Red Light Wavelengths, Explained
Last updated: September 17, 2026 · This site contains affiliate links. Read our disclosure.
Every red light device lists a string of numbers — 660nm here, 850nm there — as if they were self-explanatory. They aren't. Here's what the commonly listed specs actually describe, in plain English, with no promises attached about what any of them will do for you.
Nanometers: the unit everything is measured in
Light is measured in nanometers (nm) — billionths of a meter — which describe where a color sits on the light spectrum. When a device lists "660nm," it's telling you the peak color of light its LEDs emit. Think of it like a paint swatch number: it identifies the color, not the quality of the paint.
The two spec categories you'll see
Red light (commonly listed as 630–660nm). This is visible red light — the part you can see glowing when the device is on. Most consumer devices list one or two red wavelengths in this range (660nm is the most common single number you'll see).
Near-infrared light (commonly listed as 810–850nm). This is just past visible red on the spectrum — invisible to the human eye. Devices that list an 810–850nm wavelength are telling you they include near-infrared LEDs alongside the visible red ones. You won't see these LEDs glowing the way you see the red ones, which is normal, not a defect.
That's the whole story of the two categories: one you can see, one you can't. Brands list both to describe the LED mix in the device. A device that lists only red wavelengths has visible-red LEDs only; a device listing both has a mix.
Irradiance: the other number everyone argues about
Irradiance, listed in milliwatts per square centimeter (mW/cm²), measures light intensity at a specific point — how much light power is landing on a given area. Here's the catch buyers miss: irradiance is measured at a distance, and intensity drops quickly as you move away from the LEDs.
Most brands measure and advertise irradiance at the LED surface, which is the highest possible reading. Your actual session happens inches away, where the number is lower. Two brands' advertised irradiance figures may not even be measured at the same distance, so comparing them at face value is unreliable. Treat irradiance as a rough indicator of claimed output — useful for spotting brands that publish nothing, less useful for ranking brands that do.
LED count and other specs
LED count tells you how many diodes the device has — relevant for panels, where more LEDs over a larger area means more coverage. Beam angle (sometimes listed in degrees) describes how widely each LED spreads its light. Flicker refers to whether the LEDs pulse; some people prefer flicker-free drivers, though this is a comfort preference, not a spec that determines results.
What a trustworthy spec sheet looks like
- Lists the actual wavelength ranges of the LEDs (not just "red light")
- States the measurement distance alongside any irradiance figure
- Lists LED count and device dimensions
- Doesn't promise medical outcomes or use disease language
A brand that hides its wavelengths behind vague claims is telling you something — and it's not good. The numbers themselves are just descriptions of the hardware; what matters is whether the brand is transparent about them.