In this case, information may be acquired from scattered light as well as from absorbed light. Optical imaging is capable of spatial resolution and spectral resolution. Spatial resolution is the visualization of a physical entity, such as a tumor. Spectral resolution yields information about the identity or quantity of tissue constituents. Optical measurements are often analyzed with theoretical mathematical models of photon migration.
Light energy is deposited in tissue as the photons are absorbed in the tissue.
The absorbed energy is largely converted to heat. The body's vascular system removes heat from the treated area over a period of time. However if the heat builds up quickly, such as a large optical power over a short illumination time, sections of the tissue may boil, vaporize, burn or even explode. On the other hand, the same total amount of light applied very slowly to allow heat dissipation by the vascular system leads primarily to photochemical reactions. Therefore, the rate of application of light is as important as the total amount of absorbed light.