FAQ - The Best Laser ...Continued

QUESTION: More on laser penetration depth, wavelength and power.

Depth of penetration is an extremely complicated topic. I've been considering this issue for many years and as yet I've only scratched the surface!

The main point I want to convey in the following discussion is that the physical design of the handpiece/probe and the technique used to apply it are as, if not more, important than wavelength and power in determining penetration depth.

The optical properties of tissue are determined by four main parameters: reflection, absorption, transmission and scattering, and the 'effective' depth of penetration of optical radiation in living tissue can be simplistically defined as that depth at which there remains sufficient energy (photons) to stimulate and maintain a photobiological reaction.

Recently, in the vetrehab discussion forum, [...name deleted...] wrote:

   > Fitted onto the head of the laser was a plastic basket to keep the head at
   > least 2 -4 cm from any contact.
   > Was this because of the potential for tissue heating that many seemed
   > concerned about with the class 4?

The short answer is, "Yes". The power of Class 4 lasers is such that the beam needs to be allowed to diverge (spread out) before contacting the hair and skin of the animal or it will cause burning.

However, when you diverge the beam sufficiently to lower the power density to a safe level, you also increase the amount of energy that is reflected and back-scattered (collectively termed 'remittance'), therefore significantly reducing the amount of energy that actually enters the tissue. The total remittance is in the order of 50-80% of the incident beam.

A better solution to a single high-power emitter, such as those in the Class 4 lasers being marketed to vets, is to actually use multiple lower-powered emitters that can be applied directly to the skin. This results in a much higher percentage of the incident energy actually entering the tissue (losses can be kept below 10% with an optimized delivery device).

In other words, if you purchase a $20000 Class 4 laser and then lose up to 80% of the incident energy through remittance - because it simply can't be designed to optimize energy delivery without burning holes in the patient - essentially you're wasting $16000!

   > The class 4 rep still maintains that his laser has deeper penetration. I'm
   > under the impression from Laurie Edge-Hughes and others that wavelength, not
   > power, determines penetration. I think both of these devices have similar
   > wavelengths.

As indicated above, there is much more to it than wavelength! However, even if we just look at wavelength it is not so simple...

Just recently there was some 'discussion' in the laser forum regarding one manufacturer's claims about wavelength and depth of penetration. This manufacturer stated that:

> My diagram is simplified outside the area of interest, but apart from
> that it reflects all the common knowledge that is readily available out
> there , all leading to the same conclusion: the deepest penetration is
> in an optical window centered around 800 nm.

My response was as follows:

Whilst simplification of a complex issue can be a valuable educational tool, it is nothing more than misinformation when it is skewed to categorically state specifics that are as yet unknown.

Your statement above, that "common knowledge" leads to the conclusion that "the deepest penetration is in an optical window centered around 800 nm", is an example of this.

I admit, however, that I too have previously - and inappropriately - stated that the range approximate range from 790-840nm is the deepest penetrating for continuous wave lasers.

The optics of human skin has been the subject of study since Nichols' seminal work in 1893 (1). Through more recent work, by the likes of Anderson and Parrish (2), Gemert and Jacques (3), and many others, it is now known that the optical window of human tissue falls within a range from approximately 600nm to approximately 1300nm - which is also clearly shown by the absorption curves to which you refer.

The most simplistic conclusion which can be drawn from this is that, because the optical widow is actually centered around 950nm, this wavelength and those nearest to it must be the deepest penetrating of all. However, this is no more the case than the previous claim.

For example, Anderson and Parrish (2) found the maximum approximate depth of penetration of optical radiation in fair Caucasian skin to be at 1200nm. Zhao and Fairchild (4) measured the transmittance of laser energy through Asian, African American and Caucasian tissue. They tested 532, 633, 675, 807, 911 and 1064nm, and found 1064 to be the deepest penetrating, although the actual transmittance was influenced by both race (more accurately, skin color) and beam diameter. This is not to say, however, that 1064nm is THE deepest penetrating wavelength.

The actual penetration depth of light in tissue is influenced by numerous factors and, all other factors being equal, wavelength is the most important. However it is simply not the case that there is one 'deepest penetrating' wavelength.

The literature suggests that wavelength alone is perhaps less of a determining factor for the actual or effective depths of penetration than are the type of emitter and its operating mode, the physical design of the applicator, and the technique with which the applicator it is used. Then there is the tissue itself (muscle, adipose, bone), its location on the body, the skin color (as we've seen), and so on.

And, as with many other aspects of laser and light therapies, penetration depth, its accurate definition, its measurement, and even its importance in laser therapy, are hotly debated topics that will, no doubt, remain so for some time to come.

1. Edwards EA, Duntley SQ (1939): "The pigments and color of human skin"; American Journal of Anatomy 65:1-33.

2. Anderson, RR, Parrish JA (1981): "The Optics of Human Skin"; The Journal of Investigative Dermatology, 77:13-19.

3. Van Gemert MJC, Jacques SL, Sterenborg HJCM, Star WM (1989) ‘‘Skin optics’’; Transactions on Biomedical Engineering, 36(12), 1146–1154.

4. Zhao ZQ, Fairchild PW (1998): "Dependence of light transmission through human skin on incident beam diameter at different wavelengths"; SPIE Proceedings of Laser-Tissue Interaction IX. Volume 3254, 354-360.

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