More Factors Affecting Light Penetration in Red Light Therapy

In our previous post, we covered why 50 mW/cm² isn't equal across all red light therapy devices—focusing on beam angle, divergence, reflection, and even talked about melanin. Yes the darker your skin, the more effect it has on the light that you illuminate yourself with.

The previous blog was a start, but penetration is more complex. Let’s look at the biological and physical factors that affect how far red and near-infrared (NIR) light actually reaches your cells.

Wavelength: Depth Depends on the Color

Wavelength is one of the strongest predictors of depth:

• 630-660 nm Red Light: Ideal for skin-level issues. In light skin (Fitzpatrick I–II), it penetrates 1–2 mm. In darker skin (Fitzpatrick V–VI), melanin absorbs 20–50%, reducing depth to 0.5–1 mm.
• 810–850 nm NIR: Much less affected by melanin. It reaches 3–5 cm in lighter skin and 2–4 cm in darker skin. Best for muscle, joint, and nerve therapy.

Choose torches or panels with 810–850 nm if your goal is deeper tissue repair. 810nm is currently my favorite and has some very favorable characteristics for bone penetration and joint work. The for joint work, I would highly recommend the "Norway's Best" Torch that can be used on joints or internally with the fiberoptic attachment - grab yours now 

Beyond the 800's nanometer range where penetration seems to be at it some of its most deep, there are wavelengths in the 1000's which seem to have some deeper penetration but are also absorbed largely by water as the main chromophore.  Hopefully you can see by now, there are many options in red light therapy (photobiomodulation) and each can be a good addition.

We have now discussed that not all light is created equal

Skin Thickness: Not All Areas Are Equal

Skin isn’t uniform. It’s thickest on your back, soles, and palms (~1.5–2 mm), and thinnest on your eyelids, face, and neck (~0.5–1 mm).

• Thicker skin scatters and absorbs more light. Even 660 nm may not reach below the dermis in those areas.
• Thinner skin allows both red and NIR light to pass through more efficiently.

Torches excel in thicker areas. Panels are more than enough for other areas..

Tissue Type: Fat, Muscle, and Bone All Respond Differently

• Fat: Highly scattering. NIR performs better than red here.
• Muscle: Moderate scattering. Water content absorbs some NIR light.
• Bone: Highly dense. Even high-powered NIR struggles to get through. The right wavelength is essential.

Targeting joints or deep muscle? Go for focused torches with 810–850 nm to handle fat and deeper structures.

Blood Flow: Hemoglobin Can Get in the Way

Hemoglobin, the protein in red blood cells, strongly absorbs 660 nm. NIR light fares better.

• Inflamed areas with increased blood flow absorb more red light, reducing penetration.
• Areas with low blood flow allow better light transmission.

Close-contact wraps reduce losses from blood absorption and skin reflection by delivering light directly.

Device Power and Delivery: Raw Numbers Aren’t Enough

Higher irradiance helps push light past biological barriers. But the delivery method matters just as much.

• Panels: Often these are used at a distance (10–30 cm). Even high-output panels can lose 10–30% of energy to skin reflection and ambient scatter. This is made up for by
• Torches and Wraps: With direct contact and irradiance up to over 100 mW/cm², you get targeted penetration and photons where they need to be.

I personally measure and verify the irradiance and spectral output of every device I sell. The irradiance you see in your personal guide is tested under real-world conditions—not inflated lab figures from a Chinese factory.

Honest Numbers: Beware of Fake Irradiance Claims

Some companies advertise unrealistic irradiance numbers— sometimes over 200 mW/cm²—measured inches from a sensor. Like I mentioned in the previous blog post, this done by using a solar meter and not a spectrometer.  As well as this, the measurement is often in ideal conditions that don’t reflect skin contact or real usage. Many don’t even disclose their testing method, so be aware.

I’m the only seller who tests real irradiance and wavelength readings for each product—verified with a spectrometer, tested on skin. I also include this data in a personal guide for each customer. You deserve truth, not marketing fluff.

How to Choose the Right Tool for the Job

Ask yourself:

• Do you have light or dark skin? Darker tones might need longer duration or higher power or more NIR.
• Is your issue close to the surface (e.g., wrinkles) or deeper (e.g., joint pain)?
• Are you treating fatty areas or lean muscle?

Panels for surface. Torches for precision. Wraps for hands-free, consistent use.

Final Thoughts

Red light therapy isn’t just about using a powerful light. It's about using the right wavelength, in the right area, with the right delivery. From skin tone to skin thickness, from fat to blood flow—every layer matters. Does that mean that it won't work unless you get the right one - no. Red and infrared light done sensibly up to ridiculously high levels has been shown to be positive, but you may not be optimizing your sessions. 

In an industry filled with overhyped specs, you can trust that every product I offer is tested and used by me personally. Nothing leaves my store unless I’d use it myself!

Have a lovely day

Dominic Lamb

Title image :Avci, Pinar & Gupta, Asheesh & Sadasivam, Magesh & Vecchio, Daniela & Pam, Zeev & Pam, Nadav & Hamblin, Michael. (2013). Low-Level Laser (Light) Therapy (LLLT) in skin: stimulating, healing, restoring. Seminars in cutaneous medicine and surgery. 32. 41-52.