Absolute light calibration using S-type thermocouples

Abstract

Absolute light calibration in optical measurements requires the use of a calibrated light source. The most common sources are blackbody furnaces or tungsten lamps, which are both problematic due to their expense and the requirement for frequent recalibration. In combustion research, the availability of an absolute light calibration makes possible the quantitative measurement of several quantities of interest (e.g., soot volume fraction and number density of species measured with fluorescence or chemiluminescence). In this study, a low-cost, simple, yet high-accuracy method is described that uses widely available S-type thermocouples to do absolute light calibration. The spectral emissivity of Pt and Pt–10% Rh wires was measured in the visible range. The measured spectral emissivity, temperature, and diameter of the heated thermocouple wires allow them to serve as a light source with spectral radiance that can be calculated by Planck’s law. The proposed absolute light calibration method with S-type thermocouples was used to determine the number density of excited-state CH radicals in a lifted, coflow laminar diffusion flame of methane and the soot volume fraction in coflow laminar diffusion flames of ethylene with varying amounts of nitrogen dilution. The results were compared with previous measurements and excellent agreement was achieved.