Clothing reduces the amount of red and near-infrared light that reaches your skin, and how much it reduces depends on the fabric’s color, thickness, and weave. Even a single layer of thin, light-colored cloth can cut near-infrared delivery by roughly 16%, while a thicker or darker fabric can block around 65% or more of the light before it ever touches your body. That means red light therapy can partially work through some clothes, but treating through fabric always means less energy arriving at your tissue, and in many cases the reduction is large enough to undermine the treatment.
How Much Light Fabrics Actually Block
A 2025 study measured optical power arriving through fabric layers at near-infrared wavelengths and found that a single layer of thin, light-colored cloth reduced the delivered power by an average of about 16% compared to bare skin. A black cloth of greater thickness, by contrast, cut the delivered power by roughly 65%. Those numbers came from measuring actual optical power received on the other side of the fabric at multiple LED output settings, so they reflect real-world attenuation rather than theoretical modeling.
1Scientific Reports. Impact of a textile layer on joint optical data and power transfer to in-body devices: a study on an ex vivo approachA 16% loss might sound tolerable, and in some contexts it is. But red light therapy dosing is already a balancing act. Devices are calibrated to deliver a specific power density (measured in milliwatts per square centimeter) at the skin surface, and the therapeutic dose depends on maintaining that power density for a set amount of time. Losing even a sixth of the light means either extending your session or accepting a lower dose. And that is the best-case scenario with the thinnest, lightest fabric. Most everyday clothing does far worse.
Why Wavelength Matters for Fabric Penetration
Red light therapy devices typically emit wavelengths between about 630 and 670 nanometers (visible red) or between 810 and 850 nanometers (near-infrared). One of the properties that makes these wavelengths useful for therapy is the same property that gives them a better chance of passing through fabric: longer wavelengths of visible and near-infrared light penetrate materials more readily than shorter wavelengths do.
Research on how textiles interact with visible light found that fabrics generally block shorter wavelengths (like blue and violet) more effectively than longer ones (like red). The protection a fabric offers against visible light is better at the blue end of the spectrum and weaker at the red end, partly because longer wavelengths scatter less as they pass through fibrous structures.
2Journal of the American Academy of Dermatology. Protection from visible light by commonly used textiles is not predicted by ultraviolet protectionThis means red and near-infrared wavelengths have an inherent advantage over, say, blue light when it comes to getting through a T-shirt. But “better than blue light at penetrating fabric” is not the same as “penetrates fabric well.” Even with this wavelength advantage, fabric still absorbs and scatters a meaningful fraction of the photons. The advantage simply means the losses are not as catastrophic as they would be with shorter-wavelength light therapies.
Near-infrared light also has superior tissue penetration once it gets past the fabric and hits your skin, because absorption by melanin and water is relatively low in the 700 to 900 nanometer range and scattering within tissue is reduced.
3Nature Communications. Wearable textile-based phototherapy platform with customized NIR OLEDs toward non-invasive hair loss treatmentSo the issue is not that the light cannot reach deep tissue if it reaches your skin. The issue is making sure enough of it reaches your skin in the first place.
Which Fabric Properties Matter Most
Not all clothing is equal here. Three properties dominate how much light a given fabric lets through: color, thickness, and the tightness of the weave or knit.
Color is the most intuitive factor. Dark fabrics absorb more light across the spectrum. The study measuring NIR transmission through fabrics showed the black cloth blocking roughly four times as much light as the thin, light-colored one.
1Scientific Reports. Impact of a textile layer on joint optical data and power transfer to in-body devices: a study on an ex vivo approachWhite, cream, or pale-colored garments transmit more, but they still absorb and scatter a portion of the incoming photons.
Thickness and tightness work together. A thicker fabric presents more material for the light to interact with, and a tighter weave or knit leaves fewer open gaps between fibers for light to pass through directly. Research on knitted fabrics for seamless sportswear found that cover factor, which describes how tightly the yarns fill the fabric’s surface, was one of the strongest predictors of light transmission. The thinnest fabrics with the loosest structure transmitted the most light, while adding elastomeric yarns that tightened the loops and shrank the mesh openings reduced transmission substantially.
4Textile Research Journal. Light transmission of weft-knitted fabrics for seamless garmentsIn practical terms, that means a sheer, light-colored athletic top lets far more light through than a heavy black cotton sweatshirt. Layering makes things worse, too. Each additional layer multiplies the attenuation. Two thin white layers do not just double the 16% loss from one layer; each layer absorbs and scatters light independently, so the compound effect is steeper than simple addition.
How Fabric Losses Affect Your Actual Dose
Most red light therapy protocols are specified in terms of energy density, often stated in joules per square centimeter. You get a certain energy density by holding a device at a certain distance for a certain number of minutes. If a fabric barrier removes a chunk of the light, you are getting a lower dose than the protocol calls for, even if you follow the timing instructions exactly.
Consider a typical home panel rated at 100 milliwatts per square centimeter at six inches from the LEDs. If a thin white shirt removes about 16% of the light, you are effectively receiving around 84 milliwatts per square centimeter. That might still fall within a therapeutic window for certain applications, but it might also push you below the threshold for others, especially protocols targeting deeper tissues like joints or muscles where the light already has to travel through centimeters of skin, fat, and connective tissue before reaching the target.
With a thicker or darker garment blocking 50 to 65% of the light, you are looking at receiving only about 35 to 50 milliwatts per square centimeter from that same panel. That is less than half the intended dose. Doubling your session time could theoretically compensate, but most people will not sit in front of a panel for 40 minutes instead of 20, and some biological responses to photobiomodulation follow a biphasic curve where more is not always better anyway.
Surface Power Density and Penetration Depth
There is an important reason the loss at the skin surface matters disproportionately: how deep light penetrates into tissue is directly related to the power density arriving at the surface. A higher power density at the surface means a detectable amount of light can reach deeper structures. A lower surface power density means the light runs out of steam at a shallower depth.
5PubMed Central. Photobiomodulation: Lasers vs Light Emitting Diodes?This relationship means that clothing does not just reduce the total dose; it effectively reduces the penetration depth. If you are treating a superficial skin condition like a wound or mild acne, losing some surface power density may not change the outcome much, because the target tissue is right at the surface. But if you are treating a deep knee joint, a shoulder injury, or muscular pain, every percentage point of surface light lost to clothing translates into less energy reaching the deep target. The fabric barrier and the tissue barrier stack on top of each other, and the compounding can be significant.
This is also why manufacturer claims about penetration depth should be evaluated with some skepticism. Those claims assume the light is hitting bare skin at the rated power density. Adding a layer of fabric invalidates the assumptions behind the stated depth.
When Treating Through Clothing Is Reasonable
There are situations where treating through a thin layer of fabric is a practical compromise rather than a serious problem. If you are using a high-powered panel or pad and treating a superficial target like the skin of your face, neck, or forearms, a thin white or light-colored cloth will reduce the dose modestly but likely not enough to completely negate the effect. Some people use a thin barrier intentionally to reduce the sensation of warmth on sensitive skin, especially during longer sessions.
Hospital and clinical settings sometimes place LED pads over thin gowns or dressings, accepting a small reduction in exchange for hygiene or patient comfort. In neonatal phototherapy for jaundice, which uses blue light rather than red, babies are treated in only a diaper precisely because clothing would block too much light, and similar logic applies to red light therapy for adults. The thinner the barrier, the better.
Situations where treating through clothing is clearly a bad idea include any protocol targeting deep tissue (joints, tendons, muscle injuries), any fabric that is dark-colored or multilayered, and any scenario where you are already operating at the low end of a recommended dose range. If you are paying for clinic sessions or investing time in home treatments, wearing a thick hoodie during the session defeats much of the purpose.
Can You Compensate by Increasing Time or Getting Closer?
In theory, yes. If a fabric blocks 16% of the light, increasing your session duration by about 20% would deliver roughly the same total energy to the skin. If a fabric blocks 50%, you would need to double the session time. Getting closer to the device also increases the power density at the skin surface, which could partially offset fabric losses, though distance effects vary by device design and are not always linear.
In practice, this approach has limits. You may not know how much your specific garment blocks, so you would be guessing at the correction. The biphasic dose-response curve for photobiomodulation means there can be a sweet spot: too little light may be ineffective, but pushing the total energy too high in an attempt to compensate can also reduce the benefit. Researchers have observed this “more is not always better” pattern across a range of photobiomodulation applications. Extending session duration also means more heat generation, which can cause discomfort or even mild thermal injury with powerful panels held close to the body.
The simplest and most reliable approach is to treat on bare skin whenever possible. If you must use a fabric barrier, choose the thinnest, lightest-colored, loosest-weave garment you have, and treat for somewhat longer.
Wearable Devices Designed to Work With Textiles
An interesting area of development is wearable phototherapy devices built directly into textiles. Rather than shining an external panel through clothing, these devices embed flexible light sources like organic LEDs (OLEDs) into the fabric itself, placing the emitter directly against the skin. A recent study demonstrated a textile-based platform using custom near-infrared OLEDs woven into a wearable cap for treating hair loss. The device sits on the scalp with the emitters in direct skin contact, eliminating the fabric-barrier problem entirely.
3Nature Communications. Wearable textile-based phototherapy platform with customized NIR OLEDs toward non-invasive hair loss treatmentThese wearable designs represent a different philosophy: instead of asking whether light can get through clothing, they make the clothing the light source. The advantages go beyond eliminating fabric attenuation. A flexible, conformable textile can maintain consistent contact with curved body surfaces like the scalp, knee, or shoulder, which is difficult with rigid panels. Consistent contact means consistent power delivery across the treatment area, rather than the uneven dosing you get when a flat panel illuminates a curved joint from a distance.
For now, most of these devices are in research or early commercial stages, and the options available to consumers are limited compared to traditional panels and handheld wands. But they point toward a future where the “through clothes” question becomes irrelevant for wearable applications, because the emitter and the garment are the same object.
Common Misconceptions About Light and Fabric
One widespread misunderstanding is that if you can see a glow through your shirt when you hold a red light panel up to it, the therapy is working fine. Visible transmission does not equal therapeutic transmission. Your eyes are extremely sensitive to red light, so even a small fraction of photons making it through the fabric will produce a visible glow. But that visible glow could represent only 30 or 40% of the original output, which may be well below the therapeutic threshold for your intended purpose.
Another misconception involves UV protection factor (UPF) ratings on clothing. Some people assume that a shirt with a high UPF rating will also block red and near-infrared light effectively, or conversely that a low-UPF shirt lets all light through. Research has shown that ultraviolet protection and visible-light protection are not reliably correlated in textiles. A fabric can be excellent at blocking UV while being relatively transparent to longer wavelengths, or vice versa.
2Journal of the American Academy of Dermatology. Protection from visible light by commonly used textiles is not predicted by ultraviolet protectionUPF labels on athletic wear tell you nothing useful about whether that garment will transmit therapeutic red or near-infrared light.
A third common belief is that lasers somehow “punch through” fabric better than LEDs do. While it is true that some laser devices can deliver higher peak power densities than LED panels, the underlying physics of fabric attenuation applies to both. A photon of 850-nanometer light interacts with a cotton fiber the same way regardless of whether it came from a laser diode or an LED. Higher power at the surface does mean more absolute energy arriving on the other side, but the percentage lost to the fabric is governed by the fabric’s properties, not by the light source’s coherence.
5PubMed Central. Photobiomodulation: Lasers vs Light Emitting Diodes?What About Bandages and Wound Dressings
People recovering from surgery or managing chronic wounds sometimes want to apply red light therapy over a dressing rather than removing and reapplying it. Wound dressings vary enormously in optical properties. A thin, transparent film dressing (the kind that looks like clear plastic wrap) transmits a large fraction of red and near-infrared light and poses minimal concern. A thick gauze pad, an opaque hydrocolloid dressing, or anything with a silver-impregnated antimicrobial layer will block substantially more light.
If wound healing is the goal, the most effective approach is to coordinate light therapy sessions with dressing changes so the wound can be treated on exposed tissue. When that is not practical, thin transparent adhesive films are the least obstructive option. Applying a powerful light source through a thick dressing and hoping enough energy gets through is essentially the same problem as treating through a heavy shirt: you might deliver some light, but you have lost control over the dose.