Unlike penetrating ionizing radiation, optical radiation is generally absorbed very superficially. Ex- cept for a narrow band of visible and near-infrared (IR-A) radiation from approximately 400-1400 nm, skin and other biological tissues are nearly opaque to optical radiation. For this reason, volumetric or mass based concepts of absorbed dose are of little value. Additionally, the abosorbed radiant energy is conducted out of the absorbing site and for this reason thermal effects depend largely upon the size and location of the absorbing site as well as exposure and exposure rate. Con- cepts of exposure dose are therefore most useful and practical.