436
K. malek et al.
single-point mapping. As discussed above, the spatial resolution is improved in an
imaging system by incorporating a mCt linear array, but the smallest aperture is set
to 10 μm
2
for practical reasons. miller and Smith have demonstrated that the brightness of a synchrotron source is almost 10 times greater than that of a globar for a
10 × 10 μm
2
aperture, but it does not differ significantly for aperture sizes larger
than 60 × 60 μm
2
[51]. moreover, IR microscopes coupled with synchrotron-based
sources exhibit higher spatial resolution compared to bench top instruments due to
their confocal mode, i.e. by using two passes of light through the same pinhole [40].
the magnification of a Cassegrain objective is often 36 × and 74 × with NA of 0.5
and 0.65, respectively. this allows for the acquisition of spectra with an aperture
down to ca. 3 × 3 μm
2
and S/N ratio ca. 1000 times greater compared to a globar
source [56].the use of the largest commercially available numerical aperture (0.65)
enables one to achieve a spatial resolution of ca. 0.6 μm for the shortest wavelength
at 2.5 μm [57]. In addition, the chemical image contrast is effectively improved in
the synchrotron experiment that resolves substructures within a very small sampling
area. other advantages of the synchrotron light are its pulse time nature (10–100 s
of picoseconds) and a high degree of polarization, which are dependent on the beam
line construction [51].
the confocal configuration for an IR microscope is an option that can only be incorporated for an FtIR spectrometer coupled to a synchrotron light source. In addition, almost all IR synchrotron facilities around the world operate in the single-point
mapping mode, considerably lengthening the data acquisition time. For instance,
acquiring fully diffraction-limited maps of a 30 × 30 μm
2
area takes 2–4 h [58].thus,
the coupling of a bright synchrotron IR beam with an apertureless FPA detector to
maximize spatial resolution and achieve fast hyperspectral imaging has become a
need. Such a design has been theoretically proposed by Carr et al. [59] and tested
by moss, Petibois, and marcelli [60, 61]. however, these experiments did not overcome a fundamental problem of inhomogeneous illumination of all the pixels on
the FPA detector by the synchrotron beam, hence a synchrotron IR source typically
fills a 10–20 μm
2
area. on the other hand, these experiments have shown that spatial
resolution near 2.3 μm for light wavelength at 2–4 μm is achievable and spectra
collected from the illuminated area of an FPA detector showed a higher S/N ratio
compared to thermal sources [60, 61]. A comprehensive approach for the use of a
synchrotron beam with an FPA detector has been recently published by hirchmugl,
Bhargava, and gough [57, 62]. A new design for a synchrotron-based IR imaging
system was implemented at the Synchrotron Radiation Center in Stoughton Radiation, Wincosin (mid-infrared beamline IRENI). they have constructed an infrared
imaging system designed and optimized to illuminate a large field of view, and
which achieves a true diffraction-limited spatial resolution. their approach was to
couple multiple low-emittance synchrotron beams with a large FPA detector [57].
Radiation from a bending magnet is split into 12 beams by 12 toroidal mirrors,
K. malek et al.
single-point mapping. As discussed above, the spatial resolution is improved in an
imaging system by incorporating a mCt linear array, but the smallest aperture is set
to 10 μm
2
for practical reasons. miller and Smith have demonstrated that the brightness of a synchrotron source is almost 10 times greater than that of a globar for a
10 × 10 μm
2
aperture, but it does not differ significantly for aperture sizes larger
than 60 × 60 μm
2
[51]. moreover, IR microscopes coupled with synchrotron-based
sources exhibit higher spatial resolution compared to bench top instruments due to
their confocal mode, i.e. by using two passes of light through the same pinhole [40].
the magnification of a Cassegrain objective is often 36 × and 74 × with NA of 0.5
and 0.65, respectively. this allows for the acquisition of spectra with an aperture
down to ca. 3 × 3 μm
2
and S/N ratio ca. 1000 times greater compared to a globar
source [56].the use of the largest commercially available numerical aperture (0.65)
enables one to achieve a spatial resolution of ca. 0.6 μm for the shortest wavelength
at 2.5 μm [57]. In addition, the chemical image contrast is effectively improved in
the synchrotron experiment that resolves substructures within a very small sampling
area. other advantages of the synchrotron light are its pulse time nature (10–100 s
of picoseconds) and a high degree of polarization, which are dependent on the beam
line construction [51].
the confocal configuration for an IR microscope is an option that can only be incorporated for an FtIR spectrometer coupled to a synchrotron light source. In addition, almost all IR synchrotron facilities around the world operate in the single-point
mapping mode, considerably lengthening the data acquisition time. For instance,
acquiring fully diffraction-limited maps of a 30 × 30 μm
2
area takes 2–4 h [58].thus,
the coupling of a bright synchrotron IR beam with an apertureless FPA detector to
maximize spatial resolution and achieve fast hyperspectral imaging has become a
need. Such a design has been theoretically proposed by Carr et al. [59] and tested
by moss, Petibois, and marcelli [60, 61]. however, these experiments did not overcome a fundamental problem of inhomogeneous illumination of all the pixels on
the FPA detector by the synchrotron beam, hence a synchrotron IR source typically
fills a 10–20 μm
2
area. on the other hand, these experiments have shown that spatial
resolution near 2.3 μm for light wavelength at 2–4 μm is achievable and spectra
collected from the illuminated area of an FPA detector showed a higher S/N ratio
compared to thermal sources [60, 61]. A comprehensive approach for the use of a
synchrotron beam with an FPA detector has been recently published by hirchmugl,
Bhargava, and gough [57, 62]. A new design for a synchrotron-based IR imaging
system was implemented at the Synchrotron Radiation Center in Stoughton Radiation, Wincosin (mid-infrared beamline IRENI). they have constructed an infrared
imaging system designed and optimized to illuminate a large field of view, and
which achieves a true diffraction-limited spatial resolution. their approach was to
couple multiple low-emittance synchrotron beams with a large FPA detector [57].
Radiation from a bending magnet is split into 12 beams by 12 toroidal mirrors,