Light and photobiomodulation
Irradiance
Irradiance is how much light power lands on a given area of skin, measured in milliwatts per square centimetre (mW/cm²). It falls off sharply with distance from the emitter, and together with time it determines the dose.
Also called power density, intensity, mW/cm2
The plain version. Irradiance is brightness in engineering units: power per unit area. Two devices can share a wavelength and an LED count and still deliver very different sessions if one is twice as bright at the skin.
Distance dominates it. Light spreads as it travels, so irradiance drops rapidly as the emitter moves away. A figure quoted at 5 cm and the same device measured at 30 cm are not comparable. This is why a useful specification states irradiance and the distance at which it was measured. It also means that a person’s position, posture and body shape change the amount of light they receive.
Typical ranges. Photobiomodulation studies are often described as working in the region of 5–100 mW/cm² at the tissue surface — a rough orientation drawn from the field’s general usage rather than a single attributable figure. Well below it and sessions become impractically long; well above it and the exposure starts to be thermal rather than photobiomodulatory, and may fall on the wrong side of the biphasic dose curve.
Why it is worth asking about. Wavelength tells you what colour the light is. Irradiance and distance tell you how much of it arrives, and together they are what let a study dose be compared with a device.
Why it matters here
Irradiance, together with the distance it was measured at, is what tells you how much light a device delivers. Two devices with the same wavelength and LED count can deliver very different sessions.