Colours are perceptible by the human eye and are related to the spectral energy distribution of the emitting source, i.e. to its spectrum. Whereas the identification of atomic or molecular species demands a high spectral resolution across a large wavelength domain, the main features of this spectrum can however be measured from broad-band photometry only. One is nowadays trying to get this spectral information for more and more spatial elements. Combining in the same instrument an imagery capability (2 dimensions) with a spectroscopic mode (1 dimension) enables one to acquire (huge) data cubes which allow to investigate simultaneously the spatial and spectral properties of the object under study : distribution of chemical abundances, velocity maps, dynamics. etc.
Instruments and data are classified into the following types depending on the number of spectral bands/wavelengths used in the imaging process: multispectral, superspectral, and hyperspectral.
- multispectral : between 2 and 10 (or 20) spectral bands ;
- superspectral : between 10 (or 20) and 50 (or 100) bands ;
- hyperspectral : beyond 50 (or 100) bands.