The continuous spectrum of black-body radiation is emitted by all objects heated to high temperatures or otherwise supplied with sufficient energy -- a process called excitation. Any object which is not a perfect radiation source, or ideal black body, is also found to emit strongly at specific wavelengths in addition to its continuous spectrum emission. These specific wavelengths, which are sharply defined, are highly characteristic of the substance being excited; they are called line spectra.

Isolated atoms, which are the form of matter found at extremely high temperatures, emit line spectra which are characteristic of the element whose atoms are being excited. Chemists make use of this to identify the elements present in samples; the technique is called emission spectroscopy.