Nearly every red light device on the market quotes two numbers: 660 nm and 850 nm. Almost none explain what they mean.
A nanometre measures colour
“nm” is short for nanometre, a billionth of a metre. Applied to light it describes wavelength — the physical distance between one peak of the light wave and the next.
Wavelength is what our eyes interpret as colour. Shorter wavelengths appear blue and violet. Longer ones appear red. Longer still and we stop seeing them altogether.
660 nm sits in the visible red part of the spectrum. You see it clearly — it is the deep red glow.
850 nm is near-infrared. It sits just past the longest wavelength the human eye can detect, so you cannot see it. The array can be running at full output and, apart from a faint glow from the red LEDs beside it, look as though nothing is happening.
This is why the safety guidance matters more than people expect
Because near-infrared is invisible, you get none of the ordinary warnings you rely on with bright light. There is no glare, no discomfort, no instinct to look away or blink.
That is precisely why every reputable device tells you not to look into the LEDs, to keep your eyes closed for anything on the face or head, and to use eye protection. It is not a formality. Your normal instincts genuinely do not apply here.
Why devices include both
Because they are different wavelengths of light with different physical properties. Longer wavelengths are absorbed less strongly by tissue and therefore travel further into it before scattering — that is a straightforward property of optics, not a claim about what happens next.
A device offering only one wavelength is offering less than one offering both, which is why the combined figure is the one worth comparing.
What to actually check
Is it really both, and how many of each? “660 nm and 850 nm” on the box does not tell you the split. A device with ten red LEDs and two near-infrared is not the same product as one with six of each. Ours is 12 LEDs — six at 660 nm, six at 850 nm.
Is the combined irradiance published? Individual figures are useful. The combined figure is what you get in practice.
At what distance? See our article on irradiance — this is where most comparisons quietly fall apart.
What is the beam angle? Ours is 30°. A wider angle covers more area but spreads the same output more thinly.
What the numbers do not tell you
Wavelength and irradiance describe the light. They do not describe what happens afterwards, and anyone presenting them as though they settle that question is overreaching.
What they let you do is compare devices honestly — which is a genuinely useful thing, and harder than it should be in this category.
Read: what irradiance actually means →
See our measured output at three distances →