Biostimulatory windows in low-intensity laser activation: Lasers, scanners, and NASA's light-emitting diode array system

Objective: The purpose of this study was to assess and to formulate physica lly an irreducible set of irradiation parameters that could be relevant in the achieving reproducible light-induced effects in biological systems, bot h in vitro and in vivo. Background Data: Light-tissue interaction studies f ocusing on the evaluation of irradiation thresholds are basic for the exten sively growing applications for medical lasers and related light-emitting s ystems. These thresholds are of central interest in the rejuvenation of col lagens, photorefractive keratectomy, and wound healing. Methods: There is a mple evidence that the action of light in biological systems depends at lea st on two threshold parameters: the energy density and the intensity. Depen ding on the particular light delivery system coupled to an irradiation sour ce, the mean energy density and the local intensity have to be determined s eparately using adequate experimental methods. Results: From the observatio ns of different research groups and our own observations, we conclude that the threshold parameters energy density and intensity are biologically inde pendent from each other. Conclusions: This independence is of practical imp ortance, at least for the medical application of photobiological effects ac hieved at low-energy density levels, accounting for the success and the fai lure in most of the cold laser uses since Mester's pioneering work.