There are different classification method of filters. It can be classified according to spectral band, spectral types, bandwidth, wavelength, features of coating, industry application etc. The following will introduce two common classification ways.
1.Classified according to spectral band
According to the length of spectrum, filters can be divided into ultra violet filters, visible light filters, near infrared filters, infrared filters and far infrared filters.
The wavelengths are listed as follows:
Ultra violet filters—180~400nm
Visible light filters—400~700nm
Near infrared filters—700~3000nm
Infrared filters—over 3000nm~10μm
Far infrared filters—15~1000μm
2.Classified according to spectral types
When classfied accroding to spectral types, filters can be broken into bandpass filters, short-wavelength pass filters and long-wavelength pass filters.
2.1 Bandpass filters
Bandpass filters allow a selective specific band of light passing through and band of light outside this range will be blocked. Its main optical indexes are central wavelength(CWL), full width at half maximum(FWHM), center wavelength transmittance(Tp) and blocking range. Bandpass filters can also be divided into narrow band (<2nm to 10nm) and broad band (50nm and 80nm).
2.2 Short-wavelength pass filters
Short-wavelength pass filters transmit all wavelengths shorter than the specified cut off wavelength.
2.3 Long-wavelength pass filters
Long-wavelength pass filters transmit all wavelengths longer than the specified cut on wavelength.