The filter is made of plastic or glass and then added with special dyes. The transmittance of the glass piece is originally similar to that of air. All colored light can pass, so it is transparent, but after dyeing the dye, the molecular structure changes, the refractive index also changes, and the passage of some color light changes. For example, a white light passes through a blue filter, and a blue light is emitted, while green light and red light are rare, and most of it is absorbed by the filter.