Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 145

measurements, but then the measurement is somewhat different, as it will provide
information on the excitation spectrum and would still require selecting relevant
filters in front of the detector so that it will match the emission spectra of the
fluorochromes.
Another approach for illumination-type spectral imaging was demonstrated by
using a digital micromirror device or DMD [ 53 ]. A DMD is constructed of an array
of mirrors, typically 10 6 , each with dimensions of 10-15 square m that can be
rotated around their axis by applying electrostatic forces. A setup is constructed
with light source and a diffraction element such as a grating or a prism, and the
dispersed light illuminates the DMD. Each mirror is separately controlled so it will
reflect the light that impinges on it onto the entrance slit to a fiber that can be
connected to different optical devices such as an endoscope. The mirrors can be
controlled at a rather fast speed of up to 20 kHz.
The DMD unit can also be used for constructing another type of spectral imaging
system by allowing the device to illuminate a single spot at a time with a single
mirror. Once the spot is illuminated, the light that is emitted by the sample can be
detected with a normal spectrometer or spectrograph, and the DMD can now be
used to scan each of the pixels of the image. Such a device was used for spectral
imaging as well as for time-resolved measurements [ 54 ]. A somewhat similar
method was developed based on a spatial light modulator (SLM). The system called
programmable array microscope (PAM) enabled the measurement of the spectral
image of a three-dimensional object [ 45 ].
Another interesting method that was developed using spectroscopy actually
twists things up and uses the spectrum in order to gain spatial information [ 55 ]. The
method called spectrally encoded endoscopy (SEE) uses diffractive optics to encode
different spatial regions, each with a unique wavelength. Therefore, the spectrum
is dispersed along one dimension, and the readout of each wavelength provides
information on a single point in the image. In order to cover the other spatial
dimension, scanning is performed. A similar method that uses an interferometer
in a way that is similar to optical coherence tomography (OCT) was also used in
order to measure the 3D structure of an object [ 56 ].

4.5 System Requirements

The previous section covered optical and design methods defining the core of any
spectral imaging system. In order to achieve a successful measurement, the spectral
imaging system must include other optical elements such as light sources, control
mechanisms and software for acquisition and analysis.