Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 144

4.4.5.1 Advantages and Limitations of Single-Shot Spectral Imaging

The advantages of the method are clear, as it provides the full spectral image in
a time range that is limited only by the exposure time that provides a good SNR.
It is an excellent solution for applications where imaging time is a major factor.
One such example is the application of retinal imaging. It is well known that oxy-
and deoxyhemoglobin have a somewhat different spectral absorption. By measuring
the spectral image of the retina through a fundus camera, the spectral analysis can
provide oxygen saturation maps that indicate on possible ischemic regions on the
retina. This was demonstrated with CTIS with an exposure time of 3 ms [ 49 ].
The method has some limitations as the quality of the spectrum that is extracted
depends on the spatial-frequency distributions of the image and therefore should be
considered for the application. It also has a limited field of view due to the region
on the array detector that collects the higher-order dispersion information.

4.4.6 Illumination-Scan Spectral Imaging

All the spectral imaging setups described thus far share in common the requirement
of a single source of illumination (typically white light source), while the heart of
the spectral imaging system lies in the optical path beyond the sample and contains
some fore-optics.
Another possibility is to illuminate the sample with an array of light sources
in different wavelengths in order to measure spectral properties. The detector in
this case collects the light that is transmitted through the sample. It can also be the
emitted light, if it is a fluorescence measurement, but this will be a rather uncommon
setup.
The concept of such a setup was introduced recently with an application for
retinal imaging using a series of narrow-band LEDs with different peak wavelength
[ 52 ]. In this system only five different wavelengths were selected and coupled to
the illumination optics of the fundus camera through fiber coupling, but it can
certainly be extended if necessary. In this case, these five wavelengths were selected
to distinguish five different chromophores that are important for retinal inspection.
Such a method is preferred over a system that illuminates with white light and
disperses the colors in front of the detector, simply because much less light falls
on the retina of the measured subject enabling a more comfortable examination.
The LEDs that were selected are powerful enough to allow short exposure time
(approximately 50 ms). For retinal imaging, this is an important factor as the retina
is subjected to constant movements and short exposures reduces image smear.
In principle, all the wavelength-scan methods can be used on the illumination
path before the sample while not using any filter in front of the detector. Conceptu-
ally, such a measurement for bright-field transmission or reflection will be identical
to the systems that are described in Sect. 4.4.2 and with some modifications also to
those that are described in Sects. 4.4.3 and 4.4.4 . It can also be used for fluorescence