Bandpass filters are optical elements, used in any application area requiring the isolation of a narrow spectral bandwidth and a high signal-to-noise ratio. These filters reject all unwanted energy outside that region by either absorption or reflection.

CWL:Center Wavelength of bandpass filter

Tmax:transmittance of center wavelength

FWHM(full width at half of Tmax): indicates the spectral width at which the signal has fallen to 50% of the peak value.

Cut-on wavelength: the wavelength at which a long pass filter transmits 50%

Cut-off wavelength: the wavelength at which a short pass filter transmits 50%

Angel of Incidence:0 degree.

The passband shifts to blue and becomes worse when a filter is illuminated by a focussed beam or at an angle by a collimated beam.

The blue shift is caused by the reduction of the difference in path length between the transmitted and reflected beams.