Spectral Filter

A spectral filter is an optical element used to select or eliminate specific spectral characteristics of light. These filters are essential in applications that require the transmittance of an exact wavelength to an analyzer (detector), while other wavelengths are discriminated and filtered out.

Spectral filters are typically based on a substrate that has significant transmittance for the desired wavelength range. When the material’s properties are responsible for light selection, the filter is known as an absorption filter. By adjusting the material’s thickness, the light is gradually filtered to achieve the desired spectral characteristics and define a reasonable waveband.

In contrast, dielectric, thin-film, or interference filters function as reflecting filters. These filters do not absorb light; instead, they select light through reflection or transmission. They consist of several thin layers of refractive dielectric material, typically applied to the substrate via vacuum deposition. Interference effects favor the transmission of specific wavelengths. These filters must confine the waveband to the desired range while avoiding atmospheric absorption, which can affect temperature measurement accuracy. Therefore, bandpass filters are widely used to define cut-on and cut-off wavelengths, effectively limiting the light spectrum on both sides.

Before the light reaches the detector, its characteristic emission spectrum is modified due to the transmittance of the optical elements and the spectral sensitivity of the detector. In temperature measurement, this process is primarily influenced by the Planck emission spectrum for blackbody radiation. Given the broad range of this spectrum, it is common to select a specific range that aligns with both the detector’s spectral response and an atmospheric window.

In practical applications, the target material often emits peaks rather than a continuous blackbody spectrum, with emission varying significantly with wavelength. A high emission grade is often preferred, as it leads to a high level of radiation for the measurement device. By knowing this, we can limit the waveband to the relevant spectrum. For example, the P3 device measures the temperature of plastic materials at a specific wavelength. While using a standard wide band for measuring temperature in plastic foils can be misleading, measuring at 3.43 µm for polyethylene is effective.