Filter is in the plastic or glass substrate by adding special dyes or on the surface of the vapour plating optical film made of light waves used to attenuate (absorb) some of the light band or to accurately select a small range of bands of light waves through, and reflect (or absorb) the other unwanted bands through. By changing the structure of the filter and the optical parameters of the film layer, you can get a variety of spectral characteristics, so that the filter can control, adjust and change the transmission, reflection, polarisation or phase state of light waves. Filter classification method in general according to the spectral band, spectral characteristics, film materials, application characteristics and other characteristics of classification. Filters can be classified according to spectral characteristics into bandpass filters, cut-off filters, two-way spectral filters (such as half-transmission and half-reflective mirrors), neutral density filters, reflective filters, etc.; filters can be classified according to spectral wavelengths into ultraviolet filters, visible filters, and infrared filters; filters can be divided into soft film filters and rigid film filters according to the application of film material classification; filters can be classified according to application characteristics into medical biopharmaceuticals, medical equipment, and other medical equipment. Filters can be classified into medical biochemistry filters, fluorescence microscope filters, police multi-band hard film filters and so on.

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