ECE532 Biomedical Optics © 1998 Steven L. Jacques, Scott A. Prahl Oregon Graduate Institute

Absorption

A simple analogy for the absorption of light by molecules is placing two identical bells side by side. When one bell is rung, the other will sympathetically ring at the same frequency due to transfer of energy from the struck bell. The resonance of the second bell matches the frequency of the first ringing bell and hence the second bell accepts energy from the struck bell.

Photons are electromagnetic waves with a particular frequency. Molecules are a system with charge separation (negative electron field and positive nucleus). The state of the molecular charge separation can change in a quantized fashion by "absorbing" the energy of a photon. The photon frequency must match the "frequency" associated with the molecule's energy transition in order for energy transfer to occur. The relation between frequency and energy is

where Energy is in [J], photon frequency is in [cycles per s] or [s^-1], photon wavelength is in [m], c is the speed of light in vacuo (c = 3.0x10⁸ [m/s]), and h is Planck's constant (h = 6.62618x10^-34 [J s]). Unlike the bell analogy, photon absorption occurs as a quantum event, an all or none phenomenon. Example: absorption of 514 nm photon from an argon ion laser by a hemoglobin molecule.

In biomedical optics, absorption of photons is a most important event:

• Absorption is the primary event that allows a laser or other light source to cause a potentially therapeutic (or damaging) effect on a tissue. 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.