They are composed of transparent glass or quartz substrate on which multiple thin layers of dielectric material, sometimes separated by spacer layers .
Permit great selectivity.
White Light Source
630 nm BandPass Filter
620 -640 nm Light
520 nm Long Pass Filter
>520 nm Light
575 nm Short Pass Filter
<575 nm Light
Standard Short Pass Filters
They used to direct light in different spectral region to different detectors.
They are interference filters , long pass or short pass.
"dichroic" Di- is Greek for two, and -chroic is Greek for color - from Greek dikhroos, bicolored
Dichroic Filter/Mirror at 45 deg
Filter acting as a DICHROIC
Construction of Filters
Constructive and destructive interference occurs between reflections from various layers
Transmission determined by :
thickness of the dielectric layers
number of these layers
angle of incidence light on the filters
Such as coloured glass filters which absorb unwanted light.
Consist of dye molecules uniformly suspended in glass or plastic.
Remove much more of the unwanted light than do the interference filters
Will often fluoresce (not good!)
Bandpass filters: characterized by there
T max and (the Full Width at Half Maximum) FWHM
Notch filters are band pass filters in the upside down position
Long pass and Short pass filters: characterized by their T max and cuton, cutoff wavelength.