Spectroscopy covers a wide range of measurement techniques, the core concept of which being that they determine the amplitude of individual frequencies of interest in the spectrum of a complex signal. That signal is most often in the electromagnetic spectrum, but can also be acoustic. Spectroscopy may be performed qualitatively (when only the existence of spectral components is of interest) or quantitatively (when the relative and/ or absolute contributions of individual spectral components matter). The latter case is known as spectrometry.
The signal of interest may be emitted by the subject, reflected by it, or selectively absorbed by it. Spectrometric information from multiple views of a subject may be used to construct images.