Applications and working principles of optical filters

Filters are made by adding special dyes to a plastic or glass substrate or by depositing optical films on their surface, used to attenuate (absorb) certain wavelengths of light or to precisely select a narrow range of wavelengths to pass through while reflecting (or absorbing) other unwanted wavelengths. By changing the structure of the filter and the optical parameters of the film layers, various spectral characteristics can be obtained, allowing the filter to control, adjust, and change the transmission, reflection, polarization, or phase state of light waves. Filters are generally classified according to spectral bands, spectral characteristics, film materials, application features, and other characteristics. Filters can be classified by spectral characteristics into bandpass filters (such as NBF-808, BPF940), cutoff filters (such as IBG-650), dichroic filters (such as half-transmitting half-reflecting mirrors), neutral density filters, and reflection filters; by spectral bands into ultraviolet filters, visible light filters, and infrared filters; by application film materials into soft film filters and hard film filters; and by application features into filters for medical biochemical instruments, filters for fluorescence microscopes, and police multi-band hard film filters, etc.

For optical lenses, the filters on them are generally coated, allowing part of the light to pass through or blocking part of the light through special films. There are also dual-peak filters, which are dual-channel coatings, where two layers of film are coated on one lens, achieving the effect of two types of filters. For example, IR650+950 allows 650nm visible light to enter during the day, while at night it allows 950nm infrared light to enter, but the two do not interfere with each other.

Common filters include low-pass filters, infrared filters, infrared cutoff filters, narrowband filters, and beam splitters. Low-pass filters are mainly used on CCD and CMOS, and their functions are: 1. to filter out infrared light from the light, 2. to adjust the incoming light.