Figure 1.

Properties of low-level light. Sunlight is composed of a combination of noncoherent waves with wavelengths spanning the entire visible spectrum. In contrast, lasers emit waves of a single wavelength (monochromatic) that have spatial and temporal synchronization. This high wavelength coherence allows the transmission of energy at a high power density. Finally, low-level light consists of monochromatic or quasimonochromatic waves taking different paths leading to a common target point. While wavelength, radiant exposure, irradiance, and fractionation scheme are relevant for low-level light therapy applications, the authors introduce the possibility that noncoherence may be advantageous for some neurometabolic purposes. Noncoherence allows nervous tissue exposure at “therapeutic” wavelengths at relatively low power densities during the time necessary to modulate neural metabolism in response to activation or injury, even if this time is prolonged.