Wideband filter radiometers for blackbody temperature measurements

Abstract

The use of high-temperature blackbody (HTBB) radiators to realize primary spectral irradiance scales requires that the operating temperature of the HTBB be accurately determined. We have developed five filter radiometers (FRs) to measure the temperature of the National Research Council of Canada's HTBB. The FRs are designed to minimize sensitivity to ambient temperature fluctuations. They incorporate air-spaced colored glass filters and a Si photodiode detector that are housed in a cell whose temperature is controlled to AE0.1 C by means of annular thermoelectric elements at the front and rear of the cell. These wideband filter radiometers operate in four different wavelength bands. The spectral responsivity measurements were performed in an underfill geometry for a power-mode calibration that is traceable to NRC's cryogenic radiometer. The spectral temperature sensitivity of each of these FRs has been measured. The apertures for these FRs were cold-formed by swaging machine-cut apertures onto precision dowel pins. A description of the filter radiometer design, fabrication and testing, together with a detailed uncertainty analysis, is presented. We derive the equations that relate the spectral irradiance measured by the FRs to the spectral radiance and temperature of the HTBB, and deal specifically with the change of index of refraction over the path of the radiation from the interior of the HTBB to the FRs. We believe these equations are more accurate than recently published derivations. Our measurements of the operating temperature of our HTBB working at temperatures near 2500 K, 2700 K and 2900 K, together with measurements using a pyrometer, show agreement between the five filter radiometers and with the pyrometer to within the estimated uncertainties.

Spectral irradiance calibrations at the National Research Council of Canada (NRC)

“…This HTBB is designed to operate over the temperature range from 1800 to 3500 K with an emissivity of 0.999. We have recently constructed the required filter radiometers and performed our first measurements of the temperature of the HTBB at temperatures of approximately 2500, 2700 and 2900 K [21]. Although it can be seen from Equation (6) that only one filter radiometer is required, we have constructed and used five filter radiometers with center wavelengths between 400 and 800 nm.…”

Thermodynamic temperature by primary radiometry

“…Additionally, the uniformity of the filter radiometer and transfer detector has to be known as the calibration geometry differs from the measurement geometry. A description of this method can be found in [24,25]. For the hybrid method, the set-up is similar to the irradiance method, but a lens is introduced to enable the measurement of smaller radiance sources than the irradiance method.…”

Advancements in NRC’s Primary Spectral Irradiance Scale Realization

“…Now, NRC has a facility equipped with a high temperature blackbody (HTBB) that is implemented as a primary standard radiometric light source in spectral irradiance measurements. The thermodynamic temperature of the HTBB is determined using a wide-band filter radiometer [4], which has a spectral responsivity calibration SI-traceable to the NRC optical power scale through the NRC absolute cryogenic radiometer IOP Publishing doi:10.1088/1742-6596/2149/1/012005 2 [5,6], giving a source and detector-based spectral irradiance scale realization (Figure 1). The design of the NRC spectral irradiance facility, measurement procedure, and uncertainty analysis are presented.…”