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   There are three different kinds of spectra which are distinguished by their appearance: line, band and continuous spectra. Line spectra are emitted by atoms or irons; band spectra by molecules. Continuous spectra are not atom –or molecule-specific and are basically produced by the free electrons in the plasma which give off continuous radiation with free-free transitions and with free-bound transitions.

   In addition to continuous spectra there are other forms of radiation such as those from molecules, radicals and incandescent particles. The spectral back-ground thus consists of all these types of radiation as well as unresolved spectral lines.