Design and Evaluation of Large-Aperture Gallium Fixed-Point Blackbody

Abstract

To complement existing water bath blackbodies that now serve as NIST primary standard sources in the temperature range from 15 • C to 75 • C, a gallium fixedpoint blackbody has been recently built. The main objectives of the project included creating an extended-area radiation source with a target emissivity of 0.9999 capable of operating either inside a cryo-vacuum chamber or in a standard laboratory environment. A minimum aperture diameter of 45 mm is necessary for the calibration of radiometers with a collimated input geometry or large spot size. This article describes the design and performance evaluation of the gallium fixed-point blackbody, including the calculation and measurements of directional effective emissivity, estimates of uncertainty due to the temperature drop across the interface between the pure metal and radiating surfaces, as well as the radiometrically obtained spatial uniformity of the radiance temperature and the melting plateau stability. Another important test is the measurement of the cavity reflectance, which was achieved by using total integrated scatter measurements at a laser wavelength of 10.6 µm. The result allows one to predict the performance under the low-background conditions of a cryo-chamber. Finally, results of the spectral radiance comparison with the NIST water-bath blackbody are provided. The experimental results are in good agreement with predicted values and demonstrate the potential of our approach. It is anticipated that, after completion of the characterization, a similar source operating at the water triple point will be constructed.

NIST Radiance Temperature and Infrared Spectral Radiance Scales at Near-Ambient Temperatures

“…Figure 9 shows the typical spatial uniformity of the blackbody radiance temperature across the exit aperture. Results of a comparison of a WBBB with the Ga BB are presented in another paper [16].…”

“…The fixed-point blackbodies, all of which were designed, built, and characterized at NIST, operate at Ga (29.765 • C), In (156.599 • C), and Sn (231.928 • C) fixed-point [16] at this conference. In and Sn blackbodies have 7-mm diameter exit apertures and are designed to have high IR emissivity and plateaux of several hours duration to allow long measurement times.…”

Evaluation of Blackbody Cavity Emissivity in the Infrared Using Total Integrated Scatter Measurements

“…Further details of these fixed-point BBs can be found in [11][12][13]. Larger cavities and larger apertures are used for the near-ambient temperature range water bath and Ga fixed-point BBs [13]. Both the water bath and initial Ga cavities were two-section cone designs.…”

“…The water bath cavity is 108 mm in diameter and 340 mm deep, with interchangeable apertures ranging down to 25 mm. In addition, the Ga BB incorporates a concentrator (reflector) that is described in a companion paper [13], but was not used in the measurements described here. After the initial ITIS reflectance measurements were performed, the results (described in Section 3) led to a modification of the design: inversion of the cone near its end (at the 6 mm diameter point).…”

“…Outside the center, the reflectance level is about a decade lower. After these results were obtained, the cone was modified to include an inverted cone section in the center, as described in [13]. The reflectance map of the modified cavity is shown in Fig.…”