Why 50mW/cm² Isn’t Always Equal in Red Light Therapy: The Role of Beam Angle, Divergence, and Device Design

If you're exploring red light therapy for the first time—or comparing devices—you’ve likely seen the term irradiance, usually measured in milliwatts per square centimetre (mW/cm²). Don't worry, hopefully this article will help you understand why the irradiance isn't always the complete story.

Many brands use this number as a selling point: “Our panel delivers 50mW/cm² at 15cm!” or “Our torch puts out 200mW/cm² at skin level!”

But here’s the issue: 50mW/cm² from a red light panel is not the same as 50mW/cm² from a torch or a flexible wrap.

Let’s unpack why. We’ll explore beam divergence, light scatter, skin reflection, and how all these factors influence how much light actually reaches your cells. There is also elements like wavelength and light type that can also have drastically large effects on light behaviour under the skin. This will be covered in another article.

What Is Irradiance in Red Light Therapy?

Irradiance measures the intensity of light energy reaching a surface per second. In red light therapy, it gives a rough idea of how much energy your skin receives.

It sounds like a clear number, but it doesn’t account for what happens to the light after it hits your skin—or how well it reaches your target tissue. For many of us, that is an important factor as we trying to target something deeper inside.

And this is where beam angle, divergence, and contact come into play.

Beam Angle and Divergence: Why They Matter

Beam angle refers to how wide or narrow the light spreads from its source. A smaller beam angle means the light is more collimated (tightly focused). A wider beam angle scatters light in multiple directions.

Divergence describes how light spreads over distance. The further the light travels, the more it spreads—and the less concentrated it becomes. This can be both a good and a not so good thing, please read on.

Panel Example

Most red light panels use LEDs with a beam angle around 30° to 60° (Depending on the device here at home light therapy, I alternate. If you want 30 degrees, you can ask and I will do my best to make that happen for you.).

• At 5–10cm from the body, much of the light is still concentrated.
• At 20–30cm, the beam diverges significantly.

This means the center of the panel delivers higher irradiance than the edges.

Even if a panel advertises “50mW/cm² at 15cm,” much of the peripheral light is weaker.

Torch Example

A red light torch—especially one using a collimated lens or reflector—has a much tighter beam angle, often under 15°. The light remains focused even at short distances.

• It penetrates more deeply, especially into tissue like joints, jaw, sinuses, or the scalp.
• It delivers nearly all its energy to a small target area.

This is why torches are favored for spot treatments and deep tissue use.

Skin Contact and Reflection

Even before light enters the body, some is lost at the surface.

Studies show that up to 10% of red and near-infrared light can reflect off the skin. It can sometimes be more due to angle of light impact and skin conditons. Usually around 5-10% can be lost, especially if:

• The beam hits at an angle
• The skin is oily, sweaty, or dry
• There’s hair or uneven texture

Close skin contact—like with a wrap or a torch pressed to the skin—minimizes reflection. The beam enters more directly, and more photons reach the tissues.

This is a key advantage for wraps and torches over panels, which often sit 10–30cm away from the skin.

Light Scatter in Tissue

Once light enters the skin, it interacts with:

• Collagen and other proteins
• Cell membranes and blood vessels
• Mitochondrial structures, especially cytochrome c oxidase (CCO)

Shorter wavelengths like 660nm are absorbed quickly in the skin. Near-infrared (810–850nm) penetrates deeper—but also scatters more inside tissue. Here is where another aspect comes into play - melatonin level (again this for another blog post). Essentially, the more melanin you have, the more efficient your body is at scattering and absorbing wavelengths of light - even red and near infrared light. Put very simply, the darker the skin, the more light is potentially needed. The melanin effects reflection, scatter and absorption.

Collimated light (like from a torch) scatters less, so more of it reaches the brainstem, lymph, or joints.

Panels, with broader divergence, lose more energy through scatter, especially if used from a distance.

Comparing Devices at “50mW/cm²”

Let’s say three devices all deliver 50mW/cm²:

Each one suits different goals. There’s no “better”—only better matched to your needs.

Not All Irradiance Claims Are Honest

Here’s something most brands won’t tell you.

Many companies inflate their irradiance numbers using unrealistic measurement methods—like placing sensors directly against a diode or quoting peak readings from the very center of a panel.

This gives the illusion of a high-performing device, but in real use, you’re not getting what was promised.

I personally measure every device I sell using a calibrated spectrometer, under realistic use conditions, not lab tricks. I test irradiance at specific distances and check for actual wavelength output—so when you see "660nm at 50mW/cm²," you're getting exactly that, not a rough guess.

I also provide a personal usage guide for every customer, based on those measured values—so you know how long to use the device, how far to hold it, and what to expect.

I'm the only seller (to my knowledge and as of June 2025) who explains the effects of beam divergence, reflection, and tissue scatter when you buy. These factors are essential to choosing the right device—and they’re rarely mentioned elsewhere.

Choosing the Right Tool for the Job

Ask yourself:

• What are you treating?
• Pain, recovery, sleep, skin, inflammation?
• Where is the target tissue?
• Surface, joint, organ, nerve?
• How often will you use it?
• Daily, weekly, full-body or spot?

If you’re addressing systemic energy production or mood, a large panel at moderate irradiance over time may help most.

For targeted deep tissue—like nerve pain, jaw tension, or post-injury repair—a torch at close range delivers higher intensity, faster.

For hands-free daily therapy over joints, neck, or gut, a wrap provides convenience, comfort, and consistent delivery with low reflection losses.

Final Takeaway

The same irradiance number—like 50mW/cm²—doesn’t behave the same way across different devices.

Light from a panel spreads and weakens with distance.

A torch keeps its power tightly focused.

A wrap ensures close contact and uniform exposure.

It’s not just the number. It’s the delivery.

Understanding beam angle, divergence, skin contact, and light scatter helps you choose the right red light therapy device for your specific needs.

Don’t just follow the specs—follow the physics.

Want help choosing the best device for your goals?
 

Get in touch today using the contact page or email directly to admin@lighttherapy.no (It comes through to me directly either way)

Kindest regards

Dominic