Laser Pulse Duration?

by BHW
(CHANDLER, ARIZONA)

Question: Why exactly is it important for the pulse duration of the laser beam to be equal or less than the thermal relaxation time of the target?

Answer: (By Editor, LHRT)

Laser pulse duration for hair removal signifies the amount of time the tissue (in this case the hair follicle) will be exposed to the laser beams.

Thermal relaxation time is the time taken by that hair follicle to lose 50% of its heat generated by its exposure to the lasers.

Interesting question Dr. BHW! But I know that you know the answer better than me…

And in case you are not just testing me out, I will try to answer your question. Anyway, it will help other inquisitive visitors and experts as well.

Go here to read more popular questions in Laser Hair Removal Answers section.

Let me illustrate the facts about lasers step by step for a clear understanding by a commoner like me!

What is Laser?

The term LASER is actually an acronym for "Light Amplification by the Stimulated Emission of Radiation".

Digging Further Into The Laser History?

When we dissect the term LASER further, we find that the theory of SER (Stimulated Emission of Radiation) was first proposed by Albert Einstein.

To add the prefix LA (Light Amplification) to the SER, it took a long time, may be around four decades. The credit for this goes to Theodore Maiman for his works on ruby lasers.

And the application of LASER in the Medicine was pioneered by the father of laser medicine and dermatologist Leon Goldman, MD.

Boring, I know! We are slowly moving towards our question i.e. the pulse duration of the laser beam and the thermal relaxation time of the target. Please be patient and carry on with me.

One more concept, which is vital to understand this topic, is the theory of Selective Photothermolysis proposed by Anderson et al.

What Is Selective Photothermolysis And How It Relates To Laser Pulse Duration?

According to the theory of selective photothermolysis, selective thermal damage of the target pigmented structure occurs with a sufficient fluence of light or laser energy.

For this to occur that light or laser energy must be delivered at a wavelength preferentially absorbed by the target.

This sufficient fluence and wavelength must be administered within a time equal to or less than the thermal relaxation time of the target.

With this we come to a conclusion that there are three variables which can be modified to control the lasers' ability of selective destruction of target pigmented tissue.

• Wavelength of the lasers (For a non technical person its the measure of potency of lasers)
• Pulse duration of the lasers (Duration for which the hair follicle is exposed to laser)
• Energy loading of the tissue (thermal relaxation time of the target)

But Why The Pulse Duration Must Be Equal Or Less Than The Thermal Relaxation Time Of The Target?

As you have read in the beginning, Pulse duration is the time for which the hair follicle is exposed to lasers. And the thermal relaxation time is the time taken by that hair follicle to lose 50% of its heat generated by its exposure to the lasers.

At last... and finally we are close to our answer (after a lot of technical and nerdy blah blah…) Drum rolls please!

The pulse duration of the laser has to be shorter than the thermal relaxation time of the target chromophore to be effective. And in our case the target chromophore is Melanin in the hair follicle. A shorter pulse duration ensures that the majority of the heat energy and thus the subsequent damage is kept within the target tissue without adversely affecting the surrounding tissue. To keep it simple it prevents damage to any tissue other than the hair follicle…

To shed more light on this… (I think Dr. BHW was interested in this, rather than just testing me out!)

Thermal relaxation time is calculated by the following formula
TRT = d2/16Kd
d = diameter of the target chromophore
Kd = thermal diffusion constant

Now if you ponder over this formula and put in the values for d and Kd in it for different target tissues the result is going to be different.

Or say the thermal relaxation time (TRT) is different for different target tissues.

Now if we set the laser device pulse duration to be more than the thermal relaxation time (TRT) of the hair follicle, then what we are doing is, telling the lasers to target also some tissue other than just the hair follicle.

And if the the surrounding tissue's TRT comes in the range of the pulse duration we have set, then the natural outcome will be the destruction of that tissue by lasers… i.e. collateral damage inflicted by us, by giving lasers the wrong directives!

And worthy to note here is the boon of lasers, that we can finely control the three variables for a precise application over just the hair follicles. And also a large number of hair follicles in one shot. This is not possible in various other methods of hair removal.

I hope this helps you my visitor and also Dr. BHW, towards understanding the effect of laser pulse duration on the ultimate outcome. I apologize for the extreme delay in answering as I was quite busy lately.

Thanks anyway and Regards,

Editor,
Laser-Hair-Removal-Talk.com

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