Does Red Light Therapy Work With Clothes On? My Experiment

If you’ve ever wondered whether red light therapy works through clothing or if direct skin contact is essential, this experiment dives right into the answer.

This article covers every detail of setup, device specifications, and scientific reasoning. I’ll walk you through the methods and what the results mean for practical use.

The Question: Does Red Light Therapy Penetrate Clothing?

I often receive questions about whether red light therapy can penetrate clothing. Although this topic seems simple, it is more complex.

Most companies say, “For the best results, expose your skin to the light directly.” But how accurate is this advice? If you’ve ever listened to researchers in brain health, you may have heard claims that red light can penetrate not only through the skull but also into the brain itself. Given this context, I wanted to test how effective red light therapy could be when you’re wearing clothes.

Adding to this interest, I recently spoke with James Carroll from Thor Laser, a leader in photobiomodulation research. During our discussion, he showed me an image showing the surprising depth of red light penetration in the skull.

This visual showed red light applied in the oral cavity, reaching as far as the eye sockets—well beyond what you’d expect. Inspired by this, I thought, “Why not put it to the test?”

Experiment Setup: Devices, Distance, and Measurements

For this experiment, I used a red light therapy handheld device that emits a solid mix of red and near-infrared light, specifically at 60 mW/cm², from a distance of three inches. This device has red LEDs that produce visible light and near-infrared LEDs at around 850 to 860 nanometers.

I measured the irradiance reaching the light source using a spectrometer mounted three inches from it. First, I took a baseline reading with no obstruction—just a direct light path from the source to the sensor—to establish a control value for comparison.

Baseline Reading: Measuring Light Without Barriers

The baseline reading gave us an irradiance of 61 mW/cm²—an excellent number for red light therapy with the device’s full output. This value represents the energy exposure with no material in between, giving us a benchmark. I introduced various barriers—like fabrics and papers—between the light source and the sensor.

Test Materials: Clothing, Paper, and Towels

Here’s what I tested:

1. A thin, black t-shirt: This lightweight shirt has undergone multiple washes and is even see-through.
2. A thicker, new white t-shirt: This shirt was still thick and brand-new, making it an ideal contrast to the worn black shirt.
3. A heavy, thick white Sunstream towel replaced the usual draping over a body during a session.
4. Black lycra bike shorts represent typical workout attire made of lightweight and flexible material.
5. A thick, black neoprene knee sleeve: This was the densest and heaviest material in the test.

Paper Test: Gauging Setup and Distance Accuracy

I initially tested a single sheet of A4 paper to double-check that the spectrometer was accurately picking up changes. This wasn’t only for light penetration; it also validated that the setup could detect even minor shifts.

When I held the paper against the sensor (mimicking the skin), irradiance dropped from 61 to 21 mW/cm²—a significant decrease, showing that even a thin barrier can cause light loss. Then, I positioned the paper against the LED, yielding a result of only 8 mW/cm². This preliminary test confirmed the setup could detect variations based on distance and placement. Now, onto the fabrics.

Testing Clothes: Irradiance Results for Each Material

Thin Black T-shirt

Starting with the thin black shirt against the sensor, irradiance dropped dramatically to 1.5 mW/cm². Remember, our baseline was 61 mW/cm². So, despite the thinness of this shirt, the black color absorbed most of the red and near-infrared light, allowing only a tiny fraction to pass through.

The black shirt reduced the irradiance from 61 to 1.5 mW/cm². Only 2-3% of the energy reaches the skin.

This result suggests that wearing a dark shirt significantly reduces the therapeutic impact of red light therapy. Although some benefits might still be obtained, it’s considerably reduced.

Thick White T-shirt

Next, I tested the thicker white shirt. At first, I expected an even lower reading than the black shirt because of its density. But surprisingly, irradiance with this shirt against the sensor came in at 33 mW/cm². Despite being thicker, the white color reflects and allows more light to pass through, preserving over half the baseline energy.

Even though the shirt was thicker, the irradiance only dropped to 33 mW/cm², showing that light colors allow much more light to reach the skin.

Heavy White Towel

The towel’s irradiance was 12.8 mW/cm², about a quarter of the baseline value. So, even with a thick, dense fabric, red and near-infrared light can penetrate a white towel, suggesting that, in clinical settings, covering oneself with a towel would still allow a good portion of light to reach the skin.

Black Lycra Bike Shorts

Despite being black, the bike shorts provided an irradiance of 2.4 mW/cm²—higher than the black shirt but still low. Interestingly, the sensor readings showed an increase in near-infrared light, while red light was minimal. This may relate to the fabric composition; lycra likely absorbs less near-infrared, allowing some light to pass through.

I tested black Lycra bike shorts because I often wonder if red light therapy is effective during workout recovery periods, especially during intense training sessions.

For example, while I’m doing interval training to prepare for an indoor rowing record, getting red light during rest periods sounds appealing. However, it’s usually not practical to strip down for a complete treatment during a workout.

So, I wanted to find out if the red light could penetrate my bike shorts and reach the muscle tissue, helping with recovery and removing no gear. This test was a way to see if there’s a balance between convenience and effectiveness.

Thick Black Neoprene Knee Sleeve

Last, the knee sleeve presented the most challenging barrier, and the results reflected that. Irradiance dropped to a mere 0.002 mW/cm². The near-complete absorption shows that such dense materials effectively block therapeutic light, confirming that thick fabrics can halt almost all light penetration.

Additional Test: Placing Barriers Against the Light Source

One last variation I tried was putting the materials directly against the LED rather than the sensor. With this approach, the irradiance numbers were much lower:

• Paper: 8 mW/cm² (down from 21 mW/cm² when near the sensor)
• Thin Black T-shirt: 0.5 mW/cm² (down from 1.5 mW/cm²)
• Thick White T-shirt: 11 mW/cm² (down from 33 mW/cm²)
• Towel: 4.8 mW/cm² (down from 12.8 mW/cm²)
• Bike Shorts: 0.7 mW/cm² (down from 2.4 mW/cm²)

This comparison highlights that positioning the barrier closer to the light source significantly reduces the amount of light reaching the skin.

Practical Takeaways

Direct skin exposure is ideal for the most effective results from red light therapy. Aim to position the light on the skin with no fabric in between, as this maximizes irradiance and reduces treatment time. However, choosing the suitable fabric becomes essential when direct exposure isn’t an option because of privacy or comfort.

Lighter-colored fabrics, such as white, allow more light to penetrate than darker ones. In our tests, a thicker, new white T-shirt still allowed over half of the baseline light to reach the skin, while the thin black T-shirt permitted only about 2% to pass through. This difference highlights the importance of fabric color and thickness. These tests show that dark colors absorb more light, significantly reducing the amount that reaches the skin.

Another interesting point in this exploration is my conversation with James Carroll of Thor Laser. Carroll showed how red light, applied even inside the mouth, could penetrate deeply, reaching areas of the skull that one might not expect. This raises the question of how deep red light can penetrate, even when encountering barriers like bone and dense tissue. This level of depth reinforces the importance of optimizing skin exposure, as any layer can reduce the light’s intensity by the time it reaches deeper tissues.

Consider a longer exposure time or higher-powered devices if you have darker skin. Darker skin absorbs more light energy on the surface, which means less may penetrate deeper tissues. The impact may vary, but it’s something to be mindful of if you aim to achieve specific therapeutic outcomes. People with lighter skin absorb less energy superficially, allowing more to pass into deeper layers.

These insights are valuable for red light therapy in settings where total skin exposure isn’t always possible—such as a gym, clinic, or shared living space. When coverage is necessary, choose lighter, thinner fabrics to maximize penetration. You’ll still benefit from the therapy, though, with some reduction in intensity. Knowing how to navigate these factors can help you achieve effective results, even if complete exposure isn’t an option.

What’s Next? Future Experiments and Viewer Feedback.

This study is only the beginning. The following steps could include testing different wavelengths individually, shorter distances, or measuring effects on larger surfaces. If these insights spark your curiosity, you can stay tuned. More tests can help refine our understanding, and I’d love to hear what you’d like to explore next.

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Alex's Bio

This blog post was written by Alex Fergus. Alex is a ISSN Sports Nutrition Specialist, Fitness Professional and certified Superhuman Coach who continues to expand his knowledge base and help people across the world with their health and wellness. Alex is recognized as the National Record Holder in Powerlifting and Indoor Rowing and has earned the title of the Australian National Natural Bodybuilding Champion. Having worked as a health coach and personal trainer for over a decade, Alex now researches all things health and wellness and shares his findings on this blog. 

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