Multi-spectral pyrometry—a review

Abstract

In pyrometry measurements, the unknown target emissivity is a critical source of uncertainty, especially when the emissivity is low. Aiming to overcome this problem, various multi-spectral pyrometry systems and processing techniques have been proposed in the literature. Basically, all multi-spectral systems are based on the same principle: the radiation emitted by the target is measured at different channels having different spectral characteristics, and the emissivity is modelled as a function of wavelength with adjustable parameters to be obtained empirically, resulting in a system of equations whose solution is the target temperature and the parameters of the emissivity function. The present work reviews the most important multi-spectral developments. Concerning the spectral width of the measurement channels, multi-spectral systems are divided into multi-wavelength (monochromatic channels) and multi-band (wide-band channels) systems. Regarding the number of unknowns and equations (one equation per channel), pyrometry systems can either be determined (same number of unknowns and equations, having a unique solution) or overdetermined (more equations than unknowns, to be solved by least-squares). Generally, higher-order multi-spectral systems are overdetermined, since the uncertainty of the solutions obtained from determined systems increases as the number of channels increases, so that determined systems normally have less than four channels. In terms of the spectral characteristics of the measurement channels, narrow bands, far apart from each other and shifted towards lower wavelengths, seem to provide more accurate solutions. Many processing techniques have been proposed, but they strongly rely on the relationship between emissivity and wavelength, which is, in turn, strongly dependent on the characteristics of a particular target. Several accurate temperature and/or emissivity results have been reported, but no universally accepted multi-spectral technique has yet been developed.

Determination of Flame Temperatures and Soot Volume Fractions during Combustion of Biomass Pellets

“…The graybody assumption may be a reason for the higher-temperature value of the two-color method. These results illustrate that the accuracy of the emissivity model selection affects the accuracy of the temperature measurement. , …”

“…Pyrolysis of biomass pellets releases volatile matter, and therefrom soot is generated in the flame . The radiation intensity of the continuous spectrum increases with the increase of the wavelength, and the radiant intensity of the continuous spectrum is related to both the concentration of soot and its temperature . The continuous spectrum is used for the multispectral temperature measurement. ,, During the combustion of volatile matter from the three types of biomass pellets, from ignition to burnout, the spectral radiation intensity of the generated flames showed a trend of first increasing and then decreasing.…”

“…The existence of the discontinuous spectrum can cause an interference to the temperature measurement by the multispectral method . Therefore, in this study, the characteristic lines of Na (589.0 nm) and K (766.0 and 770.1 nm) were first removed and, subsequently, the 500–800 nm wavelength range was selected as the temperature measurement interval.…”

Temperature Measurement With Photodiode in Different Operating Modes

“…The methodological error has the greatest influence. It is shown in [2], [3], [5], [15] that when using wide spectral ranges and near the maxima of the spectral sensitivity of the two photodetectors, the methodological error decreases to 3.5%. The instrumental error is comparable to that of spectral ratio pyrometers using two photodetectors and narrow spectral ranges, as well as tandem and matrix photodetectors.…”

Multispectral Mid-Infrared Camera System for Accurate Stand-Off Temperature and Column Density Measurements on Flames

“…The spectral structure of gases, in contrast, is extremely complex, with thousands of discrete absorption-emission lines. For that reason, state-of-the-art flame pyrometry resorts to measuring soot emission, generally in at least two bands the visible range [10]. This approach, however, is limited to sooty flames.…”