One of the prerequisites for measuring temperature according to Planck's law is to accurately determine the effective wavelength of the precision photoelectric pyrometer. It is a function of the spectral transmittance of the pyrometer optical system, the spectral response of the photodetector and the measured temperature. Its value mainly depends on the spectral characteristics of the interference filter. The interference filter has significant inhomogeneity, and the effective wavelength of the pyrometer must be determined by the in-situ measurement method to reduce the uncertainty of the effective wavelength. Since the selected interference filter with a center wavelength of about 660 nm has a sub-peak near 830 nm, its peak transmittance is on the order of 10-4. This sub-peak will cause the effective wavelength to increase rapidly with decreasing temperature at lower temperatures.

In order to reduce the impact of the secondary peak on the effective wavelength, a heat-absorbing glass is added in front of the interference filter, so that the effective wavelength is significantly reduced with the temperature change, and the measurement uncertainty of the photodetector spectral response is also reduced. The effect of uncertainty. The spectral transmittance and average effective wavelength calculation results of the interference filter before and after adding heat-absorbing glass. After adding heat-absorbing glass, the change of effective wavelength with temperature is significantly reduced.