To discuss the dosages used in red light therapy, some important terms need to be covered:

Power intensity (also called irradiance): The amount of power delivered per square centimeter of body area per second, often expressed as mW/cm²/s (milliwatts per square cm).

Power density: The product of power intensity and duration of exposure, often expressed as J/cm² (joules per square cm).

Total energy (the “dose”): The product of power density and body area exposed, usually expressed as J (joules).

Red light therapy for eye conditions uses the lowest doses, with most studies using power intensities below 1 mW/cm² for around 3 minutes per session, corresponding to a power density well below 1 J/cm². Red light therapy for hair loss typically involves power intensities of around 3–90 mw/cm² for around 15–20 minutes per session, resulting in a power density of around 1–10 J/cm². Finally, some research suggests the effective dose for knee osteoarthritis is around 4–8 J per spot for light with a wavelength range of 780–860 nm and around 1–3 J per spot for light with a wavelength of 904 nm.