Red light therapy uses visible red light and near-infrared light, both of which sit on the non-ionizing end of the electromagnetic spectrum. These wavelengths fall roughly between 630 and 850 nanometers, placing them in the range your eyes can partially see (the red glow) and partially can’t (the invisible near-infrared portion). Unlike ultraviolet light from the sun or tanning beds, this light does not damage DNA or increase cancer risk.
Where Red Light Sits on the Spectrum
All light is electromagnetic radiation, and the electromagnetic spectrum organizes it by wavelength. At one end, you have short-wavelength, high-energy radiation like X-rays and ultraviolet light. These are ionizing, meaning they carry enough energy to knock electrons off atoms and damage cells. At the other end, you have long-wavelength, low-energy radiation like radio waves and microwaves.
Red light therapy operates in a narrow band between roughly 630 and 700 nanometers for visible red light, and 800 to 1,000 nanometers for near-infrared light. Both fall squarely in the non-ionizing category. They deliver energy to your cells without the molecular damage that UV light causes. This is the same part of the spectrum that includes the warm glow of a sunset or the invisible heat you feel radiating from a campfire.
Visible Red vs. Near-Infrared
Most red light therapy devices combine two types of light, and the distinction matters because they reach different depths in your body.
Visible red light (630 to 700 nm) is the part you can see. It penetrates about 4 to 5 millimeters beneath the skin surface. That’s deep enough to reach the upper layers of skin tissue, which is why red light is commonly used for surface-level concerns like skin texture, wound healing, and inflammation close to the surface. For comparison, blue light (used in acne treatments) barely reaches 1 millimeter into tissue.
Near-infrared light (800 to 1,000 nm) is invisible to the naked eye. It penetrates significantly deeper, reaching muscles, joints, and connective tissue well below the skin. This is the wavelength range more commonly targeted for pain, joint stiffness, and deeper tissue recovery. The two wavelengths studied most often in clinical settings are 830 nm and 850 nm.
How This Light Affects Your Cells
The technical name for red light therapy is photobiomodulation, which simply means using light to change how cells behave. The mechanism centers on your mitochondria, the structures inside every cell that produce energy.
When photons from red or near-infrared light reach your cells, they’re absorbed by an enzyme in the mitochondria called cytochrome c oxidase. This absorption triggers a chain of events: the mitochondria ramp up energy production (in the form of ATP), generate signaling molecules, and shift calcium levels inside the cell. Those changes activate pathways that promote cell growth, repair, and migration to damaged areas. In practical terms, this means faster healing, reduced inflammation, and increased cellular turnover in the treated area.
One important characteristic of this process is that dose matters in a non-obvious way. The cellular response follows a biphasic pattern, meaning too little light has no effect, a moderate dose stimulates healing, and too much light can actually inhibit the process or produce no benefit at all. Research on human stem cells found that an energy dose of 5 joules per square centimeter produced the best results for cell activity and migration, while higher doses diminished the effect. This is why more exposure time or a brighter device doesn’t necessarily mean better results.
LEDs vs. Lasers
Red light therapy devices deliver the same general wavelengths through two different technologies: light-emitting diodes (LEDs) and low-level laser diodes. The light itself is similar, but how it reaches your tissue differs.
Laser diodes produce coherent light, meaning the light waves are organized and travel in a tight, focused beam. This allows deeper, more concentrated penetration into tissue. LED devices emit non-coherent light that scatters across a broader area with less precision. LEDs cover more surface area per session but deliver lower energy density to any given point.
For skin-surface applications like improving complexion or treating mild inflammation, LED panels and masks are widely used and reasonably effective. For targets deeper in the body, like hair follicles or joint tissue, laser-based devices tend to deliver more energy where it’s needed. Medical-grade devices used in clinical settings typically use laser diodes for this reason. Home devices more commonly use LEDs because they’re cheaper to manufacture and safer for unsupervised use.
Power Density and What It Means
Not all red light therapy devices deliver the same amount of light energy, even if they use the same wavelengths. The key measurement is power density, expressed in milliwatts per square centimeter (mW/cm²). This tells you how much light energy is actually hitting each unit area of your skin at any given moment.
A device with low power density might technically emit the right wavelength but deliver too little energy to trigger a meaningful cellular response. Distance from the device also matters: power density drops as you move farther from the light source. This is why instructions for home devices typically specify holding the panel a certain number of inches from your skin. Combined with session duration, power density determines your total energy dose per treatment, the factor that drives the biphasic response described above.
Safety Compared to Other Light Types
Red and near-infrared light sit far from the dangerous end of the electromagnetic spectrum. Ultraviolet light (below 400 nm) carries enough energy to damage DNA and is a known cause of skin cancer. Red light therapy wavelengths carry far less energy per photon and do not cause this type of cellular damage. Cleveland Clinic notes that red light therapy is not toxic, not invasive, and does not use the cancer-causing ultraviolet wavelengths associated with sun exposure or tanning booths.
That said, the therapy isn’t without any precautions. At very high intensities, skin redness and blistering can occur. The light can also affect your eyes, so protective goggles are standard practice during treatment. People who take medications that increase sensitivity to light, or who have a history of skin cancer or eye disease, should check with a provider before starting treatment.