Red light vs near-infrared: what 660nm and 850nm actually mean

Pick up almost any light therapy device and you'll find two numbers on the box, often 660nm and 850nm. They look like model numbers, but they're some of the most useful facts on the label. They tell you what kind of light the device makes, and that shapes where in the skin the light is likely to end up. If you've wondered about red light vs infrared, or why so many devices use both, this guide explains what the numbers mean, how the two kinds of light differ, what the research does and doesn't tell us, and what to look for when you compare devices.

What does 660nm actually mean?

Light travels as a wave, and "nm" stands for nanometre, a billionth of a metre. The number is the wavelength: the distance from one peak of the wave to the next. Wavelength decides the colour we see. Shorter wavelengths look blue or violet, and longer ones look orange and then red.

At 660nm you're looking at deep red light. Go a little longer and you pass the edge of what your eyes can pick up. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) describes infrared as the band beyond visible red, starting at about 780nm. So 850nm sits in the near-infrared, the part of the infrared band closest to visible light. ICNIRP calls this band IR-A.

Red light vs infrared: what's the difference?

The short answer: red light is visible and near-infrared mostly isn't, and near-infrared tends to travel a little further into tissue before it's absorbed.

When you switch on a device that combines the two, the red LEDs glow a strong, obvious red. The near-infrared LEDs may show only a faint dull glow, or nothing at all, even though they're working. That's normal, and it's why "invisible" doesn't mean "off".

It's also worth knowing that "infrared" covers a huge range. ICNIRP's infrared band runs all the way from about 780nm to 1mm. Infrared is also known as thermal radiation, and ICNIRP lists sunlight, fire, heaters and infrared saunas among its sources. Near-infrared light therapy uses the short end of that range, close to visible red.

How deep does red and near-infrared light go?

Skin isn't a clear window. As light passes into it, some is absorbed and some is scattered in different directions. In a review of light therapy research, Huang and colleagues describe an "optical window" running from roughly 650nm to 1200nm, where light gets furthest into tissue. Below that range, the body's main pigments (haemoglobin in blood and melanin in skin) absorb much more light, and scattering is stronger. At the long end, water starts absorbing infrared strongly.

A 2017 computer-modelling study of light in skin found the same pattern across the wavelengths it tested, from 300nm to 750nm: the longer the wavelength, the deeper the light reached in the model. ICNIRP notes that near-infrared can reach several millimetres into tissue.

In everyday terms, red light is more likely to be absorbed in the skin's outer layers, and near-infrared is more likely to carry a bit further before it's absorbed. Neither is a beam that passes straight through you, and how far light travels also depends on the device and on your own skin.

Why do many devices combine 660nm and 850nm?

Both wavelengths sit inside that optical window, and pairing them means some of the light is more likely to be absorbed nearer the surface and some a little deeper. The proposed mechanism is also shared. In a 2017 review, Michael Hamblin explains that one of the main light-absorbing targets identified for red and near-infrared light therapy is an enzyme in the mitochondria called cytochrome c oxidase. Absorbing light is thought to set off a short chain of changes inside the cell, including a rise in ATP, the molecule cells use for energy.

That mechanism comes mostly from laboratory and animal research. One example: a 2017 cell study exposed human stem cells grown in a dish to four wavelengths. Red (660nm) and near-infrared (810nm) light both increased cell growth at the dose used, while blue and green light reduced it. It's a useful clue that red and near-infrared behave in similar ways in cells, but it's a cell culture experiment, not a trial in people, and it didn't use a home device. Research in people uses a wide range of wavelengths, doses and devices, and results vary, so any single study is only one piece of a bigger picture.

What to look for on a label

• Specific wavelengths. A number like 660nm or 850nm is more useful than a vague term like "full spectrum" or "infrared light".
• Which LEDs make which light. A ratio or LED count for each wavelength tells you how the light is split.
• Light output. Irradiance, usually in mW/cm², tells you how much light reaches the skin. More isn't automatically better; our post on why more intensity isn't always better explains why.
• A timer and clear directions. Session length matters as much as wavelength.
• The format. The same wavelengths can come in a panel or a wearable wrap. Our guide to choosing a panel or a wrap covers the practical differences.

When to see someone instead

Wavelengths matter, but they're no reason to put off getting a symptom checked. See your GP or a physio if pain follows an injury, if a joint is swollen, hot or red, if pain is getting worse or waking you at night, or if you notice numbness, tingling or weakness. You can find a physio through the Australian Physiotherapy Association's Find a physio directory. If you're sensitive to light or take medication that can increase light sensitivity, check with your GP or pharmacist before using any light device.

Where the Red Light Wrap fits

The Fringe Red Light Wrap uses both wavelengths covered here: 660nm red and 850nm near-infrared, in about a 2:1 ratio of near-infrared to red, across 360 LED chips. It's wireless and flexible, with adjustable brightness and a built-in 20-minute timer, and it can be used on the feet, ankles, knees, hips, lower back, neck, shoulders, elbows, wrists and hands. The Red Light Wrap is intended for the temporary relief of minor muscle and joint pain and stiffness, minor arthritis pain or muscle spasm relief, temporary increase in local blood circulation and temporary relaxation of muscles.

The usual guidance is one session of up to 20 minutes per area, once a day, in line with the built-in timer. When you switch it on, expect a strong red glow from some LEDs and only a faint glow or none from the near-infrared ones. If you're new to light therapy, Red light therapy 101 is a good place to start.

The contents of this blog, such as text, content and graphics, are intended for educational purposes only. The content is not intended to substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your healthcare provider. Consult your healthcare provider before use if you are pregnant, are sensitive to light, or are taking medications that may increase light sensitivity. The Red Light Wrap is included on the Australian Register of Therapeutic Goods (ARTG 469586). Always read the label and follow the directions for use.