Spectral Imaging: Methods, Design, and Applications - Biomedical Optical Imaging Technologies: Design and Applications - page 134

Fig. 4.14 Prism-based system. The prism is curved and a concave mirror reflects the light at the
rear. It can be shown that the aberrations are significantly reduced so that a high level of imaging
and spectral performance is achieved
Fig. 4.15 Spectral imaging system based on a prism-grating-prism (PGP) unit. The system
measures the spectrum for all points on a single line, so that a full spectral image is generated
by a scan along one dimension. The front lens L1 creates an image on an entrance slit and the light
is collimated again with lens L2. The PGP unit disperses the light which is focused with L3 on the
array detector
easy integration with fore-optics and a detector. Lastly, due to the grating, it has
high diffraction efficiency PGP systems are available commercially (e.g. Specim).
The design shown in Fig. 4.15 ensures the measurement of only a single line
from the image. Construction of the full image requires scanning along the other axis
of the image. Lens L1 focuses the image on an entrance slit while lens L2 collimates
the light. After passing through the PGP unit, the light is focused again through
lens L3. Therefore, each point along a single line from the image is imaged to a
single vertical line on the array detector, while the spectrum is vertically dispersed.
The design of the PGP unit has many degrees of freedom that can be optimized
for the spectral performance. The head angle of the entrance prism is optimized so
that the incidence angle of the refracted beam on the grating matches the Bragg
condition for the first-order diffraction of the grating. This can be exact only for a