The Superior Optics of Skin Contact for Red Light Therapy

Within typical LLLT and PBM (non-thermal) parameters, it is often recommended to use the skin contact method for deep tissue treatments.

For superficial applications the non-contact method (using devices at a distance) is allowed. 

The Phototherapy textbook by Tuner and Hode recommends non-contact for what they call "shallow problems", and for "deeper problems" to use skin contact method with some pressure. [1, Tuner&Hode, pg 118]

"At medium depth: Press the probe into skin. The deeper tissue to be treated, the higher the pressure." [1, Tuner&Hode, pg118]

One article makes this statement in a more technical way:

"Increased light transmission was found to correlate with tissue strain during indentation, with greater compressive strain resulting in greater light transmission increase." [37]

A strong correlation is found that increasing skin compression pressure will increase penetration (transmission) of the light through tissues. Hence, why experts like Tuner and Hode will recommend higher pressures depending on the depth of the tissue. 

So when we choose to use a non-contact PBM device (i.e. at a distance, like 6+ inches away), we are choosing to target superficial tissues - regardless of the intensity used.

The skin contact method increases penetration by:

1. Reduces the amount of Red/NIR light being reflected from the skin.
2. Compresses the tissue which physically reduces the distance the light has to travel.
3. Blanches away the superficial blood and water that competes for absorption and causes scattering of the photons.

These three reasons has been clearly summarized in several peer-reviewed articles like this quote:

"The [contact] pressure technique eliminates any power loss due to air gap and reflection from the stratum corneum, physically places the probe head nearer the target tissue, and blanches out the superficial microvasculature, thereby removing a possible absorbing medium to give better penetration and thus deeper absorption of a more clinically viable photon density." [2]

The most common technique in clinical Low Level Light Therapy and Photobiomodulation is to press the laser slightly into the skin:

"During laser therapy, it is a common clinical practice to press the laser probe firmly against the tissue to minimize any air gaps at the tissue–probe interface and, therefore, increase the transmission of the laser beam [17,18]." [39]

While it is clearly observable that skin contact compression improves light absorption, penetration, and therapeutic benefits - it is only in more recent studies do we understand the underlying optical physics that makes this technique so effective. 

What we found is the reason for the reduction of reflection losses is from what is called Optical Coupling and Photon Recycling. 

We also found that in addition to reducing blood and water absorption, skin compression during skin contact also promotes reduced scattering in the underlying tissues, which also improves penetration. Most importantly, is that skin compression will decrease the thickness of the skin that the photons need to traverse. 

Other industries like Laser Surgery and Optical Imaging are now also taking advantage of the superior penetration offered by skin contact method, which they have named Mechanical Optical Clearing.

Reflection is Wasted Light

The first law of photochemistry and photobiology states that photons must be absorbed in order to produce an effect.

In a textbook on Laser Surgery where accurate dosing is of utmost importance, they discuss how reflection and remittance losses will result in "wasted" light that does not contribute to the absorbed dose.

Remittance is when the light enters the skin but exits due to backscattering.

"Approximately 4–7% of VL is reflected by the skin surface, regardless of incident wavelength, pigmentation, or structure." [3]

Another article using 660nm and 830nm lasers notes:

"For laser beam radiation on the skin, reflectance accounts for 3% when the laser is directed toward tissue [38]. About 93%–96% of the incident radiation not returned by regular reflectance could be absorbed and scattered" [35]

True reflection from the first layer of the skin is actually very low at only 3-7%, so most of what we observe as "reflection" is actually remittance from backscattering.[3][4][35] For example, reflections occurring inside the skin at the interfaces at the epidermis, dermis, or other skin constituents like collagen scattering that will cause photons to exit the skin. 

"In laser surgery, light reflected from the surface is typically “wasted”. This “lost” energy varies from 15% to as much as 70% depending on wavelength and skin type. For example, for 1064 nm, 60% of an incident laser beam may be remitted" [4]

The quote above confirms the large reflection/remittance loss ranging from 15-70% depending on wavelength and skin phototype. Even the deepest penetration wavelengths like 1064nm have significant remittance of around 60%. This is confirmed by the graph from NIST data below.

For Red wavelengths the reflection is strongly variable based on melanin concentrations in the skin, but for NIR the reflection/remittance becomes more similar for all skin types. 

NIST: Reflectance Measurements of Human Skin

A 1999 article that pioneered the usage of skin contact method for lasers in dermatology made this comment about the superior penetration when combining the correct wavelengths with the skin contact method. 

(0.8 - 1.0 µm equals 800 - 1000 nm)

So even if we obtain "deep penetrating" wavelengths like 810nm and 1064nm, if it is not used in skin contact then it will not manifest the significantly deeper penetration often imagined.  

Can We Truly Compensate for Lack of Non-Contact Method?

Assuming about 60% reflection losses for all skin types in the Near-Infrared range.

If this was the only factor, we could easily increase the intensity or exposure time by 2.5x to simply compensate the dose. Which at this point is the bare minimum we recommend in our dosing guide.

Influencers may only discuss skin reflection to sell high intensity non-contact LED panels, while conveniently ignoring the other major advantages of skin contact. 

For the rest of the blog, it will become clear that the skin compression is what gives the significant boost in penetration.

One study with a 904 Laser increased penetration from 44.4% to 58% from non-contact to skin contact. As sizeable increase of 13.6% from doing nothing other than simply holding the device on the skin. This was with a convex lens at the tip of the laser that compressed the skin. [6]

One 2020 study looked at the differences in penetration in living dog tissue with skin contact or non-contact.

They found at 14.38mm thickness of skin/muscle there was 3x more power penetrating with skin contact compared to non-contact at 1 cm away. When used at 5 cm (~2in) away, there was no detectable power at 14.38mm tissue penetration. [7]

In other words, at too far of a distance (more than 5cm / 2in) there is no compensation factor that can reconcile the differences between non-contact and skin contact methods. Even at 1cm away, it would require an additional 3x exposure time to compensate for the loss of penetration.  

"Therefore, LILT of deeper tissue should be performed by the contact technique, such as in post-operative rehabilitation purposes. LILT of superficial tissue can be performed by the non-contact technique with a 1-5 cm tissue-laser probe distance." [7]

Their conclusion above affirms what experts have been saying for many years. That the skin contact method should be used for deep tissue applications. And it is acceptable to use non-contact for superficial applications. 

"If we administer treatment from a distance, we do not get the same effect as if we treat in contact or with pressure." [1, Tuner&Hode, pg166]

It is clear that the skin contact method with some pressure is the most recommended technique for deep tissue LLLT/PBM. There are many differences in the optics of skin contact far beyond just reflection and remittance losses. 

You can also learn more about skin contact method in this blog and this blog and this video.

Refractive Index and Reflection:

The textbook titled Lasers in Dermatology and Medicine chapter "Laser-Tissue Interactions" reviews many dosing considerations.[4] We have already cited them for quote on reflection/remittance losses being "wasted" light that will need to be considered for dosing. 

The reason for skin reflection losses can be quantified by the differences in Refractive Index of light travelling through different mediums. When light encounters a transparent clear surface like glass or water, there are some reflections and glimmer and glare we can observe from the clear surface. This is because of the differences in Refractive Index at the interface. Even a black surface will have some reflections due to the refractive index mismatch, even though we expect most of the light to be absorbed.

The Refractive Index of Air is 1.0, while the Refractive Index for the Stratum Corneum (the upper layer of the skin) is 1.55.[4] Essentially, the bigger the difference between Refractive Indices there is the more reflection expected at the interface.   

The refractive index of an Acrylic lens is about 1.49, obviously much more similar to the skin index of 1.55. Clear Epoxy is typically between 1.50 to 1.57. Clear silicone is reported to be between 1.4 to 1.6, which is what is typically used in LED face masks. So all of these would be good materials to use, luckily already being used in many LED devices. 

When you use skin contact you eliminate the air between the device and your skin. This is called Optical Coupling when objects of similar refractive index are brought in contact with each other, thus significantly reducing reflections. 

The chapter recommends using Alcohol Solution (n = 1.4), water (n = 1.33), or a sapphire crystal (n= 1.55) for optical coupling. They also note that excessively dry skin will increase reflection losses, so that is why removing dead skin and being properly hydrated will improve absorption. 

"On the other hand, the surface of dry skin reflects more light because of multiple skin–air interfaces (hence the white appearance of a psoriasis plaque)." [4]

The refractive index of skin is closely correlated to skin hydration:

"In turn, the refractive index of skin is significantly linked to its water content, and so the technique can be used for skin hydration measurements." [8]

Angle of Skin Reflection:

The angle at which the light reaches the skin also affects reflection following Snell's Law, the Law of Refraction. 

Light perpendicular to the skin (straight on) has the least reflection, light encountering the skin at an angle has increased reflection losses. 

"The angle between the light beam and the skin surface determines the % of reflected light. More light is reflected at “grazing” angles of incidence. It follows that, to minimize surface losses, in most laser applications, one should deliver light approximately perpendicular to the skin [3, 6]." [4]

They recommend that you can purposely reduce the absorbed intensity by using the laser at an angle, rather than perpendicular to the skin. 

You can imagine that an oversized modular panel being flat will only have "grazing" light reach the sides of the body. Perpendicular light straight from the panel will minimize reflection losses, and the light reaching the sides will have higher reflection losses. 

The flat modular panels or oversized flat panels will suffer from this issue. The section in the middle will likely be delivering most of the therapeutic effects and the LEDs on the sides would likely contribute more light that becomes reflected. If the panels on the sides could be angled inward only slightly, then that would provide more uniform coverage at a perpendicular angle to the sides of the body. 

Backscattering Problems:

Imagine a single photon entering the skin. It does not follow a straight line through the skin to eventual absorption.

Rather, as it encounters different molecules in the skin, it gets deflected, scattered, and transmitted. It bounces around more akin to a pinball or plinko game, however mostly in a forward-scattered direction deeper into the skin

"As it penetrates the skin, light energy is absorbed and scattered along the way, decreasing the intensity until it disappears." [10]

This quote also tells us two main properties that hinder penetration in the skin that we will be discussing more in later sections, absorption and scattering. 

A similar diagram can be found in the following article. [Link] [11] And this Link for free.

However, it has been found that backscattered light in the skin will increase the intensity and concentration of photons just below the surface of the skin. 

"There is backscattered light that can yield a higher fluence beneath the tissue than at the tissue surface [48]. This paradox of tissue optics is that the internal fluence can actually exceed that at the surface" [4]

If I did their equation they presented correctly, this means there is an increase of intensity and absorbed dose just below the skin surface by 3 to 5 times that of what was used on top of the skin. 

A diagram illustrating this backscatter is in [12]

Dr. Calderhead also notes this backscattering phenomenon occurring in the context of LLLT. Which may actually be a good thing for superficial targets. 

"an interesting phenomenon has been noted whereby the highest photon intensity in the target tissue is actually beneath the surface, exactly where it is required, as the cellular targets for LED‐LLLT lie at the stratum basale in the epidermis and in the dermal matrix." [12]

How do we avoid this backscattering and concentration of intensity below the skin surface? Optical coupling with the skin contact method.

As the book explains we can spare damaging the epidermis from high intensities caused by backscattering by using optical coupling:

decreasing the natural convergence of photons at the skin surface." [4]

As the lenses of LED devices also tend to match the Refractive Index of the skin, this is a powerful way to not only reduce reflection losses, but allows for higher intensities that won't damage the skin by reducing backscattering. 

I have subjectively noticed that I feel more heat from non-contact treatment than skin contact method even at similar intensities. But I never had any explanation for it until now especially as they note it seems almost paradoxical.

Optical coupling can be assisted with a gel, as is often used for IPL treatments.

The chapter also refers to "Photon Recycling" where reflected and remitted light can be trapped by a reflector on the skin, thus reducing wasted light and amplifying the absorbed photons.

return reflected light to the incident spot on the skin." [4]

Their estimation is that properly recycled light will amplify absorption by 3x depending on the design of the reflector and wavelength. When a device is pressed on the skin, more of the reflected/remitted light will return to the LED lens, and be reflected from the lens back to the skin. 

Assuming a LED Panel has a white or reflective coating, this could also improve photon recycling when moving closer to the panel. An LED Bed may be able to recycle more photons inside it's enclosure than an open-ended LED Panel. 

Blood Absorption Limiting Penetration:

One LLLT article noticed significant absorption taking place in the skin due to a layer of blood vessels called the Dermal Vascular Plexus. 

"At this level the dermal vascularisation was serious. Blood absorbs very strongly the light of wavelength over 630 nm (3, 6) . This means that the dermal vascular plexus absorbs the laser light" [15]

As another article shows, there are two major layers of blood in the skin. These layers will have significant absorption and hinder deeper penetration. 

[Link] [11]

If you press your finger into your skin, when you remove it then you will typically have a lighter spot from the fingerprint. As the blood (and corresponding water) gets temporarily blanched from the surface. The blood should return within a few seconds bringing back normal color. 

"Pressing the probe firmly against the skin will also move the light closer to the target and decrease absorption through the ischemia caused by pressure." [1, Tuner&Hode, pg118]

In this reference the term ischemia is a localized restriction of blood to the skin that decreases absorption, in addition to simply compressing the tissue that brings the light closer to the deeper tissue targets. 

the compressed area" [38]

Niels Finsen observed that blood absorption was a major limiting factor to penetration in his 1901 book Phototherapy. So he devised probably the first ever light therapy skin-contact device with a convex lens that presses into the skin which in his words intended to "exclude the blood from the region to be submitted to the action of light". [Finsen, pg70]

https://archive.org/details/39002010758374.med.yale.edu

When Finsen needed deep penetration he obviously wouldn't resort to something so barbaric as merely cranking up the intensity. Such barbarism is reserved for the year 2024. 

The easiest experiment you can do at home is to simply press your thumb into one of the lens of a Red LED on your LED panel. You will notice a dramatic increase in penetration depending on how much pressure is applied. 

Skin Compression & Mechanical Optical Clearing

Optical Clearing is using techniques that increase the transparency of tissues, often using chemical agents. This may be fine for tissues in a petri dish, but is often not practical on living human skin. 

Mechanical Optical Clearing (MOC) is often simply using physical pressure to compress the tissue to remove superficial water, blood, and changes the optics of the skin in favorable ways. It is considered minimally invasive and retains the integrity of the skin barrier.

"Mechanical indentation modifies tissue optical properties and may be harnessed as a minimally invasive optical clearing technique to improve optical diagnostics and therapeutics." [37]

This term is often used in Biomedical applications like deep tissue imaging (often with Near-Infrared light). 

"The mechanical compression can cause local water removal within compressed regions of tissue, i.e. the localized mechanical compression of skin decreases tissue thickness and water content and increases refractive index and OCT signal intensity 96, 97" [16]

Another article defines MOC as:

"mechanical tissue optical clearing (MOC) with a commercially available NIRS system. MOC utilizes a compressive loading force on tissue, causing the lateral displacement of blood and water, while simultaneously thinning the tissue." [17]

Not only do we displace high absorbing chromophores like water and blood, and decrease the skin thickness - but these studies also confirm favorable optics of the compressed skin by reduced scattering.

It is quite clear that this technique is used to increase penetration depth of light, particularly by moving away water inside the skin under the compressed area.

"Our previous work has demonstrated that mechanical optical clearing of tissues results in increased light penetration depth [45, 46] as well as increased imaging resolution and contrast [57]. These effects are thought to be the result of reversible lateral water displacement caused by localized mechanical loading. As the tissue is indented or compressed, a pressure gradient develops within the tissue, driving interstitial water away from the compressed region." [18]

Another article concludes that skin compression would help with light therapies and imaging. 

"These finding suggest that local compression could be used to enhance light-based diagnostic and therapeutic techniques." [19]

There are several reasons why skin compression delivers favorable penetration optics, primarily from (1) reduced absorption from blood and water, (2) reduced photon scattering, and (3) decreased tissue thickness. 

Low Scattering Improves Penetration

The current hype around 1064nm wavelength is that it can penetrate deeper. Although it has higher water absorption than 810-830nm, 1064nm has less melanin absorption to balance it out. More importantly, longer wavelengths have less scattering, which also lends to the increased penetration of 1064nm. 

"In general, scattering is decreasing with longer wavelengths [7]. Therefore, the higher penetration depth of 1064 nm light compared to 905 nm light seen in the present study may be due to the lower absorption of hemoglobin and melanin and/or due to less scattering." [20]

A similar diagram can be found here: [1, Tuner&Hode, pg689]

Assuming all other parameters are the same, then wavelengths or tissues with less scattering will allow deeper penetration. Higher scattering means more of the light is backscattered or deflected to the sides.

"Scattering leads to light dispersion in the tissue and the eventual reduction in the energy density with increasing depth [8]." [20]

Scattering is well accepted as one of the most important properties that limits photon penetration depth in the tissues:

"Among all of the complex light–tissue interactions, optical scattering limits the light penetration depth in tissue and, consequently, photon density." [21]

And another article states:

"Another critical phenomenon in photon transport is scattering. As photons travel through an optical medium, they may interact with molecules and undergo elastic scattering, changing direction after interacting with a particle in the medium." [18]

So, if we can artificially reduce scattering in the skin with simple compression, then we have also facilitated superior penetration for all wavelengths. 

In fact, it appears that shorter wavelengths gain an even greater benefit to this technique than longer wavelengths, since we are already reducing the scattering in the tissues for the shorter wavelengths. 

"Greater improvement in penetration depth at 850nm compared to 1310nm may be understood in part because light scattering in tissue is greater at shorter wavelengths (850nm) and changes induced at shorter wavelengths by water displacement are proportionately greater. " [36]

With the above showing the 850nm gained 3x better penetration depth, and the 1310nm gaining 2x better penetration depth. Likely because 1310nm doesn't need the improved scattering as much as 850nm. 

When the water is removed from the skin tissues, the remaining collagen and protein structures will become optically coupled. This creates significantly reduced scattering inside the tissue, allowing for deeper penetration.

As is described in the following two quotes:

"We hypothesize that localized mechanical compression reduces scattering by expulsing unbound water from the dermal collagen matrix, increasing protein concentration and decreasing the number of index mismatch interfaces between tissue constituents. "[19]

And:

"This clearing effect is thought to be the result of reversible lateral water displacement within the tissue, reducing local refractive index mismatch in compressed tissue regions and changing absorption and scattering behavior." [22]

So, not only have we removed highly absorbing molecules like blood and water, the remaining tissues now have less scattering. Both of which significantly improve penetration. 

Reducing The Thickness Light Needs to Travel

Perhaps the most obvious benefit of skin compression is simply the reduction of the thickness of tissue. The thicker the skin, the more difficult it will be for photons to traverse to reach the deeper tissue. Shortening the thickness improves the amount of light that penetrates through. 

"Reduction in specimen thickness generally increased the overall transmittance." [38]

In our next blog, we will discuss how most of Near-Infrared light is absorbed within the first 3 millimeters of tissue. If you press your finger into various parts of your skin, you will note it is easy to compress the skin by at least a few millimeters, thus reducing the thickness and shifting the entire penetration profile closer to the target tissue. 

View the diagrams in this study.

The Laser Dermatology textbook discusses the use of skin compression for laser hair removal that they call "compacting the dermis". As accessing the the hair follicles is one of the deepest targets often requiring multiple techniques to optimize penetration like higher doses (not intensity) that can cause pain. They note it can decrease the tissue distance by 30% to the follicle and also alter the optics of the compressed tissue favorably. [35] 

"In theory compression should decrease water content and improve dermal transmission [86]." [4] 

They also confirm the theories of reduced absorption from water and improved transmission through reduced scattering. 

And another article discussing skin contact for hair removal:

"In addition to being more efficient in heat extraction, active contact cooling offers manually controlled skin compression, diminishing the blood flow in superficial blood vessels; therefore, decreasing the oxyhaemoglobin which is an active chromophore. Furthermore, skin compression brings deeper targets like the hair follicles closer to the skin surface thus, maximising the absorption of the laser energy, so less fluence can be used to heat these targets." [23]

They can reduce the Fluence (a fancy way of saying "dose" or Energy Density, J/cm^2) when targeting deeper tissues with skin contact method. They do not use intensity (mW/cm^2) to modulate the penetration depth, since that is not how it works. 

A Laser Ablation study on rodents found this result with skin compression:

"If we assume a 0.2–0.3 mm cavity depth without optical clearing in rodent skin, theoretical ablation depth considering enhanced penetration depth and thinning is thus 1–1.5 mm" [24]

They found that the effective penetration depth was enhanced by 5 times with the mechanical optical clearing (skin compression). Which would also allow them to use significantly lower doses for deeper tissue treatments. 

This is why we recommend lower "doses" with skin contact method, not because we have merely reduced reflection losses, but that energy is already penetrating much deeper via reduced absorption, reduced scattering, and reduced tissue thickness. 

Problem 1: What is Slight Pressure?

When you pay attention when reading studies, you will finally notice that most studies use the skin contact method.  

They will make statements like the following about their technique:

"Cluster probe held stationary in skin contact with a 90-degree angle and slight pressure." [25]

And another article:

"Cluster held stationary in skin contact with a 90° angle and slight pressure (0° angle of incidence)" [26]

And another article:

"the technique used in the application, which in this case was in direct contact with the skin with light pressure." [27]

But what is "light" or "slight" pressure? This certainly is a relative term, that will be subject to variability and inconsistencies of applications. Different people will apply different pressures according to their own interpretation of the term. Some people may even try to apply excessive pressure to get deeper penetration, which we do not encourage. So we appreciate they commonly term it as "slight" pressure to not encourage too much.  

The geometry of the device will play a role as well. For example, a rounded convex lens will apply different pressure than a large flat lens. Imagine a masseuse using their elbow instead of the palm of their hand, then can apply much more pressure through the elbow as it is a smaller point. Pressure is the force divided by the area. 

One study proposed a apparatus for skin compression in Low Level Laser Therapy to control the measurable pressure applied. This could help reduce variability in pressures used in clinical trials. 

"At the maximum −30 kPa, the peak intensities (Fig. 4) increased by a factor of 2.74, 3.22, and 3.64" [28]

With 30 kiloPascal (kPA) equalling 4.35 pounds per square inch (PSI) of pressure. 

They appear to have found that at the maximal pressure applied, they achieved around 3x improved intensity penetration through 2.8mm of tissue. 

Problem 2: Not All Skin Contact Devices Are Made Equal

One article found that the 810nm Laser had 4% less penetration with skin contact. [6]

Why did skin contact have less penetration in this trial? They noticed the tip of the laser was hollow, meaning the skin didn't get compressed and in fact the opposite might have occurred as it would create a bulge in the skin in the hollow area of the tip of the laser. 

Only with the convex lens with their 904nm laser did they get significantly better penetration with skin contact. [6]

"This change was most likely not attributed to laser parameters, but to the different physical shape of the two probe tips. The 904 nm laser probe has a protruding convex lens, which squeezes the skin when the probe is in skin contact. This will cause better penetration with full skin contact than with no skin contact. On the other hand, the 810 nm laser probe has a recessed flat window. Here, the metallic ring surrounding the lens will push skin underneath the lens, which leads to less penetration with the probe in skin contact than not in skin contact (Fig. 3)." [6]

So you must look closely at "skin contact devices" and make sure the lenses are flush with the panel or slightly protruding. Some "skin contact" devices may sit on the skin, but the actual lens may not be making solid contact with the skin, nor promoting any skin compression over the treated area. Devices on the skin may gain the advantage of Photon Recycling with a reflective surface, but other devices may gain additional penetration when the lenses compress the skin. 

We have recommended the Convoy S2 with 660nm LEDs as a very affordable red light therapy torch on our YouTube Channel. However, the lens is sunken in from the enclosure. When using it with skin contact the benefit of Photon Recycling is apparent. However, I added a layer of clear Epoxy (Total Boat brand) to enhance the skin contact for better optical coupling and compression. 

Flexible LED pads and wraps and blankets and sleeping bags may be a "skin contact device", but they may not make complete contact with the LEDs if they are wrapped loosely or not fitted to the contours of the body. 

Typically, smaller torches (flashlights), lasers, and mini handheld clusters/panels seem to be the best for applying skin contact with compression. The larger the device, the more unwieldy it will become making it harder to follow the contours of the body with perpendicular pressure. 

Conclusions:

The skin contact method appears to be the gold standard in most LLLT and PBM literature. It has been rather impressive that brands and influencers have been able to avoid this topic for many years, which is convenient for selling non-contact LED panels. 

It is difficult to quantify and describe the many mechanisms skin contact method promotes deeper penetration of light into the skin. Even harder is to measure the actual penetration depth

This technique was likely prioritized out of intuition, clinical observations, and ease of usage. It is only in recent articles do we learn more about the mechanisms that enhance penetration with skin contact.

With this technique being given a name of Mechanical Optical Clearing, we will be able to follow as more research in light therapies and optical imaging will use it to increase penetration depths for their treatments. 

A 2015 article notes that it is certainly not controversial that mechanical pressure will improve penetration depths of lasers. It has been well-established for decades. Even Niels Finsen took advantage of this technique over a century ago.

"The effect of increasing the depth of laser radiation penetration into a biological tissue under the application of the external local mechanical pressure was demonstrated more than 30 years ago [173]." [29]

There are many ways of applying red light therapy, the technique used is just as important (often more important) as other parameters like wavelength, intensity, and dose. 

Researchers and WALT will often stress the importance that papers need to describe their application technique, in addition to all of the other parameters. 

• Contact mode
• Contact with pressure
• Non-contact
• Stationary or Scanning (in combination with one of the above)

Researchers like Dr. Enwemeka make clear that their recommended method of application is skin contact. 

"Whenever possible, the contact mode of treatment is preferred for the simple reason that the loss of energy is minimal—virtually every photon emanating from the applicator enters the patient’s skin or tissue." [30]

A review article on photobiomodulation parameters simply states:

"The first issue to be addressed is light reflection from the surface of the skin,²⁹ which can be minimized if the optical probe is held in firm contact with the skin.^30"[31]

One article describes the skin contact application as integral to the definition of Photobiomodulation:

"The light source is placed in contact with the skin, allowing the photon energy to penetrate tissue, where it interacts with various intracellular biomolecules to restore normal cell function and enhance the body’s healing processes [1]." [32]

It is not controversial in the clinical science of Photobiomodulation to prefer the skin contact method, as Dr. Hamblin has stated clearly in several interviews that he prefers using devices directly on the skin. In his words it would be "ridiculous" to use red light therapy devices at a distance. 

Additional references: 

One article used multiple ball lenses for skin compression and mechanical optical clearing. [33]

A review article on various optical clearing techniques also comments on the improved image resolution from skin compression. 

"In a broader sense, mechanical optical clearing by mechanical means (e.g., compression) can also achieve optical clearing, as it increases imaging resolution and contrast comparable with chemical tissue clearing." [33]

Another article clearly noting this technique increases deeper tissue penetration. 

"Mechanical tissue optical clearing permits light delivery deeper into turbid tissue, which may improve current optical diagnostics and laser-based therapeutic techniques." [34]

It is very clear that Mechanical Optical Clearing will be an important technique for deeper tissue optical diagnostics and therapeutic applications of light.

References: 

[1]

Hode, Lars. Tuner, Jan. Laser Phototherapy Clinical Practice and Scientific Background. 2014 Prima Books AB

[2]

Chiyuki Shiroto, Misako Yodono, Shigeyuki Nakaji, PAIN ATTENUATION WITH DIODE LASER THERAPY: A RETROSPECTIVE STUDY OF THE LONG-TERM LLLT EXPERIENCE IN THE PRIVATE CLINIC ENVIRONMENT, LASER THERAPY, 1998, Volume 10, Issue 1, Pages 33-39, Released on J-STAGE July 16, 2011, Online ISSN 1884-7269, Print ISSN 0898-5901, https://doi.org/10.5978/islsm.10.33, https://www.jstage.jst.go.jp/article/islsm/10/1/10_1_33/_article/-char/en

[3]

Austin E, Geisler AN, Nguyen J, Kohli I, Hamzavi I, Lim HW, Jagdeo J. Visible light. Part I: Properties and cutaneous effects of visible light. J Am Acad Dermatol. 2021 May;84(5):1219-1231. doi: 10.1016/j.jaad.2021.02.048. Epub 2021 Feb 25. PMID: 33640508; PMCID: PMC8887026.

[4]

Lloyd, A.A., Graves, M.S., Ross, E.V. (2018). Laser-Tissue Interactions. In: Nouri, K. (eds) Lasers in Dermatology and Medicine. Springer, Cham. https://doi.org/10.1007/978-3-319-76118-3_1

[5]

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