Page 95 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
Temperature rise in tissues following absorption of light energy
Some lasers, and most (if not all) IPL systems generate a sequence of pulses – a ‘pulse train ‘ when fired. So, even though it may appear that a single shot has been fired, in fact there will be a sequence of ‘sub-pulses’ which constitutes the whole ‘pulse’. You cannot see this – it’s too quick!
Figure 54 shows two sub-pulses and the resulting temperature rise in the absorbing targets. Typically, many lasers and IPLs will employ such a technique to build the total energy per ‘pulse’. A ‘single’ shot may comprise of three real laser energy sub-pulses of duration 5 milliseconds, with delays of 10 milliseconds between each sub-pulse. This is purely a technical decision made by the design engineers when designing the system.
It is highly unusual to have the ability to change any of these timings! Consequently, the overall pulse timing of such a system will be 3 times the duration of each sub-pulse (15 ms) plus the durations of each delay between those sub-pulses (20 ms). The overall pulse timing will then be quoted as 35 ms (the pulse envelope).
However, in reality, the ‘single’ shot has a laser energy duration of only 15ms!! This is very typical in modern aesthetic/medical lasers and IPLs. This can have a real, and significant, effect on treatment outcomes.
 Temperature rise during pulses T2 T1
            Pulse 1
Full pulsewidth
Pulse 2
Max Energy
          Figure 54: The first pulse of energy will raise the temperature of the absorbing site, to its peak at T1. However, during the ‘delay’ between the sub-pulses, some of this heat energy will conduct away causing the temperature to drop. The next pulse will push up the local temperature to T2.
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
© The Laser-IPL Guys 2021
Pulse energy (Joules) Tissue Temperature (C)

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