Infrared and red light therapy are often confused, but they are fundamentally distinct approaches to light-based treatment. Both utilize light from the electromagnetic spectrum for therapeutic purposes. However, the specific wavelengths they employ determine whether the light is visible or invisible, and this difference dictates the biological mechanism of action within the human body. Understanding this distinction is necessary to select the appropriate modality for a particular health or wellness goal.
Red Light Therapy: The Visible Spectrum
Red Light Therapy (RLT) uses light in the visible spectrum, typically between 600 and 700 nanometers (nm), with 630 nm and 660 nm being common wavelengths for devices. This visible red light is part of the broader category known as Photobiomodulation (PBM). RLT is a non-thermal process, meaning the light energy does not generate significant heat in the tissue upon absorption. The therapeutic effect is purely biochemical, initiated by the light particles, or photons, interacting with components inside the body’s cells.
The primary target for these red light photons is the mitochondria, specifically an enzyme called cytochrome c oxidase (CCO). When CCO absorbs the red light, it helps to release inhibitory nitric oxide, which improves the efficiency of the cellular respiratory chain. This photochemical reaction enhances the production of Adenosine Triphosphate (ATP), which is the primary energy source for the cell. This increase in cellular energy subsequently supports tissue repair, reduces inflammation, and accelerates wound healing.
Infrared Radiation: The Invisible Heat
Infrared (IR) radiation occupies a range of non-visible light wavelengths, starting just above the visible red light spectrum at about 700 nm and extending up to 1 millimeter (mm). The IR spectrum is broken down into three main categories: Near-Infrared (NIR), Mid-Infrared (MIR), and Far-Infrared (FIR). NIR is closest to red light, typically ranging from 700 nm to 1,100 nm, and MIR and FIR are the invisible rays most commonly associated with the sensation of heat.
The mechanism of action for the longer IR wavelengths, particularly MIR and FIR, is largely thermal. When these longer photons strike human tissue, the energy causes the water molecules and other molecular bonds within the tissue to vibrate rapidly. This increased molecular motion is perceived as heat, leading to an elevation in local tissue temperature. This thermal effect is utilized in applications like infrared saunas, where the goal is to induce sweating, improve circulation, and relax muscles through gentle, internal heating.
Distinguishing Mechanisms: Photobiomodulation vs. Thermal Effect
Red light therapy operates via photobiomodulation (PBM), a non-thermal signaling process where photons act as a trigger for a cascade of biochemical events. PBM’s effect is achieved by the light being absorbed by the mitochondria, directly increasing cellular energy production without causing a significant temperature rise.
In contrast, the primary therapeutic action of most infrared radiation, especially the longer FIR and MIR wavelengths, is a thermal effect, or thermogenesis. The energy is absorbed by water molecules, which convert the light energy into heat. While NIR (700–1100 nm) can have some PBM effects due to its proximity to red light and its ability to target CCO, the general application of infrared therapy, such as in saunas, relies on the heat to stimulate blood flow and induce systemic effects like sweating.
Penetration Depth and Targeted Tissue
The difference in wavelength dictates how deeply the light can travel into human tissue before being absorbed or scattered. Red light, with its shorter, visible wavelengths (600–700 nm), is largely absorbed by chromophores in the skin. This absorption limits its reach, making RLT most effective for superficial concerns like promoting collagen production in the dermis and accelerating skin wound healing.
Near-Infrared (NIR) wavelengths (around 700 nm to 1100 nm) scatter less and are absorbed less by the water in the upper skin layers. This allows NIR to travel substantially deeper, reaching underlying muscle tissue, joint capsules, and nerves. Consequently, NIR is the preferred choice for treating deep muscle soreness, joint inflammation, and pain management. Conversely, the much longer Mid- and Far-Infrared wavelengths are absorbed very efficiently by water in the skin, meaning their effect is primarily felt as heat on or just below the skin’s surface, and the therapeutic warmth then conducts to deeper tissues.