Introduction to filter

Optical components used to select the desired radiation band. There are two types: color filters, which are flat glass or gelatin sheets of various colors with a transmission bandwidth of several hundred angstroms. They are commonly used in broadband photometry or installed in stellar spectrographs to isolate overlapping spectral levels. Its main feature is that the size can be made quite large. Thin film filters are divided into two types: thin film absorption filters and thin film interference filters. The former is achieved by chemically etching a specific material substrate to position the absorption line precisely at the desired wavelength. Generally, it has a longer wavelength and is often used as an infrared filter. The latter is a low order, multi-stage series solid Fabry Perot interferometer formed by alternately forming metal dielectric metal films or all dielectric films with a certain thickness on a certain substrate using vacuum coating method. The selection of material, thickness, and series connection method for the film layer is determined by the required center wavelength and transmission bandwidth.

At present, various interference filters with wavelengths ranging from 1 to 500 angstroms from ultraviolet to infrared are available. The peak transmittance of metal dielectric film filters is not as high as that of all dielectric films, but the latter has more severe secondary peak and sideband problems. There is also a type of circular or elongated variable interference filter in thin film interference filters, which is suitable for space astronomical measurements. In addition, there is a type of dichroic mirror filter that forms a 45 degree angle with the incident light beam; Angle placement can decompose a beam of light into two different colors with high and uniform reflection and transmission, suitable for multi-channel multi-color metering. Interference filters generally require vertical incidence, and when the incidence angle increases, they move towards the short wave direction. This feature can be used to align the center wavelength within a certain range.

Due to significant changes in wavelength and peak transmittance with temperature and time, caution must be exercised when using narrowband filters. Due to the difficulty in obtaining large-sized uniform film layers, the diameter of interference filters is generally less than 50 millimeters.