Bandpass Interference Filters
Bandpass
Interference Filters are based on the Fabry-Perot effect.
Stacks of reflective thin films are arranged to reinforce transmittance
at a particular centre wavelength ( CWL) and to cancel out at other wavelengths.
Two stacks of reflective films separated by a half wave spacer form a
cavity. Two or three cavities are
usually superimposed to increase the rejection.
The
bandwidth of the filter is normally specified as the full width at half maximum
(FWHM). Typical values are 10nm and
40nm. To eliminate far off regions
of the spectrum a glass blocking layer is bonded to the filter.
These filters are designed for use at normal incidence but if tilted the
centre wavelength will shift towards shorter wavelengths.
They will operate over a wide range of temperature but as the temperature
is increased the centre wavelength will move to longer wavelength and the
bandwidth will be increased.
Standard
filter diameters are 25.4mm or 50.8mm but other sizes can be accommodated on
request.