If you think of the wavelength as the active ingredient of your machine, the combination of the remaining treatment parameters makes for the dose. The first of these parameters is the pulse length. The pulse length can be adjustable or fixed, depending on the system you may use.
As the name says, it is the duration that it takes to deliver one or one set of pulses. The idea of one pulse is easy to wrap your mind around, a pulse starts, irradiates and stop. This you find with what is commonly known as long pulse lasers. For example a diode laser for hair removal has a pulse length of 200 ms, thus the light is irradiated on the skin for the full duration of 200 ms. These long pulses are called continuous wave mode.
The concept becomes something more complex with machines that break the pulse up into mini pulses, or a pulse train. This is something you commonly find in high-end IPL’s. A machine like the VPL (variable pulsed light) puts one pulse on the counter, but delivers a range of smaller pulses with a resting period between each. The total pulse length may be 95 ms, but it is actually a range of 5 pulses of 15 ms with a 5 ms break between each.
To complicate the concept even more, we also get Q-switch and ultrashort pulse machines. Q-switch allows the production of light pulses with extremely high peak power, much higher than would be produced by the same laser if it was operating in continuous wave mode. These types of machines work in pulsed wave mode.
The pulse length in combination with the energy determines the photo reaction the pulse will have. An example that comes to mind comparing it with is a little camping gas stove. If you put your little gas stove under a big pot of cold water and puts it on the minimum level where you just have a flame going, the pot of water will be warm by the time the gas canister is empty. However, if you put that same gas stove on full power you have a good chance of boiling the pot of water. Thus, the resources remained the same, the only difference was the time in which the heat (energy) was transferred to the water.
If you have a low energy and you transfer it over a long period of time, you will generate a photochemical reaction. This means that you will not create any heat or shock to the body, but will activate and deactivate chemical processes already present in the body. In this instance we measure the pulse length in seconds to minutes.
When you have high energy and you transfer it over a relative long period, you will create a photothermal reaction. A ‘long period’ for high power lasers are measured in milliseconds to seconds. Even if you deal with a photothermal reaction, a change in the pulse length can enhance or decrease your results.
For example when you do a hair removal treatment. If you have the option to increase the pulse length on your machine, you can increase the transfer of heat from the hair root to the dermal papilla, which will leads to better results. What you do is to use the light to generate heat in the hair, this heat needs to travel to the surrounding tissue to destroy the dermal papilla. The heating process starts the moment the pulse start and also stops when the pulse stops. However, tissue has a very low conductance rate and it takes relative long for the heat to spread in the tissue. Thus, if you have a longer pulse you keep the heat on for longer and that allow it to spread more, giving you a bigger chance to destroy the dermal papilla.
When you use a machine with an extremely short or ultrashort pulse you get a photoacoustic reaction, such as what is needed to break up the ink molecules in a tattoo. Because you have high energy, combined with and very short pulse you increase the effect on the target molecule while you limit the effect on the surrounding tissue. There simply is not enough time to create heat and transfer it to the surrounding tissue.
To recap. With low-level laser or LED therapy you will have a long pulse duration. This can be continuous wave or pulsed waved. In this instance pulsed wave can range from milliseconds to seconds. The treatment time on one specific spot will be much longer than with high-power lasers.
The pulse duration for long pulse high-power lasers will typically be in the milliseconds. These are usually lasers causing a photothermal reaction that enable treatments like hair removal. You need enough time to create heat and allow for it to spread to the surrounding tissue, but not so long that you cause unnecessary skin damage due to heat.
Q-switch lasers are usually measured in nanoseconds and pico lasers in picoseconds. These extremely short pulses allow for you to target a specific molecule without causing damage to the surrounding tissue, like tattoo removal. Another type of lasers that also has very short pulses, ranging from nanoseconds to milliseconds are fractional CO2 lasers. With these machines you want to target water molecules without damage to the surrounding tissue.