Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 147

The most important acquisition settings are the detector's exposure time, the
spectral range to sample, the spectral resolution, and the region of interest and spatial
resolution. Observing a live view image of the sample to be measured is important
and enables to review and adjust parameters of ROI and exposure time as well as to
focus correctly. In some devices like the ones based on scanning (especially grating
and prism based), a preview image would be time consuming as at each acquisition
only a single line from the object is observed. One solution is to use another optical
path for measuring the actual image that will be measured through the spectral
imaging optics after scanning. Such a solution that uses an additional CCD camera
for measuring the live-image is available, for example, with the PARISS system
(Lightform Inc.)
A different problem arises with the systems that scan the spectral image one
wavelength at a time because it may not be possible to see all the details of the
object through a single wavelength band. A simple solution would be to add another
extra filter that transmits the full spectrum, so that it can be used for the focusing
process. Yet, the selection of the adequate exposure time per each wave band is
compromised this way since a broad wavelength filter gives a bright signal at every
pixel. Selection of maximal exposure time that does not saturate the pixels of the
detector would not always become useful as it is most likely that the intensity will
be much dimmer when measuring through each wavelength band separately. The
solution therefore is the experience and knowledge of the nature of the sample or a
trial and error repetitive measure.
A similar problem arises also with the Fourier-based system that scans the
OPD space before transforming it to the spectrum. Here, a typical view of the
image looks like a superposition of a series of fringes on top of the image and
disturbs the observation of a clear image that is necessary for the settings. This was
solved with the SpectraCube system (Applied Spectral Imaging, Israel) by adding
a mechanism in the interferometer that effectively removes the beam splitter out of
the interferometer, leaving a clear image on the detector. The exposure time can be
determined after placing it back again to the interferometer.
In summation, typical spectral imaging acquisition software should have the
ability to set or select the following parameters:
1. The region of interest that will be measured from the full detector area
2. The exposure time
3. The spectral resolution
4. The spectral range
5. Binning (if applicable) that can improve the SNR but decreases the spatial
resolution
6. Other parameters that are relevant for the specific hardware being used, such
as the zero filling in Fourier-based systems or the relative exposure time of
measuring through each filter with the wavelength-scan systems
Not all applications require the whole set of the abovementioned requirements.