Optics of the Human Skin

"The results of this study showed that the difference between the experimental group (Group 2) and the control group (Group 1) were statistically significant. Group 2 was efficient for increasing the random skin flap viability in rats, maybe due to the enhancement of vascular perfusion. Moreover, low-level thermal effects cannot be excluded as a potential mechanism to increase vascular perfusion, since Stadler et al.19 showed a thermal increase in rats, and thermal effects of irradiation are unlikely to explain the LLLT effect, but because they used infrared 830 nm laser, the skin color should be considered, particularly at higher flows."

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http://www.spectramedics.com/index.php?id=105

"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.

The actual penetration depth of light in tissue is influenced by numerous factors, one of which is wavelength. 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."

QUESTION: What additional factors influence the calculation of dosage timing, and how?

One of the most important factors is the depth of the tissue that you want to treat within the body. Jan Tunér and Lars Hode have proposed a very useful working solution to this, which is represented in the equation below.

I have revised the terms used to describe each parameter of the equation to more closely reflect the intention of calculating the total irradiation time required to deliver the dose 'D' at the Target site in the tissue:

Irradiation Time [secs] = ((D x A) / P) x (1 + d)

Where:

* D = Desired Dose at Target Tissue [Joules/cm2]

* A = Area of Target Tissue [sq cm]

* P = Power of Incident Beam (Watts)

* d = Depth of Target Tissue (cm)

Note: the parameter 'd' is limited to a range of 0-4cm, with values 1-4 only applicable to the deeper-penetrating wavelengths (approx. 760-860nm GaAlAs and super-pulsed 904nm GaAs).

Ref: Tunér & Hode 'The Laser Therapy Handbook' pp 75-76

There are no hard-and-fast rules regarding other factors such as skin color, tissue type, hair cover (or fur), and so on, other than to understand that, for example, oxygenated blood more readily absorbs near-infrared wavelengths, and adipose tissue is more transparent to light than muscle.

Also, for tissue repair, you must take into account the stage of the healing process (inflammatory response, fibroblastic repair, or maturation remodelling) and, therefore, the desired clinical outcome of each particular treatment session during a course of treatment for a particular injury.

You may, for example, wish to apply an inhibitory dose for managing pain and inflammation at the very acute stage of an injury, and then to reduce the dosage and the frequeny of treatment to promote healing and effect beneficial remodelling of scar tissue during the later stages. (NB. 'frequency' in this instance relates to the number of treatment sessions over a given time period, not to the pulsing or modulation of the laser beam).

In relation to skin color, I developed the following rule of thumb some years ago, based upon the Fitzpatrick Skin Type (phototype) scale, and it seems to work well.

Light penetration of darker-pigmented skin is reduced due to the absorption of light by melanin. Absorbed light energy is converted into heat. The amount of heat so generated, and therefore its potential to cause discomfort or injury, is greater when the power/power density is sufficiently high that the rate of energy delivery and absorption exceeds the tissue's capacity to dissipate heat.

So, to both reduce the possibility of thermal injury/discomfort when using higher-powered devices, and to ensure the desired dose is delivered to the target tissue, I recommend:

* a) decreasing the power density at the skin by treating in non-contact mode (if the beam is divergent), or reducing the source power (if collimated); and,

* b) increasing the irradiation duration by adding a nominal amount (0.1-0.8cm) to 'd' in T&H's equation.

As a guide, for wavelengths between 760nm and 800nm, I suggest adding:

* 0.2 for Skin Type 3 (darker white skin)

* 0.4 for Type 4 (light brown skin)

* 0.6 for Type 5 (brown skin)

* 0.8 for Type 6 (dark-brown to black skin)

For wavelengths longer than 800nm, add:

* 0.1 for Skin Type 3 (darker white skin)

* 0.2 for Type 4 (light brown skin)

* 0.4 for Type 5 (brown skin)

* 0.6 for Type 6 (dark-brown to black skin)

So, for example, when treating a patient with Type 4 skin with an 810nm laser, one would add 0.2, as follows:

Irradiation Time [secs] = ((D x A) / P) x (1 + d + 0.2)

A similar approach can be used when treating animals to account for the amount and color of hair/fur covering the treatment area, and its effect on

Now, too bad I failed math, lol. Does anyone know if .2 - .6 equates to a couple minutes difference in treatment time in our application?

Good find. Definitely something to chew on. I always thought about how each individual depending on scalp characteristics that have to vary greatly from person to person should have a differing treatment time or "sweet spot" catered to them in order to get results. Even varying powered lasers.

You may, for example, wish to apply an inhibitory dose for managing pain and inflammation at the very acute stage of an injury, and then to reduce the dosage and the frequeny of treatment to promote healing and effect beneficial remodelling of scar tissue during the later stages. (NB. 'frequency' in this instance relates to the number of treatment sessions over a given time period, not to the pulsing or modulation of the laser beam).

Yes level of fibrosis does matter. I've brought up this theoretically in another thread but they don't have any guidelines for differing treatment times.

Also what value should we use for P? It shouldn't be 5mW or 0.005W because they are diffused and at the distance of 1.5cm. Here is what the guy from the LED Museum measured:

4.7616mW (lens removed, point blank)

3.2643mW (lens removed, 1.0cm)

2.0274mW (lens removed, 2.0cm)

I am guessing at 1.5cm it would be around 2.5/2.6

Anyway here is what I think the values should be:

D = 4 J/cm^2

P = 0.0025 Watts

d = between 0.1 & 0.2 cm (1 to 2 mm), I think I will use 0.15cm

A = assuming our diffused diodes cover .5cm across and approx 2cm down this would be about 1cm^2

So far really white skin we get:

((4 x 1)/.0025) x (1 + .15) = 1840 seconds or 30 2/3 minutes.

For Type 4 we'd add 0.4:

((4 x 1)/.0025) x (1 + .15 + 0.4) = 2480 seconds or 41 1/3 minutes.

So treatment times actually need to increase for those with darker skin. This is due to the increase amount of melanin in the skin, which absorbs light.

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With all that being said I would to find out more about hair color. I found two really good articles that would show more information but couldn't get access to it. The one graph I saw, even though it was a 'preview' and was extremely small, it showed that blond hair absorbs LESS laser light than dark hair.

The optics of human skin.

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http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B75J2-4GWC6DS-D&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=935263350&_rerunOrigin=scholar.google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f454c0bb281fa099cdd3f64236e2e714

This one would be the mother load, it has a graph of absorption of blond vs black hair at different wavelengths. They have it down at the Cleveland Clinic library but I am not sure I will be able to get access to it.

Ross E.V., Ladin Z., Kreindel M., Dierickx C.

Theoretical considerations in laser hair removal

(1999) Dermatologic Clinics, 17 (2), pp. 333-355.

Here I downloaded the small graph and blew it up. I tried to sharpen the image too and replaced the text with what I could read from the original. I couldn't figure out the Y-Axis, which seems to be absorption. The bottom line is blond and the upper line is black hair.

Figure 1. Relative absorption coefficients for black (squares) and blond (circles) hairs. Note that blond hair absorption is very small by 750 to 800 nm.

25% increase in treatment time from very light (white) to medium skin. "I'm guessing" 50% increase in treatment time from light (white) to dark (black) skin. I didn't realize it was so much! I was previously guessing just a couple minutes but I was wrong.

Again, thanks for the info. Oh, and yeah, red, blonde and gray hair absorbs less light than brown or black hair based on laser hair removal. Not to mention only hairs in the anagen phase are affected by laser hair removal. MPB is characterized as having a small percentage of hairs in the anagen phase. The worse your MPB is, the fewer percentage of hairs in anagen but I guess that's for a different subject.

BTW, are you going to increase treatment time?

Again, thanks for the info.

Thanks for the info!

I think the biggest question in my calculation is the 'A' because this would change depending on the distance from the scalp. This is why OMGs design is so nice, since everything is a uniform height from the scalp and you get equal exposure.

So the question is. What takes precedence... my blond hair absorbing less light or my light skin absorbing more?

Hopefully the kind Dr. will clear this up for us after OMG talks to him.

I haven't decided yet. With the new exercises and with a really clean scalp I have been getting pretty sore and feeling a lot of stimulation. May try 25 minutes with no exercises and see what happens.

Even before knowing that I would suggest sticking to 20 minutes for at least 6 months. By then you should be able to at least make a honest assessment.