The filter is used to attenuate (absorb) certain light wavebands or select a small range of light waves to pass through, while reflecting (or absorbing) other undesired wavebands. By changing the structure of the filter and the optical parameters of the film, various spectral characteristics can be obtained, so that the filter can control, adjust and change the transmission, reflection, polarization or phase state of the light wave. A common feature of filters is that there is no filter to make the image of celestial objects brighter, because all filters absorb certain wavelengths, which makes the object darker. The filter is made of plastic or glass and added with special dyes. The red filter can only let red light pass through, and so on.

Optical filters are made by adding special dyes to plastic or glass substrates or evaporating optical films on their surfaces to attenuate (absorb) certain light wavebands in light waves or pass light waves in a small range of light waves, and reflect (Or absorb) other undesired bands. By changing the structure of the filter and the optical parameters of the film, various spectral characteristics can be obtained, so that the filter can control, adjust and change the transmission, reflection, polarization or phase state of the light wave.