3. Tissue Optics and Penetration Depth
Assimilation - Third we have the penetration depth into the tissues - how well it is assimilated into the body.
Along with the wavelengths that provide the biological benefits we'll see more of in coming chapters, finding ways to get the light deeper into the tissues is one of the main areas of research in LLLT or PBM.
So far we have described the medicine and the dose, and the penetration depth of the light is akin to how well the medicine is absorbed. For example, you could take the best supplement in the world but if it goes right through you without being absorbed it will be of no benefit. With red light therapy we are trying to do everything we can to get the light to penetrate as deep as possible over as large an area as possible.
The gist of penetration is this: As light pours into the tissue, light will be reflected, scattered and absorbed. How deeply it penetrates depends on the wavelength, irradiance and total power. As we'll see, red and near infrared light therapy devices (especially near infrared) can penetrate the deepest because of tissue optics or the optical window of the skin, blood, and tissues is most transparent to this range.
For all of these amazing benefits to take place we need to be able to get light INTO the body, which is not as simple and straight forward as it may seem due for reasons we will now discuss, but penetration depth is one of THE most important parameters along with wavelength and optimal dosage. The science of light transport theory describes scientifically the transfer of light energy into tissue more technically, but we'll now break this down as simply as possible.
Assimilation - Third we have the penetration depth into the tissues - how well it is assimilated into the body.
Along with the wavelengths that provide the biological benefits we'll see more of in coming chapters, finding ways to get the light deeper into the tissues is one of the main areas of research in LLLT or PBM.
So far we have described the medicine and the dose, and the penetration depth of the light is akin to how well the medicine is absorbed. For example, you could take the best supplement in the world but if it goes right through you without being absorbed it will be of no benefit. With red light therapy we are trying to do everything we can to get the light to penetrate as deep as possible over as large an area as possible.
The gist of penetration is this: As light pours into the tissue, light will be reflected, scattered and absorbed. How deeply it penetrates depends on the wavelength, irradiance and total power. As we'll see, red and near infrared light therapy devices (especially near infrared) can penetrate the deepest because of tissue optics or the optical window of the skin, blood, and tissues is most transparent to this range.
For all of these amazing benefits to take place we need to be able to get light INTO the body, which is not as simple and straight forward as it may seem due for reasons we will now discuss, but penetration depth is one of THE most important parameters along with wavelength and optimal dosage. The science of light transport theory describes scientifically the transfer of light energy into tissue more technically, but we'll now break this down as simply as possible.

As we mentioned in the last chapter, red and near infrared are the DEEPEST penetrating of ALL wavelengths of biologically active light. Most wavelengths hardly get past the skin if at all, while red and near infrared penetrate several centimeters and can benefit the cells, tissues and organs (and even the bones) more profoundly. But why does the research literature report millimeters and not centimeters? This is something that confused me when I originally poured through the research because there is a tenfold difference between a few millimeters and a few centimeters.
**Work In***
**Work In***

Here is one way to cut through the confusion. Take a relatively strong flashlight and go into a dark room. Most of us have done this, but perhaps you did not really pay attention to the red light coming out of your hand. Do this over both the palm and fingers and you can CLEARLY the red light is penetrating more than just a couple millimeters as the light coming out makes it clear it could have even gone farther.
The confusion comes from reporting penetration depth in the same way engineers report it which is a 1/e dropoff or about a decrease of 63%. That is penetration depth is reported as the depth below the surface of the skin where the light intensity or irradiance has dropped by 63%. So for example, if the irradiance is 10 mW/cm^2 at the suface, the penetration depth would be that depth where it decreases to 3.7 mW/cm^2. But it is important to realize the light keeps going! And you can also see why it is important to have a strong irradiance (but not too strong as we'll see in chapter 9).
The confusion comes from reporting penetration depth in the same way engineers report it which is a 1/e dropoff or about a decrease of 63%. That is penetration depth is reported as the depth below the surface of the skin where the light intensity or irradiance has dropped by 63%. So for example, if the irradiance is 10 mW/cm^2 at the suface, the penetration depth would be that depth where it decreases to 3.7 mW/cm^2. But it is important to realize the light keeps going! And you can also see why it is important to have a strong irradiance (but not too strong as we'll see in chapter 9).
The main types of scattering generally speaking are backwards scattering (like reflection), sideways scattering and forward scattering (sometimes referred to as an anisotropy factor or a scattering coefficient in literate). Hopefully it is clear for the best penetration you want as much FORWARD scattering as possible because if the light is scattering sideways it remains at that depth and if it scatters backwards it loses depth or penetration. But here is the key: as wavelengths increase, the forward scattering ALSO increases so even if light has a greater chance of being absorbed by water above 905nm, it ALSO has a great chance of remaining in forward motion. This is like pro athletes in football and basketball getting a full head of steam going forwards, they are harder to stop! The longer the wavelength the better the forward scattering into deeper tissue.
Typically the "low" point of absorption curves line up around 810nm, but we see a dramatic drop in absorption starting in the early 600nm's. This makes 810nm perhaps the deepest penetrating (according to many researchers). However one study showed 1100 nm being best given some credence that wavelengths above 905 CAN penetrate as well or better.
So wavelength relates directly to the absorption spectrum or optical window AND scattering effects, but there is another important factor for penetration: Irradiance and total optical power. Simply put these parameters relate to how many photons of light are delivered per second and statisically if you can flood your body with more photons the odds are many more will go deeper. In light transport theory this can be seen with monte carlo modeling and statistics. I is kinda like buying more lottery tickets, the more you have the better your odds. The limiting factor here is at a certain point too much causes more harm than good. This is related to the Hormesis effect we'll look at more deeply in chapter 9. For now understand the key to PBM/LLLT/RLT is we want as MANY photons as possible (without causing harm or inhibition) with the right wavelengths that have the most benefit to get the best results! More power and more surface area gives you the most possible amount of therapy which is why powerful whole body light beds are the best of the best when it comes to red light therapy! You flood your body with photons from all directions which will get the deepest possible penetration of therapeutic wavelengths of light!
Typically the "low" point of absorption curves line up around 810nm, but we see a dramatic drop in absorption starting in the early 600nm's. This makes 810nm perhaps the deepest penetrating (according to many researchers). However one study showed 1100 nm being best given some credence that wavelengths above 905 CAN penetrate as well or better.
So wavelength relates directly to the absorption spectrum or optical window AND scattering effects, but there is another important factor for penetration: Irradiance and total optical power. Simply put these parameters relate to how many photons of light are delivered per second and statisically if you can flood your body with more photons the odds are many more will go deeper. In light transport theory this can be seen with monte carlo modeling and statistics. I is kinda like buying more lottery tickets, the more you have the better your odds. The limiting factor here is at a certain point too much causes more harm than good. This is related to the Hormesis effect we'll look at more deeply in chapter 9. For now understand the key to PBM/LLLT/RLT is we want as MANY photons as possible (without causing harm or inhibition) with the right wavelengths that have the most benefit to get the best results! More power and more surface area gives you the most possible amount of therapy which is why powerful whole body light beds are the best of the best when it comes to red light therapy! You flood your body with photons from all directions which will get the deepest possible penetration of therapeutic wavelengths of light!
According to Hamblin - 4 cm!
“One of the best studies on penetration was provided by Tedford et al. in 2015. They performed a light-penetration study on human unfixed cadaver brain tissue ... They compared 660-nm, 808-nm, and 940-nm laser penetration. 808 nm achieved the best penetration, and they concluded that 808-nmwavelength light penetrates the scalp, skull, meninges, and brain to a depth of approximately 40 mm.”
Hamblin, M, et al. (2018). Low-level light therapy: Photobiomodulation. Society of Photo- Optical Instrumentation Engineers
“One of the best studies on penetration was provided by Tedford et al. in 2015. They performed a light-penetration study on human unfixed cadaver brain tissue ... They compared 660-nm, 808-nm, and 940-nm laser penetration. 808 nm achieved the best penetration, and they concluded that 808-nmwavelength light penetrates the scalp, skull, meninges, and brain to a depth of approximately 40 mm.”
Hamblin, M, et al. (2018). Low-level light therapy: Photobiomodulation. Society of Photo- Optical Instrumentation Engineers

Penetration Depth Limitations- The Light at the end of the tunnel
Several attempts have been made at careful dose modeling and physical assessment of light distribution following transcranial PBM treatment of the human brain (Yue and Humayun, 2015; Tedford et al., 2015). There is clear evidence that some, albeit miniscule, amounts of light are effectively transmitted to deeper parts of the brain following external PBM treatment to the head. There is surprising evidence that the human cells in the visual system are capable of detecting only a single photon (Tinsley et al., 2016; Ala-Laurila and Rieke, 2014).
Yue, L., Humayun, M.S., 2015. Monte Carlo analysis of the enhanced transcranial penetration using distributed near-infrared emitter array. J. Biomed. Opt. 20, 88001. Available from: https://doi.org/10.1117/1.JBO.20.8.088001.
Tedford, C.E., DeLapp, S., Jacques, S., Anders, J., 2015. Quantitative analysis of transcranial and intraparenchymal light penetration in human cadaver brain tissue. Lasers Surg. Med. 47, 312322. Available from: https://doi.org/10.1002/lsm.22343.
Several attempts have been made at careful dose modeling and physical assessment of light distribution following transcranial PBM treatment of the human brain (Yue and Humayun, 2015; Tedford et al., 2015). There is clear evidence that some, albeit miniscule, amounts of light are effectively transmitted to deeper parts of the brain following external PBM treatment to the head. There is surprising evidence that the human cells in the visual system are capable of detecting only a single photon (Tinsley et al., 2016; Ala-Laurila and Rieke, 2014).
Yue, L., Humayun, M.S., 2015. Monte Carlo analysis of the enhanced transcranial penetration using distributed near-infrared emitter array. J. Biomed. Opt. 20, 88001. Available from: https://doi.org/10.1117/1.JBO.20.8.088001.
Tedford, C.E., DeLapp, S., Jacques, S., Anders, J., 2015. Quantitative analysis of transcranial and intraparenchymal light penetration in human cadaver brain tissue. Lasers Surg. Med. 47, 312322. Available from: https://doi.org/10.1002/lsm.22343.

Zimmerman - 8 cm!
Majority of energy coming from the is infrared.
If I can feel the warmth, you are receiving it.
Zimmerman Study showed regardless of Melanin, the near infrared light was penetrating up to 8cm!!
Can find veins.
Near Infrared light deeply penetrating, large amount of cells in human body accessible to near infrared.
Near Infrared can even penetrate bone.
Sunlight can penetrate the skull.
Majority of energy coming from the is infrared.
If I can feel the warmth, you are receiving it.
Zimmerman Study showed regardless of Melanin, the near infrared light was penetrating up to 8cm!!
Can find veins.
Near Infrared light deeply penetrating, large amount of cells in human body accessible to near infrared.
Near Infrared can even penetrate bone.
Sunlight can penetrate the skull.
Less Absorption and More Reflection Leads to better Penetration [See above]
It seems counterintuitive that the wavelengths of Red to NIR have the MOST reflection and LEAST absorption of the entire light spectrum – it does make sense because we don’t want them absorbed too quickly and allow them to penetrate deeper than other wavelengths.
In some studies, having high reflection in our skin composition is a good thing because allows for more “scattering” of the light once it passes through the skin. And the easy way to bypass the reflection losses is to use skin contact.
It seems counterintuitive that the wavelengths of Red to NIR have the MOST reflection and LEAST absorption of the entire light spectrum – it does make sense because we don’t want them absorbed too quickly and allow them to penetrate deeper than other wavelengths.
In some studies, having high reflection in our skin composition is a good thing because allows for more “scattering” of the light once it passes through the skin. And the easy way to bypass the reflection losses is to use skin contact.

We know that when light comes into contact with different mediums such as moving from air to water, some component is reflected, some is transmitted (penetrates), and some is absorbed. Here we see an example with the reflection and transmission of two mediums. You can imagine that human skin is made up many layers of various mediums, so it is a complicated interaction of forward and backwards reflections and transmissions.
It might be ideal if the light approaches the skin at a perpendicular (straight on), compared to the light coming from an angle. Especially since we can assume most lasers are positioned at a perpendicular angle with the skin.

Problem with LED Panels at any distance - Diffuse reflection
Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered in all directions or at many angles rather than at just one angle as in the case of specular reflection.
The key is what YOU absorb, not how much the device is kicking out. Close contact and full wrap around with ALWAYS beat separating the LED device from your skin and/or using a localized panel. I am all for localized Wraps, LED panels, LLLs, etc., but I am also a huge believer in treating the whole body as well.
Also panel companies saying will be as good as a full body LED bed with comparable irradiance is a joke. There is no way you can touch a full 360 wrap around of LED with an upright panel. I am not saying you need to invest in an expensive LED bed, but just know you get what you pay for in general.
Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered in all directions or at many angles rather than at just one angle as in the case of specular reflection.
The key is what YOU absorb, not how much the device is kicking out. Close contact and full wrap around with ALWAYS beat separating the LED device from your skin and/or using a localized panel. I am all for localized Wraps, LED panels, LLLs, etc., but I am also a huge believer in treating the whole body as well.
Also panel companies saying will be as good as a full body LED bed with comparable irradiance is a joke. There is no way you can touch a full 360 wrap around of LED with an upright panel. I am not saying you need to invest in an expensive LED bed, but just know you get what you pay for in general.
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