is used as a foreoptic unit. Radiation passes through the CVF and is focused on the detector with help of the ellipsoidal mirror. Radiation from a low‐temperature (77 K) blackbody is used as a reference level of radiation. The instrument has a field of view of 8.9 mrad with a distance to the object being equal to 2250 mm. The spectral and temporal characterizations of the spectroradiometer are reported. The facility for CVF calibration is described. Temperature resolutions for the calibrated sources are given for different temperatures and for different wavelengths. A brief description of the design, operation principles and specifications of the main parts of the spectroradiometer such as the Cassegrain telescope, CVF and detector assembly are presented. The measurement uncertainties of radiance temperature associated with spectral bandwidth are discussed.