Guides / Red light therapy irradiance explained (and why brand numbers don't compare)
Red light therapy irradiance explained (and why brand numbers don't compare)
Irradiance is the single most advertised spec on a red light panel - and the most misleading. Here is what it means and how to read it without getting fooled.
What irradiance actually measures
Irradiance is optical power per unit area, written in milliwatts per square centimetre (mW/cm²). It tells you how much light energy lands on your skin each second. More irradiance means a stronger dose in less time - but only if you know where it was measured.
Why distance changes everything
Light spreads out as it travels, so irradiance drops fast with distance. A panel can read 200 mW/cm² pressed against the LEDs and under 100 mW/cm² at 15 cm. That is why a number with no stated distance is meaningless: one brand quotes it at the surface, another at 15 cm, and the figures look very different for the same hardware.
Dose = irradiance × time
What your tissue receives is a dose (J/cm²), and dose equals irradiance multiplied by exposure time. A weaker panel can deliver the same dose with a longer session. So a huge irradiance headline is not automatically better - it mostly shortens session length.
How RedLightMeter normalizes it
We record the irradiance a brand claims and the distance they measured it at, then project every panel to a standard 15 cm so the numbers line up. Where independent spectrometer data exists, we use that and mark it verified. Claims with no distance stay flagged as not comparable. See the database to compare normalized figures.
FAQ
What is a good irradiance for a red light panel?
At a realistic 15 cm distance, roughly 50-150 mW/cm² is typical for quality home panels. Much higher figures are usually quoted at the surface, not at a usable distance.
Is higher irradiance always better?
No. Higher irradiance mainly shortens session time. Because dose equals irradiance times time, a moderate panel used a little longer can deliver the same energy.
Why do two panels with the same irradiance look different?
Usually because they measured at different distances. Always compare irradiance at the same distance - we normalize everything to 15 cm.