In this talk I discuss my 1-dimensional computer model which looks at the temporal evolution of the heat distribution, during and after irradiation of hair melanin.
During longer pulsewidths (> 1ms) heat can ‘escape’ from the absorbing melanin into the surrounding tissues. As a consequence, the temperatures in the melanin are not as great compared with shorter pulses.
Hence, longer pulses generate lower temperature in the hair, for the same fluence (and wavelength).
But, is this important? If the real targets are the germ cells in the bulge. Located on the follicle wall, then we must consider the temperature at those locations too. The heat energy diffuses from the melanin in the hair shaft, and eventually reaches the bulge. In doing so, the values of the temperatures change significantly.
My talk discusses this change in the temperature distribution and the consequent effects on the germ cells.
My conclusion is that the initial pulsewidth has no direct effect on the temperature-time histories of the germ cells in the bulge. So, the pulsewidth is not important. The total energy absorbed by the melanin is the important characteristic.
I hope you find this interesting.
Mike.
PS Our next MAsterClass will be in Liverpool on June 2nd and 3rd, 2024. If you would like more info please write to DermaLaseMasterClass@gmail.com.
Mike Murphy's Blog 
Hello Sir. I have been reading your blogs on Laser Hair Removal with great interest. You have made difficult Physics concepts and theories easier to understand for the layman. Recently I came across this article where they concluded that using very long pulse durations near the tissue damage time TDT (400 ms) creates better efficacy in treating unwanted hairs while avoiding unwanted damage. I would like to know your take on this.
Article link –
https://sci-hub.ru/10.1089/pho.2010.2895
Thanks and regards,
Dr. Nikita Patel
Hello Nikita,
Thank you for your kind comments.
Unfortunately, I cannot open that link you sent! I’m not sure why.
As for the premise, as you probably saw in my BMLA presentation, my computer model shows that the pulsewidth of the laser/IPL is not really so important. That’s because the process which is important here is the chemical denaturation of the hair germinative cells. The cooling times – TRT or TDT – are quite irrelevant.
As long as sufficient energy is delivered to the germ cells, over a reasonable period of time, they will ‘cook’ properly’. That is the end goal we are trying to achieve. How quickly, or slowly, the energy diffuses out to the germs cells is really not important at all.
I hope this answers your question but I would like to see that paper you referenced. Can you give me its title please?
Best regards,
Mike.