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There are two general types of spectral filters for thermal imaging cameras.
One is a neutral density filter (ordinary filter), which attenuates the energy of the entire wavelength range to the same degree. For example, neutral density filters can calibrate thermal imaging cameras to higher temperatures and radiation ranges, such as 3000°C.
Another type of filter is a spectral filter, which shields the radiant energy in a specific band, which is helpful for you to test in the band of interest—for example, shooting and tracking a chemical gas plume visible only in a narrow area of the spectrum or depicting a target through the target flame. Under normal circumstances, the heat generated by the flame will occupy the main part of the image, but the spectral filter can shield it, allowing you to see what you want to see.