An imaging spectrograph presents spatially resolved spectra with all acquired wavelengths simultaneously. When used with a spectrum detector such as a CCD or CMOS camera it becomes an imaging spectrometer. The goal is to enable point to point imaging (see Figure 1)
Traditional spectrometers use diffraction gratings in a Czerny Turner geometry (yes, there are others). Aberrations make it impossible to image a point on the entrance slit as a point on the detector due to significant aberrations. This problem can be obviated with the use of aspheric optics.
More recently holographic gratings with aberration correcting properties made spectral imaging feasible. Using a prism with curved sides provides an elegant alternative solution with state of the art light throughput efficiency. (See figure 2)
Figure 3 shows an example of hundreds of spatially resolved spectra acquired simultaneously with the PARISS imaging spectrograph
Go here to compare the spectral properties of prism vs gratings