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Basic Concepts and Parameters of Optical Filters

1. The basic concept of optical filter

An optical filter is simply an optical device used to selectively filter the required radiation band. The substrates are mostly optical materials such as white glass, quartz, colored glass or plastic resin.

2. Optical filter parameters

1) Distinguish the filter according to the spectral band:

The filter is divided into ultraviolet filter, visible light filter, near-infrared filter, infrared filter, and far-infrared filter according to the distribution length of the spectrum (that is, the area where the spectrum is located).

The spectral wavelength range is as follows:

UV filter 180~400nm

Visible light filter 400~700nm

Near infrared filter 700~3000nm

Infrared filter 3000nm~10um above

2) Distinguish filters according to their spectral characteristics:

Bandpass filter, short-wave cut-off filter, long-wave cut-off filter.

Bandpass filter: select the light of a specific band to pass, and cut off the light outside the pass band. The optical indicators are mainly center wavelength (CWL), half bandwidth (FWHM), center wavelength transmittance (Tp), cut-off degree and cut-off range. Divided into narrowband and broadband according to the bandwidth, usually distinguished by the value of the bandwidth than the center wavelength, less than 2% is defined as narrowband, and greater than 2% is defined as broadband. For example, the narrowband BP808-10 filter, and the wideband filter such as BP650-80.

Short-wave pass type (also called low pass): Light shorter than the selected wavelength passes through, and light longer than the wavelength is cut off. Such as infrared cut filter, IR-CUT-650.

Long pass type (also called high pass): Light longer than the selected wavelength passes through, and light shorter than that wavelength is cut off. For example, infrared transparent filter, LP-700.

3) Explanation of terms related to filters:

Central Wavelength (CWL): The wavelength used by the filter in practical applications. If the main peak of the light source is an 850nm led lamp, the required central wavelength is 850nm.

Peak transmittance (TP): Assuming that the initial value of light is 100%, part of it is lost after passing through the filter. Through spectrum measurement, it is only 85%, then the optical transmittance of this filter can be set as (Tp)>80%.

Half bandwidth (FWHM): Simply put, it is the wavelength corresponding to 1/2 of the highest transmittance, and the left and right wavelength values are subtracted. For example, the peak value is preferably 90%, 1/2 is 45%, the left and right wavelengths corresponding to 45% are 875nm and 825nm, and the half bandwidth is 50nm.

Blocking: The transmittance corresponding to the cutoff zone. Since it is very difficult to achieve a transmittance of zero, the only choice is that the transmittance is close to zero. However, usually the transmittance of more than 10-5 can meet most of the requirements of use, and it is converted into an optical density value, which is expressed by OD>5.

Cut-off band: The minimum acceptable range of unwanted wavelengths. Since the response range of most photosensitive devices for electronic imaging is 350-950nm, in practice, the range can be slightly wider than this range. The cut-off range of ultraviolet and infrared is more complicated than this, and needs to be determined according to the response range of the probe used.

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