Abstract
In the temperature range from room temperatures to above the Ag, Au or Cu points, lowest achievable calibrations of radiation thermometers are possible with direct measurements of ITS-90 fixed point blackbodies. As compared to fluid-bath or heat-pipe blackbodies, both temperatures and emissivities of ITS-90 blackbodies are known with lower uncertainties. However, commercially available radiation thermometers that operate in the 8 μm to 14 μm which are capable of direct measurements of these fixed points with < 6 mm diameter openings do not exist.
We describe the design, construction and characterization of an 8 μm to 14 μm thermal infrared RT which has sufficiently low size-of-source effect (SSE), small target size, and wide temperature range of operation so that water-bath BB and ITS-90 fixed-point BBs from In- to Al-point with 6 mm diameter cavities can be used for calibrations. The RT is constructed using ZnSe lenses and a spectrally filtered pyroelectric detector. The detector and interal optics are temperature stabilized to reduce thermal noise. The target size of the RT has been designed to be about 4 mm at a distance of 40 cm using a field-stop diameter of 1.38 mm. The field stop which is made of mirror-polished stainless steel is tilted by 11˚ from the optical axis. This tilt also functions to divert the reflected thermal radiation away from the objective lens in order to reduce the SSE and also to direct the visible radiation to an optical telescope. The objective is adjusted to focus the thermal radiation and the visible radiation is used for direct viewing of the target through the same objective. The optical telescope at the side is compensated for chromatic focal shift from the ZnSe lens and is used for visual alignment of the small target onto openings of ITS-90 fixed-point BBs. To reduce the SSE, a Lyot stop is also used.
Using this RT, freezing plateaus of In, Sn, Zn and Al points were measured. The use of a lock-in amplifier (LIA) results in digitization of the temperature measurements especially at higher temperatures such that the smallest temperature interval which can be measured at the Al point is about 50 mK. To reduce this digitization, a 24-bit digitizer is used for a software-based implementation of a LIA to achieve 2 mK or a factor of 25 increase in resolution over the commercial LIA. Temperature measurements of using both a conventional lock-in and the digitizer-based implementation will be discussed.
8 μm to 14 μm Thermal Infrared Radiation Thermometer Calibrated Using ITS-90 Fixed Points From In to Al
In the temperature range from room temperatures to above the Ag, Au or Cu points, lowest achievable calibrations of radiation thermometers are possible with direct measurements of ITS-90 fixed point blackbodies. As compared to fluid-bath or heat-pipe blackbodies, both temperatures and emissivities of ITS-90 blackbodies are known with lower uncertainties. However, commercially available radiation thermometers that operate in the 8 μm to 14 μm which are capable of direct measurements of these fixed points with < 6 mm diameter openings do not exist.
We describe the design, construction and characterization of an 8 μm to 14 μm thermal infrared RT which has sufficiently low size-of-source effect (SSE), small target size, and wide temperature range of operation so that water-bath BB and ITS-90 fixed-point BBs from In- to Al-point with 6 mm diameter cavities can be used for calibrations. The RT is constructed using ZnSe lenses and a spectrally filtered pyroelectric detector. The detector and interal optics are temperature stabilized to reduce thermal noise. The target size of the RT has been designed to be about 4 mm at a distance of 40 cm using a field-stop diameter of 1.38 mm. The field stop which is made of mirror-polished stainless steel is tilted by 11˚ from the optical axis. This tilt also functions to divert the reflected thermal radiation away from the objective lens in order to reduce the SSE and also to direct the visible radiation to an optical telescope. The objective is adjusted to focus the thermal radiation and the visible radiation is used for direct viewing of the target through the same objective. The optical telescope at the side is compensated for chromatic focal shift from the ZnSe lens and is used for visual alignment of the small target onto openings of ITS-90 fixed-point BBs. To reduce the SSE, a Lyot stop is also used.
Using this RT, freezing plateaus of In, Sn, Zn and Al points were measured. The use of a lock-in amplifier (LIA) results in digitization of the temperature measurements especially at higher temperatures such that the smallest temperature interval which can be measured at the Al point is about 50 mK. To reduce this digitization, a 24-bit digitizer is used for a software-based implementation of a LIA to achieve 2 mK or a factor of 25 increase in resolution over the commercial LIA. Temperature measurements of using both a conventional lock-in and the digitizer-based implementation will be discussed.