Bandpass interference filters are used for isolation of wavelength range.

 

The main parameters of an interference bandpass filter are the peak transmission wavelength, and the full-width-half-maximum (FWHM or bandwidth) of the transmission range. Narrowband filters have 10 nm or less, medium-band filters have about 40 nm, and wideband filters have about 80 nm FWHM.

 

Principal curve of bandpass filter designed for 800 nm is shown.

 

 

 

Hot mirrors reflect infrared (heat) radiation and transmit visible radiation and in this way separate these spectral ranges. Cold mirrors reflect visible radiation and transmit infrared radiation.

 

Spectral curve of hot mirror, designed to reflect 670-1150 nm and transmit 400-630 nm is shown.

 

 

 

 

 

 

Cut-off filters make a rapid transition from virtually total blocking to virtually total transmission.

Long-pass cut-off filters transmit longer wavelengths and block shorter wavelengths. Short-Pass cut-off filters transmit shorter wavelengths and transmit longer.

 

Spectral curve of long-pass cut-off filter is shown.