Basics
The effects of red and near-infrared light have been researched for over 50 years. This page answers three key questions: what is red light therapy, what happens in the cell, and what does research show about possible applications? Clearly explained – without unnecessary jargon, but with scientific sources.
01 What is red light therapy?
In short
Red light therapy is the application of red and near-infrared light at specifically selected wavelengths. The technical term for this is photobiomodulation. Unlike a conventional heat lamp, the focus is not on warming the tissue. Instead, the light can be absorbed by certain molecules in the cells, where it can influence biological processes.
The technical term already describes the principle quite well: Photo means light, Bio stands for biological processes and Modulation for influencing or regulating them. No tissue is removed or burned in the process. What matters instead is a suitable light dose that can trigger biological responses in the cell.
The effect was discovered more or less by chance at the end of the 1960s. The Hungarian physician Endre Mester noticed that weak laser light accelerated wound healing in laboratory animals. This is why this form of light therapy was referred to for decades as ‘low-level laser therapy’. Today we know that what matters most for photobiomodulation is the wavelength and the dose – not whether the light comes from a laser or an LED. A photon at 660 nm has the same energy in both cases1. This is why the more neutral term ‘photobiomodulation’ has become established.
Even though both are lamps that glow red, they work differently. A conventional heat lamp (infrared lamp) emits a broad spectrum of light and contains large amounts of long-wave infrared radiation. This light is almost completely absorbed by the water in the tissue and converted into heat. This heat is clearly noticeable – and that is exactly what the application is designed for.
| Heat lamp | Red light therapy | |
|---|---|---|
| Principle of action | Heat | photobiological signal |
| Spectrum | broadband | defined wavelengths |
| Absorbed by | water in the tissue | enzymes in the mitochondria |
| What you feel | noticeable warmth | little to nothing |
| Dosage | based on how warm it feels | calculable in J/cm² |
You feel a heat lamp. Photobiomodulation has to be measured.
If it's not about heat: what does the light do in the body?
02 How it works
In short
Red and near-infrared light is absorbed in the mitochondria by an enzyme: cytochrome c oxidase. This enzyme sits at the end of the so-called respiratory chain – a series of reactions the cell uses to produce energy. When it absorbs light energy, this chain can work more freely again13.
Almost every cell in our body contains mitochondria – the “powerhouses of the cell”. They produce ATP, the universal energy molecule. Whether a muscle is working, collagen is being formed or a damaged structure is being repaired: all of these processes require ATP.
This explains why research covers such a wide range of topics – from skin and muscles to wound healing. The mechanism involved is not located in a single organ but in a component of practically every cell15.
Red and near-infrared light penetrate the tissue to different depths. Near-infrared light reaches deeper than red light67.
Cytochrome c oxidase contains four metal-containing centres – two with iron, two with copper. These centres absorb light particularly well in narrow ranges of the red and near-infrared spectrum23.
Under stress, oxygen deficiency or inflammation, nitric oxide attaches to the enzyme and blocks the site for oxygen. Light can release this bond. Oxygen can bind again, energy production runs more freely and nitric oxide is released into the tissue14.
ATP production increases, and the released nitric oxide is available in the tissue as a signalling molecule – among other things as the body’s own vasodilator. The changed energy state is reported to the cell nucleus. There, different genes are read and different proteins are formed, which are involved, for example, in repair processes, cell migration and antioxidant defence15. This sets processes in motion whose effects may become noticeable later.
The best way to picture it
Light is not a charger, but rather a wake-up call. The energy for ATP still comes from food and oxygen. Light removes a brake and signals the cell to start up its own programmes.
Cytochrome c oxidase is considered the best-documented light acceptor – that is, a molecule that can absorb light and respond to it. However, it is probably not the only one. Light-sensitive ion channels in the cell membrane and possible effects on water structures in and around cells are also being discussed1. Also still open are the optimal dose for individual areas of application and the question of how strongly skin type, age and tissue thickness influence the results. So the state of research has not yet been fully clarified on every point.
And why red, of all colours? Because only a narrow part of the light spectrum gets far enough into the body at all.
03 Why red and near-infrared
In short
Between roughly 600 and 1000 nanometres there is a range in which light is absorbed only slightly by skin pigment, blood or water. This “optical window” is the reason why red light therapy uses red and near-infrared light – and not blue or green.
Light that is meant to reach into the body has to get past three important absorbers. Melanin (the skin pigment) mainly absorbs short-wave light. Haemoglobin (the blood pigment) absorbs strongly in the blue and green range. That is why blood appears red: red light is absorbed less strongly. Water (our main component) mainly absorbs long-wave infrared.
In between there remains a range of low absorption. This is precisely where practically all research on photobiomodulation takes place.
Both work via the same mechanism in the cell. The difference lies in how far they reach into the tissue.
| Red light | Near-infrared | |
|---|---|---|
| Range | approx. 600–700 nm | approx. 700–1000 nm |
| Visible? | yes, vivid red | no, almost invisible |
| Mainly reaches | Skin, collagen, hair roots | Muscles, joints, tendons |
| Often studied for | Complexion, wound healing, hair density | Recovery, musculoskeletal system |
On penetration depth: please read critically
“Penetration depth” cannot be expressed as a single number. It depends on how strongly the light is attenuated on its way through the tissue. Penetration depth is often defined as the point at which about 37 % of the original light intensity is still present. This value can vary considerably depending on skin type, tissue and blood flow67. What is certain above all is that near-infrared light reaches deeper into the tissue than red light. Statements such as “penetrates 10 cm deep”, on the other hand, are misleading. Individual fractions of light may still arrive at such depths, but the amount of light available there is significantly lower. That is why we deliberately avoid blanket figures in millimetres.
And the dose?
One principle is particularly important here: with photobiomodulation, “more is better” does not apply. The effect follows a curve with an optimum. Too little light may have no effect, too much can weaken the desired effect again8. How wavelength, irradiance, distance and duration combine into a sensible dose is explained on the technology page.
That explains the mechanism. Which leaves the most important question: what does this mean in practice?
04 What it does
Photobiomodulation is a broad field of research with very varied evidence. We sort by the strength of the scientific evidence, not by what sounds particularly appealing. We also show where the scientific evidence is still limited.
A good point of reference is a scientific assessment from 2025 published in the Journal of the American Academy of Dermatology9. In it, an expert panel assessed the clinical use of photobiomodulation. For some areas – including certain forms of chronic wounds, nerve damage, radiation-induced skin reactions and hereditary hair loss – the panel found robust scientific evidence.
Important context
The studies cited were predominantly carried out with clinical devices or lasers, often in people with a diagnosed condition and under medical supervision. They show that and how this field of research is being studied. They are not a statement about what a home device will do for an individual person. heilys® devices are not medical devices.
05 An honest assessment
A field that attracts both serious research and exaggerated promises needs a clear distinction.
We consider this clarity an advantage. Anyone who can explain the limits of their product can also credibly explain how it works.
On safety: Red and near-infrared light is non-ionising. Each light particle does not carry enough energy to damage DNA – unlike UV or X-rays9. Nevertheless, you should bear a few points in mind – such as the recommended eye protection, photosensitising medication, pregnancy and pre-existing conditions. You will find more detailed information on the safety page.
06 Frequently asked questions
This cannot be answered with a single number. The usual reference value is the depth at which about 37 % of the light intensity remains. This value depends on skin type, tissue and blood flow67. What is reliable above all is the order: red light acts mainly in the skin, near-infrared reaches into the underlying structures.
Because the sensitivity of the human eye practically ends above about 780 nm. An 850 nm LED can run at full power and still appear almost dark. This is normal and not a defect. That is precisely why safety goggles are important with near-infrared: the glare that would otherwise make you look away is missing.
UV light has a completely different mechanism. UV photons carry enough energy to break chemical bonds, damage DNA and accelerate skin ageing. The effects of photobiomodulation, by contrast, work via the absorption of light in an enzyme. This requires red and near-infrared light. Photobiomodulation devices therefore operate without any UV component.
For the mechanism described, according to current understanding, yes. The special focusing of laser light does not play a decisive role in absorption in the cell. What matters are wavelength and dose1. This is exactly why the term photobiomodulation has prevailed over “low-level laser therapy”. LEDs have the advantage of irradiating large areas evenly.
Usually you only feel a slight warmth. A strong immediate sensation is not part of how photobiomodulation works. That distinguishes it from a heat lamp. It is to be expected that you feel little. It is not a sign that nothing is getting through. Conversely, a strong feeling of heat is not a mark of quality, but usually a sign that the distance is too short.
There is no blanket recommendation, because the right dose depends on wavelength, distance and target tissue, among other things – and because more is not automatically better8. Studies usually examine regular, short sessions over several weeks. So stick to the recommended duration and frequency for your device.
Six wavelengths in the optical window, documented irradiance, developed in Germany and a five-year guarantee.
15 papers from the scientific literature, each with a DOI or PubMed ID for verification.
Note: This page is for information and educational purposes. It does not constitute medical advice and does not replace diagnosis or treatment by medical professionals.
heilys® products are not medical devices within the meaning of Regulation (EU) 2017/745 and are not intended to diagnose, treat, cure or prevent any disease. The studies cited describe the current state of scientific research on photobiomodulation. They were predominantly carried out with clinical devices or laser systems under controlled conditions and do not constitute a statement about the effect of heilys® products on individual users. Individual results may vary.
If you have any existing medical conditions, are pregnant, are taking photosensitising medication or are unsure about use, please consult your doctor before using the device.
