Why 50mW/cm² Isn’t Always Equal in Red Light Therapy

Why 50mW/cm² Isn't Always Equal in Red Light Therapy

By Dominic Lamb - LightTherapy.no

Someone contacted me recently to ask why the panel they had bought "wasn't working" - they had compared the irradiance spec on two devices, bought the one with the higher number, used it for six weeks and felt nothing had changed. When I asked a few questions it turned out they were sitting 40cm away, lights angled off to the side so it didn't glare into their screen, for twenty minutes a day. The irradiance number they had bought on was measured at 15cm directly in front of the panel. At 40cm, with that angle? They were probably receiving about a quarter of what they thought.

Not because they did anything wrong. Because nobody told them this matters. So let me.

What irradiance tells you - and what it doesn't

Irradiance measures how much light energy hits a surface per second, in milliwatts per square centimetre. It's the right number to look at when deciding whether a device can deliver a therapeutic dose at all - too low and you're not stimulating anything meaningful, too high for too long and you can tip into the biphasic dose response territory where more becomes less (a whole separate conversation, but worth knowing about).

What irradiance doesn't tell you is what happens to the light after it touches your skin. And if you're trying to treat a knee joint, a nerve, a jaw, a sinus - what matters is how much energy reaches that tissue. Not how much landed at the skin surface. The gap between those two numbers is where beam angle, skin contact and tissue scatter live. And it's a meaningful gap.

Beam angle - the number they don't put on the box

Most red light panels use LEDs with a beam angle of around 30-60 degrees. At 5-10cm from the panel, the light is still reasonably concentrated. At 20-30cm - which is where most people sit - the beam has spread considerably. The centre of the panel delivers meaningfully higher irradiance than the edges. And if you angle it off to the side to avoid screen glare, you've added another layer of loss.

I alternate between 30 and 60 degree LEDs depending on the device I'm building, and the choice has real consequences for how the device is used. If someone specifically wants tighter beam angles, I can arrange that - message me. But the point here is that you need to know which you have, because the difference in effective dose at typical use distance is significant.

A torch with a collimated lens works differently. Beam angle under 15 degrees. The light stays focused at skin contact and delivers most of its energy to a tight target area. That's exactly what you want when you're trying to reach a joint capsule, a nerve root or deep musculature. It's not better than a panel in any universal sense - it's the right tool for a different job.

Skin contact and reflection

Some light is lost before it even enters the body. Studies show 5-10% of red and near-infrared light reflects off the skin surface - more if the beam hits at an angle, or the skin is oily, dry, or there's hair in the area. Direct skin contact - torch pressed to the skin, wrap covering a joint - minimises this. The photons enter more directly, fewer are wasted.

That's a meaningful practical advantage for devices used in contact versus panels sitting at a distance. I'd rather use a lower-spec torch in contact than a high-spec panel at arm's length for treating a specific joint. The numbers on the box don't tell you that. The physics does.

What happens once light enters the tissue

Inside the skin, light interacts with collagen, cell membranes, blood vessels and - what we're ultimately trying to reach - cytochrome c oxidase in the mitochondria. Shorter wavelengths like 660nm are absorbed quickly in superficial tissue. Near-infrared (810-850nm) penetrates deeper but also scatters more as it goes.

Collimated light from a torch scatters less on the way in - more of it reaches the deeper structures you're targeting. Panels with broader divergence lose more energy through scatter, especially used at distance. Melanin in the skin is another variable that affects absorption and scatter - darker skin tones may need longer exposure for equivalent tissue dose, which almost nobody in this industry talks about honestly. That's a separate post.

Three devices, same number, different realities

None of these is better. They're matched to different applications. The right one depends on what you're treating and where in the body the target is.

The irradiance claims you can't trust - and why I test everything

Most brands in this space will not tell you what I'm about to tell you. Many of the irradiance numbers you see marketed are measured using methods that bear no resemblance to how you'll actually use the device. Sensor placed directly against the diode. Peak reading from the single best point on the panel, not an average. Lab conditions that nobody replicates at home. The result is a number that can be two to three times higher than what you actually receive sitting in front of it at a normal distance.

Not slightly inflated. Materially misleading.

I measure every device I sell with a calibrated spectrometer under realistic conditions. The irradiance I report is measured at the distances people actually use. The wavelength output is verified, not assumed from the LED spec. When I tell a customer "660nm at 50mW/cm² at 20cm," that is the number they will actually receive. And I give every customer a personal usage guide based on those measured values - so they know exactly how far to sit, how long to run a session, and what to expect.

I'm the only seller in Norway doing this (to my knowledge, as of 2025). I think it should be standard. It isn't.

So - what do you actually need?

Ask yourself three things: what are you treating, where in the body is it, and how often will you realistically use it?

For systemic energy, mood, sleep and general mitochondrial support through the Norwegian winter - a large panel used consistently every morning. For targeted deep tissue - nerve pain, arthritis, jaw tension, post-injury recovery - a torch at close range. For hands-free daily therapy over a specific joint, the neck or the gut - a wrap. These aren't competing products. They're a toolkit.

Browse the panel range and the portable and specialist devices. Not sure what fits? Get in touch - that's genuinely what I'm here for. Norwegian version of this post: Hvorfor 50mW/cm² ikke alltid er lik i rødlysterapi. For a deeper look at penetration depth, wavelength and melanin: More Factors Affecting Light Penetration in Red Light Therapy.

- Dominic Lamb, Home Light Therapy