Glass surface temperature can be measured using a radiation thermometer operating at a mid-IR wavelength, typically the 3–5 μm band, where the glass is opaque. For optical fiber preforms, the temperature measurement requirement may exceed 2,200°C. Scale realization at national measurement institutes at these temperatures is usually carried out at short wavelengths, typically less than 1 μm. The mismatch in wavelength can lead to significant uncertainties when calibrating a radiation thermometer working at 3–5 μm. To overcome this, a narrow band 3.95 μm radiation thermometer has been built that is designed to be used from 1,000 to 2,500°C. It is calibrated by measurement of high-temperature metal–carbon eutectic fixed-points. The instrument is based on silicon lenses, with a liquid nitrogen (LN2)-cooled InSb detector, and narrow-band interference filter. An anti-reflection coated objective lens/aperture stop focuses onto a field stop giving a 1 mm target, then a collimating lens, and glare stop. All parts visible to the detector, other than the target area, are either at LN2 temperature or are part of a temperature-stabilized housing. A relay-operated shutter that blocks the field stop is used to subtract the background. The size-of-source effect of the instrument has been measured. Gold-point measurements have been made to assess the stability. The device has been calibrated using high-temperature fixed points. A three-parameter fit has been applied and the resultant scale compared to an ITS-90 realization.
