Spectro-imagers

Spectral imaging, born of the combination of imaging and spectrometry, aims to generate spectrally resolved images, or, equivalently, spatially resolved spectra. It is therefore a generalization of color imaging, with the difference that the number of spectral components associated with each image pixel is no longer limited to three, but can exceed a thousand. When this number is low, around ten, we speak of multispectral imaging, and hyperspectral imaging when the number of bands exceeds a few dozen. Today, the term spectro-imaging has become virtually synonymous with hyperspectral imaging, and it is in this sense that we will use it. However, the boundary between the multispectral and hyperspectral domains has yet to be precisely defined and accepted by all authors, although most agree that an instrument must deliver images in narrow, contiguous spectral bands to qualify as hyperspectral. The term can also vary according to the user community. For example, in astronomy, the terms 3D spectroscopy, integral field spectroscopy or integral field spectroscopy are commonly used, and in biology or chemistry, chemical imaging.

Spectroscopic imagers can be used in a wide range of applications, from microscopes to the focal points of the largest telescopes. The first chapter of this article provides a brief overview of the main current uses of hyperspectral imaging.

The range of devices for acquiring both spectral and spatial information is even wider. This diversity is partly due to the diversity of spectrometric techniques, but also to the different ways of scanning an image. Few instruments can acquire all spectral and spatial information in a single exposure. It is therefore often necessary to introduce a temporal scan, which can involve both the spectral and spatial dimensions. This leads to a classification of spectro-imagers according to their spatial acquisition mode. We can thus distinguish four major instrument families:

• whiskbroom" instruments, for which the instantaneous field of view is reduced to a single point;

• pushbroom" instruments, for which the field of view is one-dimensional;

• instruments in "staring" or "framing" mode, for which the field of view extends in two dimensions and remains fixed;

• instruments in "windowing" mode, where the field of view is two-dimensional, but scans the scene continuously in one direction.

This is the distinction we have adopted to present our article, since the second chapter will be devoted to single-point mode instruments, the third to linear field scrolling instruments, the fourth to dotted-field mode instruments, and the fifth to those...