Thermal Relaxation Time
The thermal relaxation time (TRT), usually measured in milliseconds,
is the length of time it takes tissue to cool to 50% of the temperature
to which it has been heated. The TRT of the epidermis, for example,
is 3-7ms, this means that between 3 – 7ms after the application
of energy the epidermis will begin a natural cooling process and will
no longer contain the heat energy but will begin to diffuse it. The
TRT of a hair follicle is 40-100ms, much longer than that of the epidermis.
Since the hair follicle will continue to hold the heat much longer than
the epidermis it is possible to balance the length of the exposure time
of the energy between the two, allowing the destruction of the desired
target (in this case the hair follicle) without harming the surrounding
tissue, particularly the epidermis.
Therefore, if the pulse is set longer than the TRT of the epidermis,
but shorter than that of the hair follicle, the burst of energy will
be applied over a long enough period of time to allow the epidermis
to cool while the hair follicle continues to absorb the light and heat
energy.
