Red light therapy uses visible red light and near-infrared light, both part of the non-ionizing portion of the electromagnetic spectrum. The visible red wavelengths fall between 620 and 700 nanometers (nm), while near-infrared wavelengths sit between roughly 800 and 1,000 nm. Neither produces ultraviolet radiation, and neither generates significant heat at therapeutic power levels.
Where Red Light Sits on the Spectrum
The electromagnetic spectrum ranges from extremely short, high-energy wavelengths (gamma rays, X-rays) to extremely long, low-energy ones (radio waves). Visible light occupies a narrow band in the middle, and red light sits at the long-wavelength end of that visible band. Just beyond red, wavelengths become invisible to the human eye and enter the near-infrared range.
This positioning matters because it means red and near-infrared light carry far less energy per photon than ultraviolet light, X-rays, or gamma rays. Those shorter-wavelength forms of light are “ionizing,” meaning they carry enough energy to damage DNA and break chemical bonds. Red and near-infrared light cannot do that. They interact with tissue through gentler, non-destructive mechanisms, which is why red light therapy doesn’t cause sunburn or increase skin cancer risk the way UV exposure does.
The Two Wavelength Windows
Most red light therapy devices deliver light in one or both of two ranges. The first is visible red, typically centered around 630 to 660 nm. You can see this light as a deep red glow. The second is near-infrared, most commonly around 850 nm. Near-infrared light is invisible, so a device emitting only this wavelength looks dim or completely dark to your eyes even when it’s on.
These two windows aren’t arbitrary. They correspond to absorption peaks of a specific protein inside your mitochondria (the energy-producing structures in every cell). That protein absorbs photons most efficiently at red and near-infrared wavelengths, which is why those ranges were selected for therapy rather than, say, green or blue light.
How Red Light Interacts With Cells
The key target is a protein called cytochrome c oxidase, the fourth step in the chain your mitochondria use to convert food into cellular energy (ATP). Under normal conditions, a small molecule called nitric oxide can bind to this protein and slow the energy production process. When red or near-infrared photons are absorbed, they knock that nitric oxide loose, allowing the protein to work at full speed again. The result is increased energy output at the cellular level and a rise in the electrical charge across the mitochondrial membrane, which further supports energy production.
This mechanism is why the formal scientific name for red light therapy is “photobiomodulation,” literally meaning light that modulates biological activity. The effect depends on the photon being the right wavelength to be absorbed. A photon that passes straight through tissue, or one that’s absorbed by water or melanin before reaching the target protein, won’t trigger the same response.
How Deep the Light Penetrates
One common misconception is that red and near-infrared light penetrate deep into the body. In reality, low-power light sources under 6 watts are limited to roughly the first 3 millimeters of human skin. One study using an 850 nm source at 0.10 watts found that only 34% of incident light made it through less than 0.8 mm of skin. Even at 820 nm, analysis shows penetration of less than 2.2 mm from a low-power laser diode.
Near-infrared wavelengths do penetrate somewhat deeper than visible red because longer wavelengths scatter less in tissue. But the difference is a matter of millimeters, not inches. This means red light therapy is most effective for superficial targets: skin cells, shallow nerve endings, and tissue close to the surface. Claims about treating deep organs with a handheld LED panel should be viewed skeptically.
LEDs vs. Lasers
Red light therapy can be delivered by two types of light sources: LEDs and lasers. Lasers produce a very narrow, coherent beam where all the light waves are synchronized. LEDs produce a slightly broader spread of wavelengths (typically a few tens of nanometers wide versus 1 to 2 nm for a laser diode) and the light waves are not synchronized.
For years, some practitioners argued that laser coherence was essential for therapeutic effects. The current body of evidence doesn’t support that claim. Published reviews have concluded that photobiomodulation is a photobiological phenomenon, not dependent on coherence. LED devices, and even broader-spectrum filtered light sources, produce measurable physiological effects at the same wavelengths. Since LEDs cost roughly one-hundredth as much per milliwatt of optical power and can be arranged in large flat panels to cover broad treatment areas, they dominate the consumer market. Lasers remain more common in clinical settings where precise, focused delivery to a small spot is needed.
How It Differs From Infrared Heat Therapy
Red light therapy is often confused with infrared heat lamps or infrared saunas, but they work through different mechanisms. Far-infrared devices (wavelengths well above 1,000 nm) generate heat that you can feel on your skin. That heat raises tissue temperature, increases blood flow, and promotes sweating. The therapeutic effect is thermal.
Red light therapy at 630 to 850 nm does not produce meaningful heat at the power levels used therapeutically. Its effects come from photon absorption by specific cellular proteins, not from warming tissue. If a red light device feels hot on your skin, that heat is a byproduct of the electronics, not the therapeutic mechanism. The distinction matters because the biological responses triggered by each approach are fundamentally different: one is a heat response, the other is a photochemical one.
Power and Dose
The therapeutic dose in red light therapy is measured in joules per square centimeter (J/cm²), which accounts for both the power of the light and how long it’s applied. Clinical devices used in skin studies typically deliver irradiance levels between about 5 and 55 milliwatts per square centimeter in the red wavelength range. Treatment times vary from a few minutes to 20 or 30 minutes depending on the device’s output.
The FDA classifies clinical photobiomodulation devices as Class II medical devices, which means they require premarket review for safety and effectiveness when marketed for specific medical uses. Many consumer panels and handheld devices are instead marketed as “general wellness” products with lower power levels, placing them outside that regulatory framework. This distinction means consumer devices vary widely in actual light output, and not all of them deliver enough energy to replicate the doses used in clinical research.