660nm vs 850nm: What's the Difference for Hair Growth?

Every red light device you’ll come across for hair uses light somewhere in the 600–900 nanometre range, and most name two specific numbers on the box: 660 and 850. They’re not interchangeable, and they’re not competing with each other either. They do different jobs, which is why most serious devices, Theracap included, use both.

What the number actually means

A nanometre is a unit of wavelength, and it’s what determines a light’s colour and how deep it travels into tissue before it’s absorbed or scattered. Visible red light runs from roughly 620 to 700 nm. Once you get past about 700 nm, the light is still real and still has effects on tissue, but your eyes can no longer see it. That’s near-infrared, and 850 nm sits comfortably inside it.

Neither number was picked by accident. There’s a rough sweet spot in the 600–1000 nm range, sometimes called the “optical window,” where light passes through skin without being absorbed too quickly by water or blood, which is what lets it actually reach cells below the surface instead of just warming the top layer.

660 nm: red, shallower, well studied

660 nm sits near the middle of the visible red range. It penetrates skin to a depth of roughly a few millimetres, which happens to line up well with where hair follicles sit in the scalp. It’s also the wavelength most of the actual hair-growth research has used, including the Lanzafame trials that found increased hair counts after 16 weeks. If you had to pick one wavelength to test to see whether this category works at all, this is the one that’s been tested the most.

850 nm: invisible, deeper, less scalp-specific data

850 nm sits further into the near-infrared range and reaches noticeably deeper into tissue, on the order of a centimetre or more depending on the tissue type. It’s widely used in near-infrared therapy for joints and muscle because it can get past skin and fat to reach structures underneath. For hair specifically, there’s less dedicated clinical research than there is for 660 nm, but the physiological reasoning carries over: it reaches the dermal layer where follicles and their blood supply sit, from a slightly different angle than 660 nm does.

Why combine them instead of picking one

Hair follicles aren’t sitting at a single fixed depth. They vary by person, by scalp thickness, and by where on the head you’re looking. A device using only 660 nm reliably reaches the shallower ones. Adding 850 nm extends that reach further down, and it means the follicle itself, along with the small blood vessels feeding it, gets light from more than one angle. That’s the reasoning behind dual-wavelength panels generally, not something specific to any one brand.

Not more light. Light doing two different jobs at once.

Does more wavelengths mean better results?

Not automatically, and be skeptical of anyone implying that stacking five or six wavelengths must be superior to two. The dose and the exposure time matter as much as the number of wavelengths present. A device that blasts you with red, near-infrared, blue and green light for two minutes isn’t necessarily doing more than one that delivers a properly dosed 660 and 850 nm combination for ten. Past a certain point, adding wavelengths is a spec-sheet argument, not a biological one.

What Theracap uses

The cap combines 660 nm red and 850 nm near-infrared LEDs across the full inside of the crown, so both wavelengths reach your scalp from hairline to the back of your head in a single 10-minute session. Neither wavelength is visible to the naked eye as anything dangerous, and both sit well within ranges used in skin-contact consumer devices generally, but this device specifically hasn’t been independently tested for hair growth by a regulator. See our Health & Safety Information page for precautions before you start.