Biophotonics_Concepts_to_Applications

reflecting) adjacent wavelengths over extended spectral ranges. The wavelength
bandwidth often is described in terms of thefull-width half-maximum(FWHM)
response of afilter. As shown in Fig.5.11, the FWHM is the width of the spectral
transmission window at the half-power point.
A basic application of a dichroicfilter (or a TFF) is to separate a beam of light
into two beams with differing spectral regions, as Fig.5.12illustrates. Reciprocally,
by reversing the arrows in Fig.5.12the dichroicfilter can combine two optical
beams from different spectral bands into a single optical beam. Such a device

Av er ag e

Center transmission

wavelength

50%

Cut-off

50%

Cut-on

Longpass filter Bandpass filter Shortpass filter

Transmission (%)

Relative wavelength region

0

100

(a) (b) (c)

50

Average

Center transmission

wavelength

50%

Cut-off

50%

Cut-on

Fig. 5.11 Three categories of dichroicfilters and some terminology

450

450

450

Antireflection

coating

Dichroic

filter coating

Substrate

Incident beam

Reflected

beam

Transmitted

beam

Fig. 5.12 Characteristics of a
generic dichroic mirror for
beam splitting

5.6 Optical Filters 139