Image Mapping Spectrometry
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An Image Mapping spectrometer (IMS) is a snapshot hyperspectral imager. The IMS replaces the camera in a digital imaging system, allowing one to add high-speed snapshot spectrum acquisition capability to a variety of imaging modalities such as microscopy, endoscopy, macroscopy, and ophalmoscopy to maximize the collection speed.
The motivation to develop the IMS technique originates from the high temporal resolution requirement in time-resolved multiplexed biomedical imaging. Conventional spectral imaging devices acquire data through scanning, either in the spatial domain (as in confocal laser scanning microscopes) or in the spectral domain (as in filtered cameras). Because scanning instruments cannot collect light from all elements of the dataset in parallel, there is a loss of light throughput by a factor of Nx × Ny when performing scanning in the spatial domain over Nx × Ny spatial locations, or by a factor of Nλ when carrying out scanning in the spectral domain measuring Nλ spectral channels.
Fig. 1. Principle of IMS
The IMS is a parallel acquisition instrument that captures a hyperspectral datacube without scanning. It also allows full light throughput across the whole spectral collection range due to its snapshot operating format. The IMS uses a custom-designed mirror, termed image mapper, which comprises multiple angled facets to redirect portions of an image to different regions on a detector array (Fig. 1). By redirecting slices of the image so that there is space between slices on the detector array, a prism or diffraction grating can be used to spectrally disperse light in the direction orthogonal to the length of the image slice. In this way, with a single frame acquisition from the camera, we obtain a spectrum from each spatial location in the image. The original image is reconstructed by a simple remapping of the pixel information.
To reflect the image zones into different directions, individual facets of the image mapper have different tilt angles with respect to the two axes in the plane of the slicer. This mapping method establishes a fixed one-to-one correspondence between each voxel in the datacube (x,y,λ) (x,y, spatial coordinates; λ, wavelength) and each pixel on the camera. The position-encoded pattern on the camera simultaneously provides the spatial and spectral information within the image. Since the acquired data results directly from the object’s irradiance, no reconstruction algorithm is required, and simple image remapping produces the image and data displays.
Fig. 2. IMS system and hyperspectral imaging of multiple-fluorophores-labeled cells