Hair removal using Diode lasers vs Nd:YAG, Alex lasers & IPLs

Hair Removal Systems

These days there are a number of different systems available for hair removal, which can make it a bit tricky when deciding which to purchase.

In particular, diode lasers are a different type of laser when compared to the other systems (and IPL). Here I outline some of the differences between these devices:

Diode LasersNd:YAG, Alex Lasers / IPLs
  
Diodes are continuous  These systems output long pulses, typically longer than 1 millisecond
  
Energy/fluence increases with pulsewidth. Diode lasers can usually only increase their output energy/fluence by using longer pulses.  Lasers/IPL are powered by flashlamps, so, can change fluence independently of the pulsewidth. Higher voltages on the flashlamps means more energy without changing the pulsewidth.  
  
Longer pulses generate higher temperatures in the absorbing targets (hair).  Shorter pulsewidths generate higher temperatures in the targets.  
  

Diode Lasers and ‘Powers’ (Watts)

I often see diode lasers being quoted with ‘powers’ and I get confused by what they mean – like many people!

There are two main ‘powers’ to be aware of:

The first of these is the devices electrical power requirements. This is purely an electrical power requirement. It means that the laser’s power supply unit can output a maximum power of some level (maybe 1000W or 3500W, or something similar).

The second ‘power’ refers to the laser’s actual optical power output – the stuff that hits the skin!! This is NOT the same as the laser’s electrical power (see below). This is the power that is actually important. It tells us how quickly the energy or fluence may be supplied to the skin surface. A 1000W laser can deliver 1000 Joules of energy per second, whereas a 5000W can deliver five times that rate.

Device electrical power

A percentage of the laser’s electrical power is used to ‘drive’ the diode lasers (perhaps between 50 and 80%). So, the unit might have a power rating of 5000 Watts, but only between 2500 and 4000W is actually used to ‘fire up’ the laser. (The rest is used to pump the water, run the screen etc…

A percentage of this electrical power is converted into laser (light) power. This is the “output” power – it is NOT the same as the “electrical” power of the device! Now, lasers are never 100% efficient so not all of this power will be converted into the laser power. It depends how good the diodes are, but this might be between 75 and 90% (or thereabouts).

So, a 5000W (electrical power) device might deliver an optical power (to the skin) of around 2500W – this is the power we should be interested in. This is what actually hits the skin surface.

As a consequence, higher powered lasers deliver more energy/fluence in same pulsewidth than lower power lasers

For a pulsewidth of 20 milliseconds in three different lasers:

Laser Output Power (optical) (Watts)Maximum Fluences (J/cm^2)
100020
250050
5000100
Maximum fluence assuming a 1 cm^2 spot size

So, we can see that a higher powered laser will deliver more fluence to the skin in the same time (pulsewidth) than a lower powered laser. This means that the temperature rise in the target hair will be higher – simple because a higher fluence results in higher temperatures.

If the same fluence is delivered to the skin using a longer pulsewidth (say 40ms instead of 20ms), then the resulting temperature will be lower because more of the heat energy will have conducted away from the hair during the pulse!

This all makes it a bit tricky when comparing diode lasers of different powers. A 20J/cm2 fluence from a 1000W will generate a lower maximum temperature in the hair compared with a 20J/cm2 fluence from a 5000W laser – because the 5000W laser will deliver the energy much faster!

Repetition Rate

The repetition rate is the ‘number of shots per second’ that are fired to the skin. It is usually measure in Hertz (Hz).

In many diode lasers, increasing the repetition rate also alters the fluence/pulsewidths too – usually by lowering these values!!! So you might have to ‘compromise’ between repetition rate, fluence and pulsewidth to achieve a good result.

When applying the ‘stamping technique always use the 1Hz setting – that will allow you to fire a single shot with ease.

For the SHR, gliding, dynamic mode, use higher repetition rates – between 5 and 10 Hz.

Combination…

In reality, the three main settings – fluence, pulsewidth and repetition rate – are often “tied” in many diode laser systems. This means that when any one of these settings is changed, the one, or both, of the other settings also change.

So, if you want to increase your fluence, then the pulsewidth will also increase – to allow more energy to be output. But, the repetition rate might fall. This is simply because the laser has a fixed limit in terms of it maximum output – it just cannot go above a certain level.

Likewise, if you want to fire more shots per second at the skin (higher repetition rate) then, quite often, the fluence will drop!

This makes it a little difficult sometimes to carry out the treatment with the settings that you want to use.

Of course, a higher powered laser will give you more fluence…

Finally, sometimes the lasers do NOT output what they claim. This is not unusual for lasers and IPLs. Check with an engineer…

Summary

Diode lasers are not the same as the other hair removal lasers, or IPLs. They are a different technology with their own quirks. This can make things a little confusing when trying to get the best out of them. I am aware that some companies clearly don’t understand their own devices – I recently spoke to a diode user who had been ‘trained’, by her supplier, with some completely useless, and negative, information.

I am thinking about running some seminars on this topic – laser hair removal using diode lasers. If you are interested in joining, please let me know.

Hope this helps,

Mike.

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