LED Spacing for Optimal Penetration in Red Light Therapy: Mind the Gap

Red Light Therapy devices come in many forms. But is there an optimal design? Or do they all bring us to the same level of effectiveness in the long run?

Is it better to have continuous, uniform coverage over an entire treatment area - or are some gaps and spaces between LEDs acceptable?

For example, consider these pictures.

Will the "uniform" coverage on the left always be better, or would the more "spotty" coverage on the right would be sufficient?

The left side may additionally benefit from the convergence of beams from multiple LEDs and wavelengths. The right side the individual LEDs are essentially working alone. Leaving gaps on the skin that are not touched by light within the treatment area. 

For the rest of this blog, we will refer to the left side concept as the Uniform style, and the right side as the Spotty style of device design or treatment. 

Intuitively, our brains seem to prefer the Uniform coverage. Because... why not?

The sales narrative for years is to cover every square inch of skin possible for the maximum full-body benefits. So, according to the mainstream logic, then we should never tolerate intentional gaps in treatment area coverage.

Thus, to become unbiased and remove our ego, we must first be self-aware of where these assumptions would have originated. Are they based on evidence, or logic? Data, or peer-pressure? Commercially available products, or products used in clinical studies?

Many things in Photobiomodulation science have been proven to be counter-intuitive. Especially when many of these logical, intuitive claims have been produced by manufacturers and influencers to promote a specific sales narrative. 

As usual, these assumptions are rarely supported by evidence. In fact, the top RLT Influencers make clear that they are happier to promote sales narratives without any evidence. That coercing people into believing their speculative design preferences will boost their ego, rather than sticking to what has been established in the science. 

• The vast majority of laser treatments and laser devices have spotty coverage. Quite literally due to the small spot size of the laser and how it is used in small, individual areas. There are even laser hair caps and other laser arrays with obviously spotty coverage.
• The vast majority of consumer flexible LED pads and flexible skincare LED masks also have coverage gaps. Many consumers are getting great results with those types of devices, despite false claims that they fail to be effective. Many of these style devices have also been clinically studied for effectiveness.
• Distance-treatment LED Panels have gaps in coverage when the beam angles are narrow (<30 degrees) and the user is within <12 inches of the device. In many modern LED panels the user needs to be quite far away to get truly uniform coverage. Again showing that most consumers are unwittingly exposed to spotty coverage by default, yet are getting good results. 

Ultimately, focusing on "uniform" versus "spotty" coverage is vague and misleading. It may be purposely vaguely defined so that influencer's can play favorites with preferred brands.

The best dosing theory that equilibrates these differences is the Total Joules dosing theory. This way we can focus on specifics that are quantifiable, and not an arbitrary sales gimmick about LED spacing. 

For example, target dosing with total Joules would be:

• Spot Treatments: 6 - 300 Joules (per point)
• Large Areas (LED pads, masks, helmets): 500 - 6,000 Joules
• Full Body: 50,000 - 200,000 Joules

According to this Total Joules dosing theory, different devices can have the same effect if they achieve the same total energy absorption. Regardless of LED counts, spacing, gaps, beam patterns, or coverage. 

An ideal, evidence based design may intentionally use Spotty coverage to optimize penetration, coverage area, and minimize heating on the skin. Whereas a Uniform design will cause limitations in safe intensity levels, excessive heating, excessive energy absorption, oversaturation of the skin with photons, and even less penetration. 

Thus, it is easy to make an evidence-based case that Spotty coverage with large gaps is the superior design for Red Light Therapy. And even easier to debunk the latest mainstream sales gimmick of making an issue over Uniform coverage. 

What Does the Expert Say?

Dr. Hamblin has said in multiple interviews that he prefers to use LED Panels or Flexible LED Pads directly on the skin, which would imply that he is not concerned by the gaps in coverage. In fact, we will learn later there are technical advantages to the spotty coverage. 

A recent 2025 interview on the Squint Podcast affirms that Dr. Hamblin's position has not changed over the years:

Interviewer: "And you are saying five watts or ten watts?"

Dr. Hamblin: "Yeah, you know, if you've got a little handheld device , even one or two watts might be worth having."

Interviewer: "Ok, and that would be kind of an overlay, an LED overlay."

Dr. Hamblin: "Yeah if you just had something that you put on a sore elbow or sore wrist, you know, these handheld ones. But flexible wraps are good because, you know, most of the light goes into the tissue."

[About 1hr & 3 min timestamp]

Once again, Dr. Hamblin affirms his preference for flexible LED wraps and the skin contact method of treatment. Which implies that the gaps in coverage from those types of devices are indeed a non-issue.

And for small targeted handheld devices Dr. Hamblin reiterates his preference for low powered devices that are only 1-2 Watts. As opposed to the handheld devices being pushed by influencers are often 10s of Watts or even almost 100 Watts.

The topic of LED spacing and gaps and spotty coverage is a non-issue. Thus, explaining why this topic is "rarely discussed" - for good reason. It does not play a significant role in effectiveness, as long as we focus on quantifiable dosing metrics. 

Misleading Marketing or Inadequate Education?

 

[5] Photo Credit: https://pmc.ncbi.nlm.nih.gov/articles/PMC9721157/ 

The Total Joules for this treatment was reported as 5.4 kJ (kiloJoules), which is 5,400 Joules. Treatment time was 15 minutes. [5] A perfectly reasonable power, if not, on the upper end of the recommended range. 

Another study on muscle growth also used a Flexible LED wrap with spotty coverage.[6]

They showed the pictures of this wrap as below:

[6] Photo Credit: https://pmc.ncbi.nlm.nih.gov/articles/PMC5026559/

There were only 50 LEDs in a 34cm x 18 cm area (~13 inches by 7 inches). They do not specifically note the spacing. They say the treatment points are only 0.2 cm^2 each. [6]

The total power was 5 Watts, and treatments were only 15 seconds. That was a Total Joules dose of 75 J per application. [6] A low dose that produced impressive results for muscle hypertrophy.

One study investigated the biphasic dose response in humans with a flexible LED pad.

They reported the device had 200 LEDs with an area of 0.2 cm^2 each. The total area of the pad was 612 cm^2. That means only 200 x 0.2 = 40 cm^2 was true treatment area. So we only have 40 / 612 = 0.065 fraction of the area covered. [8]

That means this pad only "treated" just 6.5% of its entire area, and the rest of the area was gaps in coverage. The total device output power was only 2 Watts. 

They studied two doses of either 800 Joules or 1,920 Joules, in addition to a control group. They found the group with 800 Joules had a better response to metrics like glycemia. [8]

So, we know that "spotty" coverage is so effective, that it can even cross the biphasic dose response and become less effective with too much energy. 

If influencers and brands are arbitrarily pushing for more Uniform coverage with excessive LED coverage, then it could likely only lead to less effective treatments. 

8. LED Pad for Cellulite

LED Masks for skincare are worn directly on the face, and typically have spacing between LEDs of up to 1 inch between LEDs. 

We don't need to belabor this point. Thousands of consumers use these types of masks with great benefits. Like most things, masks don't work for everyone.

Some consumers may respond well to non-contact LED panels for their skincare needs, so the consumer should simply be provided with awareness of their options. 

You can check out the GoalToGetGlowing blog for a deep dive into the science of RLT for skincare. Which was very gracious of her to have given credit to ourselves (GembaRed) for using our blogs as a resource. A rare sign of humility these days for influencers to provide credit for where they found some of their research. 

Here are a few studies that used typical skincare LED masks with success. Again proving that LED spacing is a non-issue. 

Skincare Studies with Conventional Style LED Masks:

1. https://pubmed.ncbi.nlm.nih.gov/37418018/

2. https://pubmed.ncbi.nlm.nih.gov/39960921/

3. https://onlinelibrary.wiley.com/doi/10.1111/srt.13391

4. https://pmc.ncbi.nlm.nih.gov/articles/PMC7992664/

It would be reckless and irresponsible to assume that a mask with greater LED density would produce better results (or shorter treatment times, faster results, etc). Excessive energy may create more heat on the device and the user, or simply reach a biphasic dose response faster.

To settle some debates about "optimal" masks, then the studies and brands should be quantifying their Total Watts output and Total Joules doses. This way it would make a more standardized metric to follow. 

For now, the issue is that most LED masks only advertise the Intensity (mW/cm^2), and the consumer is left to make assumptions about total power based on how many LEDs there are in the device. 

One study compared a Brain Helmet versus a Full-Body Bed for treating Brain Fog from a popular 2019 virus. 

This is one example that directly compared Uniform (Full Body Bed) to a more Spotty (Helmet) treatment. 

Both devices claimed to emit 24mW/cm^2 and the treatments were 14 minutes each for a "dose" of 20.2 J/cm^2. [15]

Both treatment groups received similar benefits, with no statistical significance between differences in group. 

Thus, this is one rudimentary study that shows that relatively more Uniform and Spotty treatments can have the same effect. And in fact, the helmet used in the study turned out to be emitting much less intensity at closer to 2mW/cm^2, and still got comparable results. 

The Vielight helmet system has famously used individual LEDs with wide spacing to target specific brain areas. It has been utilized in many clinical trials, and has been a popular consumer brand for many years.

This way the Vielight can utilize high intensity (>100mW/cm^2) at specific points for deep penetration in targeted regions of the brain. [16]

For example, if the same 100mW/cm^2 was applied uniformly over the head in a more traditional helmet style, then that would likely be excessive power and total joules. 

In one interview, the owner of SunPowerLED brand indicates that they had to reduce the power of their transcranial helmet device. [Link] Their helmet device is a combination of their handheld pieces that can emit around 150mW/cm^2. 

Their helmet was initially set at 100mW/cm^2. and the owner said they had to cut it in half to reduce heating and warmth. So now with "uniform" coverage they are limited to about 50mW/cm^2. That is a pretty big adjustment they had to make, and shows that there is indeed a limit to intensity especially for brain helmets. 

https://www.youtube.com/watch?v=swrerUdm1k0 

In their clinical trial the SunPowerLED device reported 60mW/cm^2. Meaning that with "Uniform" treatment they had to significantly reduce the intensity to make it appropriate for brain treatments. [17]

Even at 60mW/cm^2, they had to limit the treatment time to only 180 seconds (3 minutes) to avoid heating and discomfort. [17]

Which shows the problematic ideal that these "walls of light" of Uniform coverage are creating new limitations for intensity, heating, and treatment times.

The SunPowerLED device was unable to be used at its full capacity of 100mW/cm^2 because of it's large coverage area and heating. However, Vielight is able to use >100mW/cm^2 because it uses single diodes in select areas for targeting deep tissues in specific brain regions. 

The Vielight, Neuronic, and SunPowerLED brain PBM devices are a good representation of these two design methods. Spotty (Vielight) versus more Uniform (Neuronic, SunPowerLED) coverage.

Although they have not been directly clinically studied head-to-head* in the same trial, we can see that both approaches have been independently studied to be effective both in published research and consumer anecdotes. 

So, it would likely be an assumption for marketing if an influencer were to claim that one methodology were to be superior at this point. Especially when it is clear that there are limitations on intensity with Uniform coverage due to heating and overdosing. 

*Section footnote: gold star to myself for this pun.

One study used a large full-body commercial LED panel at 12 inches away. This was an older model with 90 degree beam angle lenses. Thus, we can assume there was a uniform "wall-of-light" of the reported 46 mW/cm^2 at this distance away. [18]

This was the highest powered device on the market at the time. Thus, the treatment time to reach a clinically studied dose from previous trials was 2.5 minutes on each side of the body. [18]

If influencers and brands were correct, then this dose with a Uniform treatment area should have produced a superior result. Instead, the doctors reported that they were perplexed as to why the study had failed, unlike previous studies that used the same dose with success.[18]

The researchers speculated that it could have been too much Total Joules dose of energy to engulf the entire body. Whereas previous studies used a more targeted approach with the same dose. 

Thus, we have a clear case where Uniform treatment was unable to match previous studies' effectiveness with smaller targeted treatments. 

Optimal Spacing for LED Devices:

We do not want to maximize any parameter like Uniform coverage area without considering the possible downsides. 

We may be able to utilize LED Gaps in a way that optimizes penetration depth, rather than speculating that it always leads to inferior results. As usual, the same influencers that claim to care about deep penetration are the same ones promoting sales narratives that reduce penetration. 

To understand the best penetration design. We must understand a few concepts and more importantly - limitations. 

Beam Width:

The penetration depth is affected by the width of a beam. Not just wavelength, power, or energy. 

This is rarely discussed because LED devices are typically much larger than Lasers. So it has been irrelevant up until this point except now to debunk a new sales gimmick. 

But this is a commonly known aspect of lasers. A laser beam width can be quite small, then it can suffer from reduced penetration from scattering. As a laser beam becomes wider, the penetration depth can benefit from internal convergence during scattering in the tissue. 

Picture adapted from: https://pubmed.ncbi.nlm.nih.gov/28900751/ 

However, according to one study, they found that the maximum penetration was at 10mm beam width. Beams wider than 10mm then plateau in their penetration depth. [19]

This indicates that with wider coverage and excessive intensity - there will be significant over-saturation of photons in the tissue, without anywhere else for the photons to go but to be converted into heat. 

Temperature vs Coverage Area:

Temperature is an obvious limitation for Photobiomodulation for safety, efficacy, and penetration depth. 

A tiny beam of 100mW/cm^2 won't produce much heat on the skin. As the total power and energy can be quite low and managed easily by dispersion in the tissue and thermoregulation.

As we noted above, the 100mW/cm^2 will have room to scatter to the sides, thus preventing oversaturation and heating. But 100mW/cm^2 from a large uniform area emitter will certainly produce significant heating on the skin. 

This has been well documented as a principal of laser therapy.

One article notes:

"Larger spots also enhance thermal load, potentially increasing pain." [20]

Another article notes:

"Larger spots also can provide for greater subsurface target heating at equivalent device fluences." [21]

And this article:

"As a result, as spot size increases, the light penetrates deeper. Consequently, a larger spot size allows more effective heating, and conversely deeper heating can be achieved with lower fluences when delivered with a larger spot size (17)." [22]

This is also documented in the radiant heat therapy science called Water-Filtered IRA. They find that heating increases with coverage area up to about 10cm x 10cm. Which is only about 4 inches by 4 inches. So we know we can reach maximum saturation of the skin and heating with just a 4x4 inch coverage device. [23]

Once again, we are dealing with diminishing returns and tradeoffs especially with heat as a limiting factor. We can optimize penetration depth with a beam with of ~10mm diameter, and going wider does not enhance penetration and will only increase the thermal load on the skin. 

If RLT Influencers are promoting high intensities with wide uniform area, then they would not be enhancing penetration depth and only be risking more overheating. 

Penetration Depth Diminishing Returns:

The RLT influencers have finally gotten the memo from us breaking down the science in excruciating detail in our previous blog. The penetration depth vs intensity chart follows an exponential function. 

Increasing the intensity has diminishing returns on the "effective" penetration depth.

The exact inflection point may be up for debate. The inflection point may occur somewhere between 50 to 100mW/cm^2 for conventional devices depending on many nuances.

Since we like promoting safety, we prefer to recommend around 50mW/cm^2 for the optimal tradeoff of penetration depth and minimizing heating. 

Even doubling the intensity from 50mW/cm^2 to 100mW/cm^2 we calculate only a 15% increase in "effective" penetration depth (17mm to 20mm), so it would not be worth the risk of promoting a recklessly high intensity with minimal return on penetration depth.

However, we know that with small spot sizes then 100mW/cm^2 is OK, it is only with large uniform coverage areas that 100mW/cm^2 becomes problematic.

Temporal Spacing vs Geometric Spacing

Consider pulsing. The rapid on and off of a light. The gaps in time between pulses allow for the tissue to cool. Obviously we would otherwise prefer continuous wave, since that would maximize the energy dosage delivery time. 

A continuous light of 100mW/cm^2 may produce unacceptable heating, but when pulsed the average intensity of 50mW/cm^2 may be more allowable to have peak intensities of 100mW/cm^2. 

Consider the scanning method. Where a device may be waved back and forth over the skin. This creates both a temporal gap and geometric gap in treatment area to manage the heat from high intensity. 

In other words, we are proposing that intentional gaps in space may help create an optimal environment for a tradeoff between penetration depth, heat, and high intensity. In this case, the gaps are effective in a fixed position.

Again, the influencers can parrot that pulsing and scanning are good techniques to manage heat from high intensity, but they don't do enough independent thinking about the benefits of intentional spacing between LEDs or lasers. 

This means that an optimal penetration design may be:

• Up to 10mm beam area per point
• Intensity of 20-100mW/cm^2 (depending on wavelength and heat)
• a spacing of at least 5mm between beams to allow diffusion and thermoregulation cooling

Thus, we can utilize spacing to optimize penetration. A grid of individual LEDs with adequate power and heatsinking for the device could produce the optimal penetration depths at many points over an area.

Which is quite literally the way most cold-laser treatments have been conducted, but now you can have many LEDs covering more area simultaneously to save time. Rather than treating "point-by-point" - a large LED array like a pad, mask, or panel can deliver similar dosing with intentional gaps to properly mimic laser treatments. 

So we would not want Uniform coverage like this unless the intensity is very low:

Our proposed ideal design looks like this:

 

Both designs could theoretically deliver the same Total Joules dose and penetration depth. Except our design also minimizes skin heating and oversaturation and overdosing the surface of the skin. 

As mentioned earlier. If an influencer insists that 100mW/cm^2 delivers the optimal penetration, then it would be better implemented with Spotty coverage to reduce heating. A uniform coverage device of 100mW/cm^2 over a decently large area would produce unacceptable heating. 

Conclusions:

Uniform coverage with a "wall-of-light" is unnecessary. Influencers and brands are, once again, taking a concept that has been a non-issue for decades and trying to spin it into a marketing gimmick.

As usual, the gimmicks run the risk of excessive heating, oversaturating the skin, excessive total joules dosing, and actually reduces penetration depth. Not much different than previous marketing gimmicks that promoted excessive intensities and non-contact treatments. 

A real expert or researcher would appreciate the history of LLLT has been using lasers with spotty coverage for many decades. As well, we can easily confirm that LED pads and masks with wide spacing have been proven to be effective in clinical studies and many more anecdotal reports. 

When we understand the optics and physics of the situation, it becomes clear why Spotty coverage with intentional gaps has been so effective over the years. It allows for deep penetration and gaps between points that allow the skin to thermoregulate. It allows for treating a wide area without excessive energy (Total Joules) delivery.

To reduce confusion and improve dosing standards, then more devices and studies should be reporting the Total Watts optical output, and the Total Joules dosing.

This way we can study in the future if the distribution of power has a significant effect. For example, delivering the same amount of Power (Watts) either in a Uniform wall of light (lower intensity), or delivering it in small spots with gaps (higher intensity). 

So far, there is no indications that either method would produce a significantly different response. As long as the doses are adjusted properly and are mindful. 

This means that conventional LED Pads, Masks, and other wearables on the market are indeed effective. The users reporting benefits from them are not placebo, and indeed following clinically verified treatment methods. 

It is clear that the narrative of Uniform coverage is much more speculative. There may be cases that it can be implemented effectively, but reckless coverage will lead to excess heat and energy. 

We recommend following the clinically studied path, and not the sensational hype from influencers. There is nothing special about these low powered flexible pads and masks, and that simplicity is exactly what makes them so safe and effective. 

 

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December 2015 | Volume 14 | Issue 12 | Original Article | 1437 | Copyright © December 2015

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^aScripps Clinic, La Jolla, CA
^bCynosure Inc., Westford, MA

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