The importance of pulse length

In the quiz on our website to test your knowledge on laser hair removal we have the following question:

The thermal relaxation time of the human hair varies between 10 ms and 100 ms. To achieve the best possible hair removal results, the pulse length should be:

a) Less than 10 ms.

b) More than 10 ms, but less than 100 ms.

c) Longer than 100 ms.

d) Pulse length does not matter.

This seems to be a question that a lot of people who have taken the quiz are uncertain about. Originally when the theory of selective photothermolysis was defined, thermal relaxation time (TRT), was seen as the a main factor in a successful treatment. Thermal relaxation time is the time taken for the target to dissipate about 63% of the incident thermal energy.

However, various scientists have since delivered findings that dispute the importance of the TRT and instead propose the thermal damage time (TDT) as the main factor that determine treatment success. Thermal damage time is the time required, for the entire target, including the primary chromophore (e.g. melanin) and the surrounding target (e.g. hair follicle), to cool by about 63%.

The TDT is significant longer than the TRT. Where the TRT for a hair range between 10ms to 100ms, the TDT of a hair range between 200ms to 400ms.

From a paper: Simulation of Heat Distribution and Thermal Damage Patterns of Diode Hair-Removal Lasers: An Applicable Method for Optimizing Treatment Parameters. Leila Ataie-Fashtami et al, 2011.

The simulations show that longer pulse durations cause an effective temperature rise in the deep part of the hair follicle, whereas the epidermis experiences an insignificant rise in temperature. As is shown in the image below, a pulse duration of 200 ms is accompanied by a heat level of ∼60°C in the adjacent epidermis versus the 40°C level created by a pulse duration of 400 ms. This occurs despite the fact that there is no significant difference between these two pulse durations in terms of the effective heat distribution in the hair follicles.

To come back to our question. Given that the TRT for hair is 10ms to 50ms (the time needed for the heat to spread from the hair to the surrounding tissue) and the TDT is 200ms to 400ms (the time the target tissue, dermal papilla, need to suffer sufficient damage). The correct option is: Longer than 100 ms.

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