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Optical filters and it’s basic parameters

An optical filter is an optical component used to selectively filter the required radiation band . The substrate is mostly transparent glass , fused silica , colored glass , or plastic resin and other optical glasses.

Optical filters can be classified according to spectral distribution ,spectral type , bandwidth ,wavelength ,coating characteristics , applications and so on . 

1. Classified as per spectral distribution

As per spectral distribution , we usually classify the optical filters as UV band filter , visible band filter , Near-infrared band filter , infrared band filter ,and far-infrared band filter .

Spectral wavelength range as per coating :

UV : 180nm – 400nm

Visible: 400nm – 700nm

Near-infrared : 700nm – 3000nm

Infrared : 3000nm – 10um over

2. Classified as per spectral type

As per spectral type , we usually classify as bandpass filter , shortpass filter , longpass filter , dichroic mirror etc. 

Bandpass filter : The required wavelength pass through and blocking the wavelengths out of passband . Optical parameters of bandpass filters are center wavelength (CWL), bandwidth (FWHM), transmittance (Tpeak),blocking value(OD) and blocking wavelengths range . As per FWHM , we can also classify it as narrow bandpass filter and wide bandpass filter . How to judge a filter is narrow band filter or wide band ? It generally can CWL divided by FWHM . If value smaller than 2% , it is narrow band filter . Otherwise is wide band filter . For example , BP808-10 is a narrow band, and BP650-80 is a wide band filter .

Typical spectrum curve of bandpass filter :

Shortpass filter (Lowpass filter): An optical filter that pass the band lower than required band wavelength band , and at the same time block the band higher than required wavelength band , then it is a shortpass filter (Lowpass filter) . For example : IR CUT 700nm 

Typical spectrum curve of IR CUT 700nm : 

Longpass filter (Highpass filter): An optical filter that block the band lower than required band wavelength band , and pass the band higher than required wavelength band , then it is a longpass filter (Lowpass filter) . For example : LWP680nm

Typical spectrum curve of Longpass filter 680nm:

3.Explanation of basic parameters of optical filters

Center of wavelength (CWL): The wavelength of a filter used in a practical application , for example 850nm LED , we can say it’s CWL is 850nm .

Peak transmittance (Tpeak) & Bandwidth (FWHM):

Assume that the initial value of the light is 100% and part of it is lost after passing through the filter, the left and right wavelength values are reduced . For example ,the peak value is 90% , 1/2 of it is 45% , and the left and right wavelengths are corresponding to 45% is 875nm and 825nm, and then we can say the FWHM is 50nm .