480
S. Biswas et al.
16.3.1.1 IR Mapping
For several decades now, IR spectrometers have used thermal sources coupled to
microscopes equipped with single-element detectors for acquiring point spectra.
Spectra recorded could be assigned to pixels in a sequential fashion with the help of
raster scanning, to create a point-by-point mapped image. there were two big disadvantages to this process. First, the signal to noise ratio (SNR) was poor because
benchtop thermal sources are of relatively low power. Spatial resolution was poor
as well due to using IR opaque apertures to define the spatial resolution. In general, the IR absorbance images formed lacked clarity and accuracy, especially when
compared to optical microscopy. Second, due to raster scanning a single pixel, data
acquisition was an extremely slow process and scanning tissue sections of reasonable size was not feasible.
Recent advances in scanning technology, noise filtering and better hardware
have resulted in some impressive advances. New products are still being introduced
with single element detectors that provide exceptional performance. one advance
has been to use smaller AC-coupled detectors, in which low noise can be further
rejected by electronic filtering. the SNR of such systems can be exceptionally high
and can provide exceptional spectra even from small aperture sizes due to better engineered systems that are dedicated to single element data recording. A notable exception has always been the use of synchrotron sources, which allowed for small aperture sizes and exceptional quality data [14–16]. this coupling essentially provides
a large signal [17]. While there is no doubt that the large signal improves throughput
and is very effective for small aperture sizes, simply using a brighter source is not
a guarantee of the best performance. For example, the brighter source may require
faster mirror scanning, leading to larger Fourier frequencies and other sources of
noise that may overwhelm detector noise. Shot noise in the source may also start to
become apparent in some cases on using a brighter source. hence, we posit that any
instrument that is designed around a specific source or detector carefully also consider data acquisition. While this is a fine point and a simplification is often made
that using a “better” source or detector will improve the measurements, it is truly the
integrated system in which every matched part is optimally designed, that will provide the best performance. It turns out that the flux provided by a synchrotron does
not lead to unstable recording or encroachment into regimes where the intensity is a
problem, emerging laser sources must be cautiously integrated.
Synchrotron sources are of significant importance because synchrotron beams can
be collimated very efficiently and can form bright, very high intensity sources that
are almost point-like. Covering point areas of the sample and, as with globar sources,
single-element detectors used for acquiring point spectra by raster scanning can create
a mapped image. this makes them an ideal source to be used to validate fundamental
theoretical advances in IR microscopy. In some ways, the synchrotron is an ideal
source for IR microscopy. While it is obvious that the cost and availability of such
systems cannot compete with the globar, synchrotron-based systems nevertheless represent a useful avenue for proof-of-principle studies and provide a performance yardstick for all of IR microscopy. Even under these conditions, there is one disadvantage
S. Biswas et al.
16.3.1.1 IR Mapping
For several decades now, IR spectrometers have used thermal sources coupled to
microscopes equipped with single-element detectors for acquiring point spectra.
Spectra recorded could be assigned to pixels in a sequential fashion with the help of
raster scanning, to create a point-by-point mapped image. there were two big disadvantages to this process. First, the signal to noise ratio (SNR) was poor because
benchtop thermal sources are of relatively low power. Spatial resolution was poor
as well due to using IR opaque apertures to define the spatial resolution. In general, the IR absorbance images formed lacked clarity and accuracy, especially when
compared to optical microscopy. Second, due to raster scanning a single pixel, data
acquisition was an extremely slow process and scanning tissue sections of reasonable size was not feasible.
Recent advances in scanning technology, noise filtering and better hardware
have resulted in some impressive advances. New products are still being introduced
with single element detectors that provide exceptional performance. one advance
has been to use smaller AC-coupled detectors, in which low noise can be further
rejected by electronic filtering. the SNR of such systems can be exceptionally high
and can provide exceptional spectra even from small aperture sizes due to better engineered systems that are dedicated to single element data recording. A notable exception has always been the use of synchrotron sources, which allowed for small aperture sizes and exceptional quality data [14–16]. this coupling essentially provides
a large signal [17]. While there is no doubt that the large signal improves throughput
and is very effective for small aperture sizes, simply using a brighter source is not
a guarantee of the best performance. For example, the brighter source may require
faster mirror scanning, leading to larger Fourier frequencies and other sources of
noise that may overwhelm detector noise. Shot noise in the source may also start to
become apparent in some cases on using a brighter source. hence, we posit that any
instrument that is designed around a specific source or detector carefully also consider data acquisition. While this is a fine point and a simplification is often made
that using a “better” source or detector will improve the measurements, it is truly the
integrated system in which every matched part is optimally designed, that will provide the best performance. It turns out that the flux provided by a synchrotron does
not lead to unstable recording or encroachment into regimes where the intensity is a
problem, emerging laser sources must be cautiously integrated.
Synchrotron sources are of significant importance because synchrotron beams can
be collimated very efficiently and can form bright, very high intensity sources that
are almost point-like. Covering point areas of the sample and, as with globar sources,
single-element detectors used for acquiring point spectra by raster scanning can create
a mapped image. this makes them an ideal source to be used to validate fundamental
theoretical advances in IR microscopy. In some ways, the synchrotron is an ideal
source for IR microscopy. While it is obvious that the cost and availability of such
systems cannot compete with the globar, synchrotron-based systems nevertheless represent a useful avenue for proof-of-principle studies and provide a performance yardstick for all of IR microscopy. Even under these conditions, there is one disadvantage