More Factors Affecting Light Penetration in Red Light Therapy

By Dominic Lamb – LightTherapy.no

You've bought the panel. You're standing in front of it ten minutes a day. The knee still aches. Or the shoulder. And you start to wonder whether the light is actually reaching the part of you that needs it, or just bouncing off the surface. It's a fair question, and the honest answer is: it depends on more than most sellers will tell you.

Wavelength comes first — but it's not the whole story

In the previous post on why 50 mW/cm² isn't equal across devices, we covered beam angle, divergence and reflection. Wavelength was touched on there, but it deserves more space because it's the single strongest predictor of depth.

Red light at 630–660 nm is the visible range, the glow you see from the panel. In lighter skin (Fitzpatrick I–II) it penetrates roughly 1–2 mm, which covers the epidermis and upper dermis. That's the right range for skin-level work: collagen stimulation, surface inflammation, wound healing, acne. In darker skin (Fitzpatrick V–VI), melanin absorbs 20–50% of that red light, reducing penetration to around 0.5–1 mm. Still useful for skin, but the depth margin is tighter.

Near-infrared at 810–850 nm is a different story. It's invisible and you won't see it, but it reaches 3–5 cm in lighter skin and 2–4 cm in darker skin, and melanin barely touches it. Research on near-infrared penetration confirms its effectiveness at reaching deep muscle and joint tissue in a way that red light simply cannot. If your goal is a knee, a hip, deep shoulder tissue, or anything below the dermis, 810–850 nm is where you start.

810 nm specifically has particularly favourable characteristics for bone and joint work in the literature. Worth noting for anyone treating articular joints rather than muscle. Beyond the 800s there are wavelengths in the 1000s with some deeper penetration potential, but they're also heavily absorbed by water as the primary chromophore, and the relationship between depth and absorption becomes more complex at that end of the spectrum.

Skin thickness and tissue type

Skin isn't uniform. Your back, palms and soles are the thickest, roughly 1.5–2 mm, while your face, neck and eyelids are as thin as 0.5–1 mm. That matters because thicker skin scatters and absorbs more light before it reaches anything underneath. Even 660 nm may not reliably reach below the dermis on your back or palms. A torch pressed against the skin directly outperforms a panel at 20 cm in those situations, not because the panel is weak, but because contact and focus change the delivery geometry entirely.

Once light is past the skin, the tissue type it meets determines what happens next. Fat is highly scattering and near-infrared handles it better than red light does. Muscle has moderate scattering and some water absorption of NIR. Bone is dense enough that even high-powered NIR struggles to penetrate, which is why wavelength selection matters particularly for joint work: you want the wavelength that loses the least energy to the tissue layers it has to cross before it gets there.

Does this mean it won't work unless everything is optimised? No. Red and near-infrared light used sensibly across a wide range of intensities has shown consistent positive effects in the research. But understanding these layers means you can stop guessing and start making deliberate choices about which tool you reach for.

Blood flow and haemoglobin

Haemoglobin absorbs strongly at 660 nm. In inflamed tissue where blood flow is elevated, that absorption reduces the penetration depth of red light considerably, which is somewhat counterintuitive because inflamed tissue is often exactly what you're trying to treat. Near-infrared at 810–850 nm is much less affected by haemoglobin, which is another reason it tends to outperform red light for deeper inflammatory conditions.

Areas with lower blood flow allow better light transmission. Wraps and direct-contact devices reduce losses from both blood absorption and skin surface reflection by eliminating the air gap between emitter and skin. The portable and specialist range covers these for localised use.

Device design and delivery

Higher irradiance helps push light past biological barriers. But how it's delivered matters as much as how much of it there is.

Panels used at 10–30 cm are effective for broad surface coverage: back, chest, full torso. Even high-output panels lose some energy to skin reflection and ambient scatter at distance, but the coverage area compensates. For the joint on your knee or the specific point on your shoulder, a torch changes the equation entirely. Direct contact, 810–850 nm, irradiance above 100 mW/cm² focused on a small area: that's a different kind of dose to what you get standing in front of a panel.

I measure and verify the irradiance and spectral output of every device I stock, with a spectrometer rather than a solar meter, and at real usage distances rather than the optimistic close-range readings some manufacturers publish. I've sent devices back when the measured output didn't match the claimed specs. That conversation with a supplier is never fun. But an underpowered device with inflated marketing numbers isn't something I'll sell. The irradiance data in your personal protocol guide is what you'll actually receive in a session. The irradiance post covers this in more detail if you want to understand what to look for before you buy.

Matching tool to target

The practical question is: what are you treating, and where is it? Surface skin work, whether collagen, texture, acne or scarring, is well served by a panel at the right distance. Broad muscle recovery after training, back tension, general mitochondrial support: panel again. For anything deep, a joint, dense connective tissue, thick muscle on the back or thighs, a focused torch at 810–850 nm or a wrap delivering direct contact is the right tool. Darker skin tones may benefit from slightly longer sessions or higher NIR emphasis to account for melanin absorption at the red end of the spectrum.

If you're unsure which device fits your specific situation, the device selection guide is a good starting point, or message me directly. I'd rather spend five minutes helping you choose the right thing than have you buy something that doesn't reach what it needs to.

See the full panel range or the portable and specialist devices depending on what you're working on.