Light and photobiomodulation
Fluence
Fluence is the total light energy delivered per unit area, measured in joules per square centimetre (J/cm²). It equals irradiance multiplied by exposure time. In photobiomodulation it is the closest thing to a dose, and more is not automatically better.
Also called radiant exposure, energy density, dose, J/cm2
The arithmetic. Fluence (J/cm²) = irradiance (W/cm²) × time (seconds). So 20 mW/cm² for 300 seconds gives 6 J/cm². This is why a session length means little without an irradiance figure: ten minutes at one intensity is a different session from ten minutes at another.
Typical study doses. Much of the literature is described as working in the range of roughly 1–20 J/cm² at the tissue surface.
The catch that makes fluence unusual. In most of medicine, dose scales predictably. In photobiomodulation it does not. A review by Huang, Chen, Carroll and Hamblin (Dose-Response, 2009) documents that low doses often stimulate and repair tissue better than high ones, and that overshooting can abolish the effect or reverse it. Doubling the session length is not a reliable way to double the effect.
Reciprocity does not fully hold either. The same fluence delivered briefly at high intensity and slowly at low intensity does not always produce the same result, so J/cm² alone, without the irradiance it was delivered at, is still an incomplete description.
Why it matters here
Fluence is the closest thing photobiomodulation has to a dose. It is why session length on its own tells you little: ten minutes under a weak light and ten under a strong one are not the same session.