Thermal Relaxation Time – what is wrong with it?

The theory which has underpinned many skin-laser treatments since 1981 is called ‘Selective Photothermolysis’. It is based on matching the laser wavelength to the absorbing chromophore. It also matches the pulse duration of the laser to the thermal relaxation times of those targets.

However, there is a problem with this approach. The thermal relaxation time is a measure of the cooling rate of an object – small objects cool faster than large objects, and hence, have shorter relaxation times. The original theory attempted to minimise collateral damage to adjacent tissues by restricting the laser energy pulse duration to one thermal relaxation time.

The problem with this idea is that it does not consider the actual heating time of the target tissues. Given that the whole point is to destroy selected tissues using heat, delivered by laser or IPL light, then why do we restrict the heating time according to the cooling time of the target? This does not make sense!


The aim of photothermal treatments is to denature a sufficient quantity of target cells to render the tissue non-viable. This requires the correct combination of temperature and time. Unfortunately, the current theories of Selective Photothermolysis only consider the temperature ‘half’ of the whole process – they do not properly consider the physical denaturation half. For this we MUST think about the chemical processes required to ensure sufficient denaturation. Hence, the need to ensure the required supra-threshold temperature, for the required time to achieve the desired result.

Our studies have shown that the target tissue must be maintained at, or above, some temperature for a given time to ensure complete denaturation of those cells. This usually requires heating times longer than the thermal relaxation times of those targets. We published a report in the journal ‘Lasers in Medical Science’ in 2014 explaining our results (for a copy)

Unfortunately, the original idea about relaxation times was erroneous. The theory did work quite successfully in the early days, but this was merely a coincidence!

It was just luck that the chosen pulse durations delivered some reasonable results in some of the treatment. But it did not explain the success of the later IPL systems, which typically output much longer pulse durations than lasers.

In addition, most of today’s lasers (and all IPLs) output a ‘train’ of pulses – when the manufacturers’ claim a “20 ms” pulse, they actually mean an overall pulse envelope of 20 ms containing a series of shorter pulses within that envelope. But that’s for another post in future….

In summary, many skin conditions such as larger blood vessels and hair follicles require long pulse durations of laser/IPL energy. These can be much longer than the supposed thermal relaxation times. Plus, we have to properly consider that ‘real’ targets when dealing with these conditions.

Watch this space…



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