Photons are electromagnetic waves with a particular frequency. Unlike the bell analogy, photon absorption occurs as a quantum event, an all or none phenomenon. Without absorption, there is no energy transfer to the tissue and the tissue is left unaffected by the light. Absorption of light provides a diagnostic role such as the spectroscopy of a tissue. Absorption can provide a clue as to the chemical composition of a tissue, and serve as a mechanism of optical contrast during imaging.
Absorption is used for both spectroscopic and imaging applications.
OMLC homepage .