Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 141

4.4.4.2 Advantages and Limitations of Fourier-Based Spectral Imaging

This method has the advantage that the intensity at each wavelength is collected
throughout the whole duration of the measurement. Therefore, it increases the
efficiency of light collection for a typical spectrum. It also allows selecting the
required spectral resolution without any changes to the hardware, simply by setting
the total measured OPD. This provides an important advantage because the required
spectral resolution may vary significantly from one application to another. It also
has an effect on the spectral image acquisition time, which gets shorter when a
lower spectral resolution is selected. The Sagnac interferometer provides another
degree of freedom for the spectral resolution. As shown in Eq. 4.6 , the dependence
of the OPD as a function of the rotation angle depends also on the constant C .This
constant depends on the optical alignment of the interferometer elements and it can
be tuned for optimal measurements. Therefore, if very high spectral resolutions are
required, the system can be aligned to have a relatively large C , which means that
a large total OPD can be reached and therefore also a high spectral resolution. Note
that the maximal rotation angle of the interferometer is limited (see Fig. 4.16 )and
therefore this additional degree of freedom becomes rather important.
The improvement of the spectral resolution by increasing the parameter C is
limited also because of the pixel size. While increasing C , the intensity falling on
a single pixel along one axis will also have an OPD that will vary along the pixel.
This is physically limited to a change that is smaller than OPD max <=2,whichis
the Nyquist criteria and practically even to half of that.
The advantage of collecting the intensity of all wavelengths at each OPD sets
a limitation on the Fourier method, namely, it is not possible to skip unimportant
parts of the spectral range. The Fourier-based system measures the whole spectrum
in a given range. This limitation can partially be overcome by use of sets of single
band filters. Another issue related to this limitation occurs in cases where there is a
combination of spectral features that has different intensity level, as an example
assume that the intensity in the range 400-500 nm is relatively high while the
intensity in the range 700-800 is low. At every OPD, the intensity that is collected
is a weighted sum of the whole spectrum, and because of noise, the signal from
the low-intensity part of the spectrum may be affected due to the Poisson noise of
the signal from the high-intensity part. This problem can be resolved, again, with
the use of a filter to block the more intense parts of the spectrum.

4.4.5 Full Spectral Images in a Single Shot

When the principle of measuring a spectral image was described in Sect. 4.3.1 ,it
stated that the ultimate spectral imaging device would be able to capture the whole
spectral information I.x; y; / at a single shot. It is clear, however, that this is
impossible with a 2D detector, unless some compromise is done. In some cases, the
compromise is not necessarily a disadvantage. As an example, consider a case where