An accurate instrument for emissivity measurements by direct and indirect methods

Emissivity is a quantity essential to consider when assessing the measurement uncertainty in non-contact temperature measurements. This paper presents a new instrument for measuring emissivity of opaque materials from 200 to 450 °C in the spectral range of 2.1 to 2.5 µm. These ranges are ideal for measuring the temperature of metals, such as aluminium, during manufacture or heat-treating process. The instrument consists of a pair of hemispherical cups coated with Vantablack® and gold respectively, a custom designed radiation thermometer, and a hot plate. This instrument enables both the direct and the indirect methods for measuring emissivity of materials. Use of two identical cups allowed for quantitative analysis of the uncertainty of the instrument to determine the most suitable emissivity measurement range. The expanded uncertainty of the instrument was lower than 0.058 (k = 2) over the entire measuring temperature range. Studies were undertaken using different materials with emissivities ranging from 0.06 to 1. These included: aluminium alloy 6082, stainless steel 304, and HiE-Coat 840M paint. Relative uncertainty analysis indicated that the indirect method was more accurate for measuring low emissivity materials, whereas the direct method was more suitable for all other materials. Our instrument, with experimentally determined measurement uncertainty, aims to offer accurate emissivity references for use in radiation thermometry applications.