Red Light Therapy (RLT) utilizes specific wavelengths of light to promote cellular function and healing. A common question for those starting this treatment is whether the light can pass through clothing to reach the skin. The direct answer is that some light transmission does occur, but the effectiveness is highly variable. This variability depends on the specific wavelength being used and the type of material covering the skin.
How Light Interacts with Materials
When light encounters any material, it undergoes three primary interactions: absorption, reflection (or scattering), and transmission (or penetration). Absorption occurs when the material captures light energy, converting it into heat or other forms of energy. Reflection happens when the light bounces off the material’s surface or internal structures. Transmission is the portion of light that passes completely through the material.
The degree to which light penetrates a material is fundamentally tied to its wavelength. Shorter, higher-energy wavelengths are absorbed more readily by surface materials. Longer wavelengths possess physical properties that allow them to scatter less and bypass surface barriers more effectively.
The Critical Difference Between Red and Near-Infrared Light
Red Light Therapy devices typically emit light in two distinct spectral ranges: visible red light (RL) and near-infrared light (NIR). Visible red light (630 to 700 nm) has a shorter wavelength. This light is efficiently absorbed by pigments in clothing dyes and the superficial layers of the skin. Consequently, red light has limited ability to pass through most fabrics and only reaches shallow tissue depths.
In contrast, near-infrared light utilizes significantly longer wavelengths, usually ranging from 800 to 1000 nm. These longer wavelengths scatter less when encountering particles and molecules within a medium. This property allows NIR light to more easily bypass the initial absorption barriers presented by clothing fibers and the upper dermal layers.
For therapeutic applications targeting deeper structures like muscle or joint tissues, NIR light is often the more active component because of its superior penetration capability. The ability of NIR light to transmit through fabric is far greater than that of visible red light, making it the primary source of any deeper therapeutic effect achieved through clothing.
Factors Determining Clothing Penetration
The material covering the body dictates how much light ultimately transmits to the skin. Clothing color is the most substantial factor in light absorption. Dark fabrics, such as black or navy, absorb a high percentage of incoming light, sometimes over 90%. Conversely, light-colored or white fabrics reflect a greater proportion of the light, allowing more to pass through the weave.
The density and thickness of the textile also play a substantial role in blocking light transmission. Tightly woven materials like denim or heavy fleece present a dense physical barrier that minimizes the pathways for photons to pass through. Thinner, sheer fabrics with a loose knit, such as light cotton or certain synthetic blends, allow a larger volume of light to transmit through the spaces between the threads.
Furthermore, the specific composition of the material affects the outcome. Some synthetic fibers and natural materials have different internal reflective and scattering properties. Generally, the combination of light color and low density yields the highest penetration rate for both red and near-infrared wavelengths.
Therapeutic Effectiveness and Practical Use
While near-infrared light can certainly penetrate thin, light-colored garments, the therapeutic irradiance delivered to the target tissue is significantly diminished. The fabric acts as a filter, scattering and absorbing a portion of the light, which reduces the total energy density reaching the intended treatment area. This reduction in energy can compromise the photobiomodulation effect on cells below the skin’s surface.
To ensure the maximal delivery of photons to the targeted cells, direct skin exposure to the light source remains the recommended practice. If removing clothing is not feasible for a specific treatment area, the best practical choice is to wear very thin, white, or lightly colored natural fabrics, such as a thin cotton t-shirt.