The last twenty five years have seen a very large amount of work done in the field of multispectral and spectral imaging all around the world, in industry and academia. The motivation is to collect spatially and spectrally resolved radiance information of a predefined region of space. The number and fields of application of such information are innumerable; as a result, the different optical configurations used in the design of instrumentation built for this purpose have also been innumerable, in order to provide the needed information through a method and format most suitable for the relevant application. Instrumentation has been built for laboratory and field use, industrial and military use, on land, on the sea, from the air and in space. Such types of instrumentation are based in the visible spectral range (400-750 nanometers), and in the various regions of infrared (the Near Infrared (NIR) range of 750-2500 nanometers, the Mid Wave Infrared (MWIR) range of 2500-5000 nanometers, and the Long Wave Infrared (LWIR) of 5000-14000). Instrumentation of spectral imaging has been built for analysis of microscopic samples in hospital environments, as well as of distant cosmic objects through large astronomical telescopes. The size of the analyzed region of space and spatial resolution also vary widely, as well as the spectral resolution, depending on the type of detector used (i.e. the size, speed, sensitivity and number of resolution elements (pixels) that the detector provides). All work done in this field allows the acquisition of knowledge of the spatial distribution (the imaging side) of the material constituents (the spectroscopic side) in a predefined region of space. Just one of many examples of early commercial spectral imagers is the microscope mounted SpectraCube 1000 of the early nineties (“Novel spectral imaging system combining spectroscopy with imaging applications for biology”, Proc. SPIE Vol. 2329, 180 (1995), and U.S. Pat. No. 5,539,517), sensitive in the visible range, invented at CI Systems in 1991 and transferred to Numetrix and later to Applied Spectral Imaging Ltd. for use in medical applications.
The early drive in the 1990's was to make the transition from systems based on imaging at a limited number of wavelengths using bandpass filters (below ten) used on satellites (like Landsat and similar instruments of the 1970's and 1980's), to hyperspectral imaging, providing hundreds of spectral resolution elements at each image pixel, using gratings and interferometers and sophisticated optics. The latter group, especially through advances in modern cooled or chilled CCD's and cryogenically cooled infrared detector arrays, represents a high end type of instrumentation. Such high end instrumentation provides large quantities of information, but is usually more suitable for research projects and not for practical day-to-day civilian and industrial applications, due to exceedingly high costs which may reach hundreds of thousands of dollars each.
A more recent trend is to exploit spectral imaging technology for safety, security and industrial applications, and in particular in the application of hazardous gas cloud detection and imaging. Spectral imaging technology as applied to such applications can be used, for example, in automatic detection of leaks in industrial installations without the need for manpower intensive maintenance investigations, and to identify gases liberated to the air in traffic accidents involving trucks during transport. The low price and maintenance-free operation required for this type of instrumentation is a strong motivation to use low price detectors with no moving parts. Both are significant challenges: i) the former, because most hazardous gases in safety applications are transparent in the visible range and require more expensive infrared detectors, in order to be detected; ii) the latter, because the need for simultaneous spatial and spectroscopic information needed for detection and identification is more easily achieved with a spectral scanning method of some kind, usually requiring the movement of some optical component, such as a scanning mirror of an interferometer, a set of band pass filters mounted on a rotating wheel, or other.
Recent technological advances have allowed the development of more cost effective infrared detectors and cameras not requiring cooling at all, or at most requiring thermoelectric cooling (for example microbolometers MIR detectors and MWIR PbSe arrays). As a result, the motivation to find optical configurations yielding just the right amount of information for a specific application, while maintaining low cost, has also become very strong. Compromises in this respect have been described in the literature of many different types. Dereniak et al., (“Snapshot dual-band visible hyperspectral imaging spectrometer”, Optical Engineering Vol. 46(1), 2007) and Kudenov et al., (“Review of snapshot spectral imaging technologies”, Optical Engineering 52(9), 090901 (September 2013)), discuss the use of Optical Computed Tomography (OCT) techniques to remove the use of moving parts in spectral imaging at the expense of spatial and spectral resolutions. Other snapshot methods described in Kudenov et al., have been developed. Such snapshot methods still provide a spectral image with intermediate resolution in both wavelength and space parameters, but the optical fabrication of the required exotic components is cumbersome and expensive (such as reformatting coherent fiber bundles, lenslet arrays, and multiple mirror facets).
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