Characteristics of Optimal Wavelengths Selection for High Temperature Quadrispectral Pyrometer in Near-infrared Spectral Range for Metals with Non-linear Emissivity

This article examines the behavior of the quadri spectral method in the design of a pyrometer applicable to the heat treatment of metals. The quadri spectral pyrometer incorporates four different optical filters that filter the four spectra to be used and converge them towards the four detectors of the device. The light energy from these spectra will be converted by the detectors into a processable electrical signal. The application of the nonlinear model known as Temperature by Nonlinear Model (TNL) will calculate and select these four wavelengths. This method applies inverse calculus, exploiting Planck's relation for thermal radiation by setting the temperature and then determining the wavelengths using ordinary least squares. With this model, the four wavelengths will be selected sequentially by modeling the emissivity of the metal as a second-degree polynomial. The obtained wavelengths will be subjected to various criteria to choose the best groups for a suitable pyrometer intended for high-temperature metal treatment. Those criteria are flux sensitivity to wavelength and temperature, standard deviation at temperature, and the minimum difference between two successive wavelengths. The various tests against the criteria, given the non-linearity of the emissivity of metals, characterize the model in high temperatures in order to proceed with such a pyrometer design.