Least Heating Wavelengths for Red Light Therapy: Best Penetration?

Which wavelengths are the least heating for red light therapy? Is it Red or Near-Infrared that produce the least heat on the skin? Can the least heating wavelengths tell us which ones have the best penetration?

The basic premise and definition for Photobiomodulation (PBM) and Low Level Laser/Light Therapy (LLLT) has been that it is the science of the non-thermal interaction of light on biology. [1]

The Red to Near-Infrared wavelength range from 600nm to 1100nm are typically considered to be the "optical window" of the skin that not only penetrate deepest into the skin compared to the rest of the spectrum, but also deliver the least heating to the tissue.

"LLLT consists of non-thermal red or near infrared light (600–1000 nm) which might affect many cellular processes" [2]

"The use of low level light (laser) therapy (LLLT) consisting of non-thermal red and/or near infrared light (600–1000 nm) delivered from a laser or from a non-coherent light source has been shown to have beneficial effects on a wide range of pathologies." [3]

Notice the above quotes referring to the wavelengths of PBM/LLLT as non-thermal.

However, any wavelength can cause heating at high intensities. As it often denoted by the common term "cold laser" to separate low intensity lasers from the high intensity ones that cause heating.

PBM studies are careful to use low intensities to ensure it is a non-thermal tissue response. Or they may use other methods such as pulsing or external cooling to reduce the deleterious heat from high intensities.

As this quote reminds us, it is important to use intensities that are low enough to not cause significant heating of the tissue.

However, if we can find which wavelengths have the least heating effect, then we could potentially maximize the intensity that can be used safely. For example if 810nm is less heating than 650nm (it is), then we could use relatively higher intensity of 810nm than 650nm without causing heat.

The wavelengths that generate the least heat on the skin also correspond to being the ones that penetrate the deepest. So this is a simple way to confirm which wavelengths have the best penetration.

And we will finally put to rest another myth that Near-Infrared is more heating than Red. When it is the opposite, Near-Infrared wavelengths produce less heat on the skin compared to Red.

Spoiler: Near-Infrared wavelengths around 800nm-850nm produce the least heat on the skin. Where the Red wavelengths are significantly more superficial absorption leading to more skin heating. For people with darker skin types it may be important to use longer wavelengths around 1064nm due to the higher melanin absorption with the shorter Red and NIR wavelengths.

Finding the least heating wavelengths is particularly important for people with sensitivity to heat, people predisposed to hyperpigmentation and melasma, or people wanting an evidence-based treatment for true "cold" light therapy called photobiomodulation.

As we have noted in previous blogs, heating will drastically increase the ROS production leading to a faster biphasic dose response - and increase circulation which actually hinders penetration. So for optimal PBM benefits, most studies are carefully designed to minimize heating. The first step to minimizing heating is selecting the proper wavelength.

Infrared is not Heat

Many people falsely assume that all Infrared wavelengths are intrinsically heating.

In his book Zapped, author Bob Berman reminds us of the obvious:

"Infrared radiation is not heat. Rather, infrared radiation creates heat. " (pg. 39) [5]

Bob Berman explains to the reader that Visible light wavelengths can also generate heat. The real difference is how the different wavelengths are absorbed.

Lets say we have:

• 10 Joules of Red Light (660nm)
• 10 Joules of Blue Light (450nm)
• 10 Joules of Far-Infrared light (5,000nm)

Which one is the most heating?

We know the first law of thermodynamics is that energy is normally never created nor destroyed - it only changes form. [6]

"As a representative phenomenon of light–matter interaction, the photothermal effect is generally characterized with the temperature increase in a material through the absorption of light." [2]

So 10 Joules of any light wavelength is the exact same amount of potential heat energy.

This is the same kind of riddle to say "if you have 10 kilograms of steel and 10 kilograms of feathers, which one is heavier?" In the same way we know 10 Joules of Red light is the same potential heat energy as 10 Joules of Far-Infrared.

So what is the real difference when we think about how wavelengths get transformed into heat? Absorption.

Interactions with Light and Matter

When an electromagnetic wave (i.e. light) encounters matter (anything from solid objects, air molecules, water, etc), then it has 3 fundamental choices:

1. Reflection
2. Absorption
3. Transmission

Many semi-transparent objects will exhibit a combination of all 3 of these features. Even a glass window may reflect some light as glare, absorb some light as heat, and let most of the visible light transmit though.

Obviously Reflection and Transmission would not contribute to any heating effect, and only absorption can have a heating effect.

This phenomenon is rather intuitive, but it is so crucial that it is called the First Law of Photochemistry/Photobiology; that only absorbed radiation will have an effect on the molecules. It is also called the Grotthuss-Draper Law after the researchers who cemented this concept in the early 1800's. [7] 

What Happens During Photon Absorption?

When a photon is absorbed by a molecule, the photon is annihilated and the molecule enters a higher energy state. Since energy is conserved via the 1st law of thermodynamics, the molecule must resolve this higher energy state to maintain some equilibrium and stability.

There are 3 fundamental options for the molecule to do after absorbing a photon:

1. Increase molecular motion
2. Emit Radiation
3. Reorganize the molecule

(pg 36) [8]

In laymen's terms, the result is:

1. Increase in temperature (heat is the macro-scale of molecular motion)
2. Re-emit light like in luminescence
3. Undergo a chemical reaction

The goal of Photobiomodulation is to focus on the chemical reactions that are facilitated by by photon absorption whilst minimizing heating. As we can see in the below quotes.

"PBM is devoid of thermal and ablative mechanisms and exploits the absorption of light to affect a chemical change [9]. " [10]

For example the energy provided can break the bond between Nitric Oxide and CytoChrome C Oxidase to up-regulate the Electron Transport Chain, or to impart it's energy to form EZ Water in the cell, or modulate proton gradients and ion channels.

A more common chemical or molecular change is a phase change. When water transforms into steam it consumes energy without increasing temperature. The name of this is "latent heat" which is a stored potential energy now in the higher-energy state of steam. The same way many chemical reactions will consume energy without neccesarily changing temperature.

This is indicative of the 2nd Law of Photochemistry/Photobiology. Also called the Stark-Einstein law of photoequivalence, it tells us every absorbed photon will cause a single elementary reaction to a molecule. As we have already listed the elementary changes that can undergo are heat, luminescence, or chemical changes.

Ultimately the end result promotes the production of the chemical energy currency of the cell, ATP. These are all photochemical events, and with minimal influence of photothermal effects.

Deeper Penetration means Less Heating

It is generally agreed upon that Red has more superficial absorption and Near-Infrared is deeper penetrating. Yet this is contradictory to the myth that Near Infrared is more heating than Red.

One study explains this concept well:

Only a comparable small fraction of the higher red wavelength’s energy is being absorbed, as most of it is reflected and the absorption is spread out over a larger volume due to the long wavelength’s increased penetration depth." [11]

Now this example discusses how Blue and Green light is more heating than Red light. But the concept remains the same.

The wavelengths with the deepest penetration can spread it's energy out in a wider volume. Wavelengths that are superficially absorbed will cause more superficial heating.

For example, consider heating a pot of water that is half-full compared to a pot that is completely full. Obviously the half-full pot heats up faster because it has less volume. Similarly if Near-Infrared penetrates twice as much as Red, it has twice as much volume to disperse it's energy into. Therefore, less skin heating from NIR.

Human Skin Optics

As discussed in earlier blogs, the "optical window" of the skin is where there are the lowest absorption of primary chromophores like Melanin, Hemoglobin, and Water.

[12][13][14]

Here we see the least absorption ranges are around 800-850nm and then another low absorption range 1040nm to 1100nm. We often see these ranges are popular in clinical trials due to their deep penetration.

Incidentally, the "optical window" wavelengths also correspond to the wavelengths that have the highest reflection from the skin.

[NIST Skin Reflection Spectrum]

Here we see the highest reflection from 630nm to 900nm, and another reflection peak around 1050nm to 1120nm. Interestingly the reflection spectrum is nearly the inverse of the absorption spectrums, which makes sense because they affect each other.

Having the lowest absorption and highest reflection is why these wavelengths have the least thermal impact on the skin. Which also corresponds to wavelengths that are expected to penetrate the deepest.

While many will dispair at the high reflection losses when using non-contact red light therapy (i.e. at 6 inches away), this is a good thing. The reflection properties will cause forward scattering once the photons are in the body - especially aided by using skin contact technique to overcome the initial reflection losses. 

Higher Intensities Possible with Non-Thermal Wavelengths

Now lets look at some examples. An article compared the heating effects from a standard incandescent halogen lamp versus a Water Filtered IRA (wIRA) lamp.

In the wIRA lamp, the heating wavelengths are filtered by water so only non-thermal wavelengths are passed through.

"The threshold temperature for heat pain—defined according to DIN 33403-2 as being approximately 43C (see ref. [24])—was observed at an incident irradiance of approximately 1000 W m)2 for the unfiltered IR halogen lamp and of about 2500 W m)2 for the wIRA radiator (see Fig. 10a)." [15]

With a standard halogen bulb the tolerated intensity is only up to 100mW/cm^2. However with the filtered wIRA lamp the tolerated intensity limit is 250mW/cm^2.

Confirming the premise that we could use higher intensities if we focus on the least heating wavelengths.

Higher Intensities Possible with Non-Thermal Wavelengths - With Lasers

Lets look at another example that used lasers.

"The irradiance values, that produce unacceptable heating of the tissue, are governed by the wavelength and are ∼750  mW/cm2 at 800 to 900 nm, about 300  mW/cm2 at 600 to 700 nm, and as low as 100  mW/cm2 at 400 to 500 nm." [16]

We see that the 400-500nm range (blue light) has much higher heating effects leading to "unacceptable heating" at only 100mW/cm^2.  Blue, green, and even yellow light is highly heating due to superficial absorption from melanin - similar to how far-infrared causes heating from water absorption.

And the crux of this blog is already illucidated in this quote. That you can tolerate more than twice as much NIR (800-900nm) intensity than Red (600-700nm) before unacceptable heating occurs.

We could say Near-Infrared is 2.5 times less heating than Red according to the differences in intensity here. And we can see the parallel with penetration that some studies confirm Near-Infrared has nearly twice as much penetration as Red.

So if we want to use higher intensities with less heating on the skin, then we would choose Near-Infrared (800-900nm).

Coverage Area also Affects Heat

Now these laser intensity numbers are much higher than the recommended non-thermal intensity numbers we usually recommend from large LED panels to be less than 50 mW/cm^2 to avoid heating.

Which firstly we want to stay far away from the pain or damage threshold of high temperatures as a so-called maximum tolerated dose, but maintain practically no temperature change with low intensities for true Photobiomodulation.

In the previous example with the wIRA lamp, the authors explain the impact of surface area on heating. [15]

A wide surface area from an LED panel or Incandescent bulb using 100mW/cm^2 will be a lot more total energy and total watts than a small spot of a laser delivering the same intensity.

"thermal effects in tissue will depend on the size of the area exposed. Thus, with any given irradiance, the maximum temperature reached at the exposed area increases with area size" [15]

So the larger area being irradiated, the higher the heat. With relatively lower intensities, a large area treatment will have more heat effects.

The laser may have high intensity but has low overall power, and there is more area for the laser heat to diffuse. A large LED panel will saturate a large area of the skin with intensity, leading to more energy the skin needs to manage with less volume that is unaffected.

Lets look at a diagram to explain.

So we can note that 250mW/cm^2 with non-thermal wavelengths covering a wide area will lead to deleterious heating, whereas the laser example can use 300-750mW/cm^2 of non-thermal wavelengths before causing heat damage due to the small spot size.

So this is why we often see much higher intensities used with lasers, and it is recommended to use much lower intensities with wide-area treatments like LED panels.

Least Heating Wavelengths: Mathematical Modeling

One study used mathematical modelling based on the properties of of human skin optics to look at penetration and temperature increases based on wavelength.

They found in the following order the least heating to the most heating:

1064nm < 980nm < 905nm < 905 < 850 < 808 < 1200 < 660 < 632

[17]

Surprisingly 1064nm had the least heating, while this study confirms that the Red wavelengths are significantly more heating than Near-Infrared.

However, the big drawback to this study is that it was done with mathematical modeling, which may not be able to fully calculate the complexities of human skin optics and thermoregulation processes.

Least Heating Wavelengths: In Vitro (tissue) Study

A study that compared several wavelengths was conducted on isolated tissue with real lasers.

They found the following relationship with the wavelengths they tested:

810nm <<< 650nm ≈ 1064nm < 980nm

[18]

In laymens terms this says that 810nm is much less heating than 650nm. The heating from 650nm and 1064nm are roughly equal. And the 980nm had the highest heating.

This is in better alignment with the penetration models that 810nm is expected to have the least heating. Both 1064nm (NIR) and 650nm (Red) are about the same heating, as there is a tradeoff between higher melanin absorption from Red and higher water absorption with the longer wavelength NIR. There is a peak of water absorption in the 900's, which makes sense that 980nm was actually the hottest wavelength tested.

But again there is a minor drawback that the authors mention that this was on an isolated tissue sample that doesn't fully represent all the layers of the skin or the thermoregulation as part of a body.

Why is Near-Infrared mislabelled as Heating?

By now it should be clear based on the theory and research examples that Near-Infrared is certainly less heating than Red wavelengths, despite the constant myths otherwise. This means we need to consider Melanin absorption as just as important for causing heating with shorter wavelengths as Water absorption is for longer wavelengths.

Here are some possible explanations as to why Near-Infrared is falsely associated with heating:

1. Near-Infrared often wrongfully gets associated with their siblings Mid-Infrared and Far-Infrared. But we know up to 1100nm NIR is still minimal water absorption and fairly good penetration, however past 1100nm+ is when more significant heating starts.
2. Most nerve fibers are past the epidermis and in the dermis. [19] So the deeper penetrating Near-Infrared will simulate those deeper nerve heat receptors faster, perhaps resulting in the immediate sensation of heat. However, the Red will be more superficially heating the epidermis without as much heat sensation, which is actually a more risky scenario as you are heating the skin without feeling it.
3. Organs that are extremely sensitive to heat will also be more likely to be affected by high intensity or dose Near-Infrared as it will naturally penetrate deeper to deliver more energy into those organs. For example overdosing the testies or brain with NIR will lead to unwanted overheating.[20][21]

"Another disadvantage of NIR laser neuromodulation is the accidental overheating of brain tissue, which may cause inherent injury and inhibit neural activity, producing side effects in addition to normal regulation" [21]

In this way we may still choose Red light in cases that we want to avoid accidental heating of deeper sensitive organs. Or simply use properly low intensities to avoid any problems at all.

If we want to avoid heating superficial tissues for example to avoid hyperpigmentation, then we would choose deeper penetrating Near-Infrared.

Conclusion:

We often think of selecting wavelengths in terms of the benefits it can deliver, sometimes preferring a specific penetration depth, specific mechanisms, or specific chromophore targets.

In this blog we think of selecting wavelengths in a different way, in choosing ones that cause the least heat.

"As an increased wound temperature can correlate with decreased wound healing and wound bed score [37], the choice of the wavelength used has to be carefully considered." [11]

For some therapeutic contexts, choosing the least heating wavelength will be more important for effectiveness and safety.

The deeper penetrating 810nm wavelength is expected to produce the least heating for most skin types, and in darker skin types the 1064nm wavelength will be more important for avoiding superficial melanin absorption.[22]

Near-Infrared wavelengths could still deliver unwanted heat into deeper tissues like organs that are highly sensitive to heat. In this way we must consider if we are trying to control the heating to superficial tissues or deep tissues. Choosing Red wavelengths may avoid overheating sensitive organs, even though it will have more heating to the skin.

Ultimately, utilizing low intensities will ensure there is minimal heating as is the basic definition of photobiomodulation. This helps the cells focus on the beneficial photochemical effects and minimizes risks and complications often caused by heating.

The foundation of Red Light Therapy is with deep penetrating "non-thermal" wavelengths in the Red to Near-Infrared range. Selecting the optimal wavelength is often based on many factors including bioindividuality.

We may be able to avoid many myths from being generated if we are strong in the basics of optical physics, photochemistry, photobiology, and skin optical properties. So this blog was hopefully able to convey some of these important concepts in relatable ways.

[1]

Anders, Juanita J et al. “Low-level light/laser therapy versus photobiomodulation therapy.” Photomedicine and laser surgery vol. 33,4 (2015): 183-4. doi:10.1089/pho.2015.9848

[2]

Hossein-Khannazer, Nikoo et al. “The Role of Low-Level Laser Therapy in the Treatment of Multiple Sclerosis: A Review Study.” Journal of lasers in medical sciences vol. 12 e88. 28 Dec. 2021, doi:10.34172/jlms.2021.88

[3]

Huang, Ying-Ying et al. “Low-level laser therapy (LLLT) reduces oxidative stress in primary cortical neurons in vitro.” Journal of biophotonics vol. 6,10 (2013): 829-38. doi:10.1002/jbio.201200157

[4]

Ahmed, S., Bewsh, G., Bhat, S., & Babu, R. (2013). LOW LEVEL LASER THERAPY: HEALING AT THE SPEED OF LIGHT. Journal of Evolution of medical and Dental Sciences, 2, 7441-7463.

[5]

Berman, Bob. "Zapped : from infrared to x-rays, the curious history of invisible light." New York : Little, Brown and Company, 2017

[6]

Photothermal Nanomaterials: A Powerful Light-to-Heat Converter

Ximin Cui, Qifeng Ruan, Xiaolu Zhuo, Xinyue Xia, Jingtian Hu, Runfang Fu, Yang Li, Jianfang Wang, and Hongxing Xu

Chemical Reviews 2023 123 (11), 6891-6952

DOI: 10.1021/acs.chemrev.3c00159

[7]

Albini, A. Some remarks on the first law of photochemistry. Photochem Photobiol Sci 15, 319–324 (2016). https://doi.org/10.1039/c5pp00445d

[8]

E.J. Land (1983) The Science of Photomedicine, International Journal of Radiation Biology and Related Studies in Physics, Chemistry and Medicine, 43:4, 471-472, DOI: 10.1080/09553008314550531

[9]

Choi JE.  Photobiomodulation Therapy in Recovery of Peripheral Facial Nerve Damage.  Medical Lasers 2020;9:89-94.  https://doi.org/10.25289/ML.2020.9.2.89

[10]

Leyane, Thobekile S et al. “Cellular Signalling and Photobiomodulation in Chronic Wound Repair.” International journal of molecular sciences vol. 22,20 11223. 18 Oct. 2021, doi:10.3390/ijms222011223

[11]

Dungel, Peter et al. “Wavelength-Dependent Effects of Photobiomodulation for Wound Care in Diabetic Wounds.” International journal of molecular sciences vol. 24,6 5895. 20 Mar. 2023, doi:10.3390/ijms24065895

[12] Water Absorption Coefficient Spectrum:

https://omlc.org/spectra/water/data/hale73.txt

https://omlc.org/spectra/water/abs/

[13] Hemoglobin Absorption Coefficient Spectrum:

https://omlc.org/news/jan98/skinoptics.html

https://omlc.org/spectra/hemoglobin/summary.html

[14] Melanin Absorption Coefficient Spectrum:

https://omlc.org/spectra/melanin/mua.html

[15]

Piazena H, Kelleher DK. Effects of infrared-A irradiation on skin: discrepancies in published data highlight the need for an exact consideration of physical and photobiological laws and appropriate experimental settings. Photochem Photobiol. 2010 May-Jun;86(3):687-705. doi: 10.1111/j.1751-1097.2010.00729.x. Epub 2010 Apr 16. PMID: 20408985.

[16]

Zein, Randa et al. “Review of light parameters and photobiomodulation efficacy: dive into complexity.” Journal of biomedical optics vol. 23,12 (2018): 1-17. doi:10.1117/1.JBO.23.12.120901

[17]

Chaki C, De Taboada L, Tse KM. Three-dimensional irradiance and temperature distributions resulting from transdermal application of laser light to human knee-A numerical approach. J Biophotonics. 2023 Jun 1:e202200283. doi: 10.1002/jbio.202200283. Epub ahead of print. PMID: 37261434.

[18]

Cronshaw M, Parker S, Grootveld M, Lynch E. Photothermal Effects of High-Energy Photobiomodulation Therapies: An In Vitro Investigation. Biomedicines. 2023 Jun 4;11(6):1634. doi: 10.3390/biomedicines11061634. PMID: 37371729; PMCID: PMC10295700.

[19]

Park, Bomi, and Seong Jin Kim. “Cooling the Skin: Understanding a Specific Cutaneous Thermosensation.” Journal of lifestyle medicine vol. 3,2 (2013): 91-7.

[20]

Ahn, J.-C & Kim, Y.-H & Rhee, C.-K. (2013). The effects of low level laser therapy (LLLT) on the testis in elevating serum testosterone level in rats. Biomedical Research (India). 24. 28-32.

[21]

Pan, Wei-Tong et al. “Advances in photobiomodulation for cognitive improvement by near-infrared derived multiple strategies.” Journal of translational medicine vol. 21,1 135. 22 Feb. 2023, doi:10.1186/s12967-023-03988-w

[22]