To accurately determine the total amount of energy delivered to the tissue by the laser, the duration of exposure is vital. Prolonged exposures result in tissue destruction, and too short an exposure results in an inadequate effect. The dose, or fluence, is a measure of the total energy. It is determined by multiplying Pd by the exposure time (t) and is expressed in terms of energy per unit area of incident tissue, as follows:
Fluence = Pd(t) = J/A, where J is the energy in joules and A is the cross-sectional area of beam in square centimeters.