Advanced Spectroscopy Technique for Biomedicine - Biomedical Optical Imaging Technologies: Design and Applications - page 10

a

Toroidal mirror

b

Fixed mirror

Beam
Splitter

Moving
mirror

Detector

Notch filter

Lens

Reflection grating

Entrance slit

Exit slit Entrance slit

Fig. 1.5 Schematic drawings of a dispersive monochromator ( a ) and a Fourier-transformed
spectrometer ( b )

include dispersive monochromator, dispersive spectrograph, and Fourier transform
spectrometer, as illustrated in Fig. 1.5 . Both the dispersive spectrograph and the
dispersive monochromator are based on diffraction gratings or prisms, but the
Fourier transform spectrometer is based on the Michelson interferometer. The con-
figuration of a monochromator and a spectrograph is very similar. A monochromator
scans a single wavelength or wave band to its exit slit at a time by rotating its
dispersive components usually a grating or prism, while a spectrograph images a
range of wavelengths simultaneously onto the detector plane. In monochromator-
based spectrometer, the spectral signals are captured by a single-channel detector,
such as a photomultiplier tube (PMT). For spectrograph-based spectrometer, the
whole spectrum is acquired simultaneously by an array detector, such as a charge-
coupled device (CCD) camera.

1.2.4 Filters and Polarizers

Optical filters and polarizers are the most common components for spectroscopy.
The most common filters are neutral density filters (ND), long-pass filters (LP),
short-pass filters (SP), band-pass filters (BP), and notch filters (NF). Neutral density
filters are used to attenuate the light for the full band in some applications where
the intensity is too strong for the detectors. Long-pass filter and short-pass filter
are also called edge filters that only allow the longer wavelengths for LP or shorter
wavelengths for SP than the cutoff wavelength to be transmitted. Long-pass filter
is usually used to transmit the signal and block the excitation light such as in
fluorescence and Raman spectroscopy. Short-pass filter is used to remove the longer
wavelength signal such as blocking the excitation light in the measurement of two-
photon fluorescence spectra.