Safety and penetration of light into the brain

Transcranial photobiomodulation is safe, based upon published literature. Photobiomodulation for neurological diseases primarily requires light to reach target neurons in the brain. Alternatively, light may change brain function by modulating photoreceptors, such as opsins in the skin or eye, that yield secondary effects or trigger a cascade. A critical concern is the ability of light penetration through the layers of the human cranium, including the scalp, skull, meningeal layers, and brain parenchyma. Light penetration depth depends on tissue optical properties, wavelength, skull anatomy, irradiance, and pulsing. Within the ultraviolet, visible, and near-infrared (NIR) regions, longer wavelengths and greater irradiance are associated with deeper penetration. NIR light appears to be superior in penetrating human calvaria and brain tissue compared to visible red light. Light penetration is dependent on the light source’s location. Understanding light propagation is critical in maximizing efficacy of transcranial photobiomodulation by providing sufficient light penetration into the human cranium to activate mechanisms for neuromodulation.