Fig. 3: Schematic of light tissue interactions.

Absorption occurs when light is absorbed by tissue molecules and converted into other forms of energy (e.g., heat). Reflection refers to light bouncing specularly (at a single angle) off the surface. Scattering involves deflection of light in various directions by tissue structures (e.g., organelles and fibers) which may also alter the photon’s wavelength. Fluorescence is the re-emission of absorbed light at a different wavelength. Transmission describes the process of photons passing through tissue without being dissipated. These phenomena occur simultaneously and at varying degrees depending on tissue properties and light characteristics.