Application
46 1590 99 002_1 - 21. February 2011
Pulse duration
The pulse duration of the laser should be shorter or at least not significantly longer than the thermal
relaxation time of the target (e.g. hair or blood vessel). The Thermal Relaxation Time is the time in
which the target gives off 50% of the heat to the environment.
Thin structures (hair, vessels) give off the heat much more rapidly than thick structures. To achieve
the desired temperature in thin targets it is necessary to deliver the required energy as quick as
possible. If the pulse is too long, the heat loss into the surrounding skin must be compensated by a
higher fluence, which causes by tendency more side effects.
Target
Pulse duration Pulse duration
longer than shorter than
Thermal Relaxation Time Thermal Relaxation Time
Figure 38: Thermal Relaxation Time and pulse duration of the laser beam
Figure 39: Thermal relaxation time for different target diameters (examples)
However, for an effective treatment the required fluence and spot size are defined first. Only
afterwards the shortest possible pulse is set for thin structures and light skin.
For dark skin it is important to consider that the melanin in the epidermis shall not be heated, i.e., a
long pulse duration should be selected no problem for thick structures.
A compromise must be made when treating thin hair/vessels of patients with dark skin. The pulse to
be selected is longer than for light skin and thin target but shorter than for dark skin and thicker
structures. This decision is acceptable because the development of heat of thin targets is
considerably lower than of thick targets. Moreover, the MeDioStar ensures an effective cooling of
the epidermis (see below).
The epidermis can be protected additionally by the pulse interruption (double pulse). In BASIC mode
the double pulse is always emitted, in the PROFESSIONAL mode it can be activated on the display.
Target Diameter Therm. Relaxation time
Ectatic vessels 10 µm 50 µm 50 µsec - 1 msec
100 µm 5 msec
300 µm 45 msec
500 µm 125 msec
1mm (500 msec)
Hair bulge ca. 300 µm ca. 45 msec
Melanosomen ca. 0.5 1 µm ca. 0.05 µsec
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