A ‘Brief Guide’ to some of the more important terms used in medical/aesthetic laser/IPL treatments (updated 21st Jan 2021)

Version 1.3 – updated 21st Jan 2021

There are many terms and expressions used in modern laser/IPL applications. However, some of them are not so well explained to operators. So we’ve put together a list of the more commonly used expressions with a brief explanation.

We describe these in more details in our upcoming book, which should be out soon…

Get a pdf version of this here.


Wavelength is essentially ‘COLOUR’!

The image shows the ‘visible spectrum’ – the colours humans can see.

But there is a whole range of invisible wavelengths which we can’t see including X-rays, ultraviolet and infrared. The wavelength of a beam of light is the distance between two peaks (or two troughs) of the wave (see below).

Most lasers generate just a single wavelength…

…whereas all IPLs emit a wide range of wavelengths, which are subsequently filtered!

All light is ‘electromagnetic’ meaning it is formed of an electric wave and a magnetic wave.


‘Energy’ is the stuff that makes things ‘work’ or ‘change’!!It comes in many forms – light, heat, electrical, nuclear, solar, sound, vibrations and various others…

With lasers and IPLs we use the energy ‘contained’ in their beams to heat up the targets (usually!!)

The energy is used to change something from one form to another – i.e. break down tattoo ink or coagulate blood vessels.

It is the energy which does ‘the work’!! We measure energy in Joules.

1 Joule is not much energy at all. In fact, it takes more than 82,000 Joules to heat up a cup of coffee or tea. We typically only use a couple of Joules when treating skin conditions (or less)!

Spot Size

The ‘spot size’ of a laser is usually the diameter of the laser spot on the skin’s surface. We typically measure this in millimetres or centimetres. Many lasers generate (near) circular spots shapes while most IPLs and most diode lasers generate rectangular or square spot shapes, respectively.

The spot size is critical in many laser-skin treatments because it has a direct effect on the energy density (fluence) and the depth of penetration (see below).

The spot size also has an important effect on penetration depth as can be seen in the above image. This is particularly important when treating deep targets.

You should always use as large a spot diameter as possible when treating deeper targets.

Fluence (Energy Density)

Fluence appears to be a poorly understood concept among many laser/IPL users. We’re not sure why because it really is quite simple.

Fluence is how ‘concentrated’ the light energy is in a spot. So, if we take a certain amount of light energy and put it all into a 10 mm diameter spot, then that energy is ‘spread out’ in that while spot area (see image).

Now, if we take the exact same energy but squeeze it in to a 1 mm spot diameter, then we can see that it is much more concentrated than in the 10 mm spot.

It is quite clear that the smaller spot will be more ‘forceful’ than the larger spot, because all that energy is concentrated into it. This is precisely what happens in the skin when laser or IPL energy is fired into it. If it is in a larger spot, then the effect is more ‘gentle’ than if it was in a smaller spot diameter.

Fluence is, essentially, the concentration of energy!

The ‘proper’ term for this expression is ‘radiant exposure’, and is measured in Joules/cm2.


When you fire a pulsed laser, a ‘pulse’ of energy will emerge. The duration of this pulse in time is known as the pulsewidth (it can also be called the pulse length or the pulse duration – all the same!!) It is, essentially, how long the pulse of energy is ‘ON’ for!

The pulsewidth determines how quickly, or slowly, the energy is absorbed in the target. If the pulsewidth is very short, then the energy can be absorbed very quickly, leading to a high temperature (this occurs since there is little time for the heat to ‘escape’ during the pulsewidth).

However, if a long pulsewidth is used, then the absorbing object will heat up as it absorbs the energy, but it will lose heat into its surroundings and so will not achieve a high temperature.

The pulsewidth is critical in many laser/IPL skin interactions since it determines the final, maximum temperatures.

Pulsewidth is a measure of the time duration of a pulse of energy, so we measure it in units of time – seconds, milliseconds, nanoseconds etc.


Power is defined as how quickly, or slowly, energy is ‘delivered’. It is simply the rate of delivery of energy and is measured in Watts.

So, if we fire a laser beam into the skin with 1 Joule of energy in a 10 millisecond pulsewidth (which is 0.01 seconds)), then the power of this beam is just 1 Joule / 10 millisecond which is 100 Watts.

If, however, we fire that same beam of 1 Joule in a 1 nanosecond (which is 0.000000001 seconds) pulsewidth (as with a Q-switched laser) then the power of that beam is 1 Joule / 1 nanosecond which is 1 billion Watts!!

Hence, the power is greatly dependent on both the energy and how quickly it is delivered.

Sometimes, you might hear the terms ‘peak power’ and ‘average power’. Here’s what they mean:

Peak Power’ – this is the maximum possible power a system can supply, but usually only for a short time. Generally, devices are not designed to output their peak power for very long – otherwise they would ‘burn out’.

Average Power’ –  this is the power a device will output over long periods of time. It is always less than the peak power. Most systems’ powers are quoted using this term. So, a 30 Watt CO2 laser will generate an average of 30 Watts over minutes or hours. But, if it has a ‘pulsed mode, it will be able to fire much higher-powered individual pulses – but these might be only for up to 100 milliseconds!

Power Density

Earlier we looked at ‘fluence’ or ‘energy density’. This was described as the ‘concentration’ of energy in a spot size.

Well, power density is the same idea, but replacing power for energy.

So, if we know the output power of a laser/IPL (in Watts), then, the concentration of that power in any spot size is called the ‘power density’, and is measured in Watts/cm2.

As with fluence, power density is very important in determining the outcome of laser/IPL treatments:-

  Energy –Determines the total temperature rise 
Power Density is:Pulse duration –Determines the peak temperatures
  Spot size – Determines the volume of tissue affected

The ‘proper’ term for this expression is ‘irradiance’, or ‘intensity’, and is measured in Watts/cm2.

Power vs Energy

This seems a bit confusing! Why do we have ‘power’ and ‘energy’, and their ‘densities’, when they seem to be roughly the same thing???

Well, it’s to do with the way your laser works. Basically, if your laser is generating ‘pulses’, then we generally talk about ‘energy’, because we know how long the pulses are (or, we should!!).

If the laser is generating a continuous beam (so it stays ON for as long as you press the button) then we talk about ‘power’ because the duration of the beam is determined entirely by the user.

Pulsed beams Continuous beams
Energy (densities)Power (densities)

The ‘densities’ merely follow suit…

Penetration Depth

Penetration depth is a very important issue in laser/IPL treatments. In its simplest terms, this is how deep the light can penetrate into the skin. But, this doesn’t really tell us anything useful, when thinking about treatments.

The penetration depth is determined by various factors including the wavelength, the spot size, the pulsewidth and scattering (see below).

Obviously, it is important to choose all of the above carefully when considering any treatment – there is no point using blue light (which doesn’t penetrate far) if trying to treat deep targets.

In fact, many treatments effectively ‘stop’ when the light can no longer reach the depths where the final targets are. The video above shows to get around this problem.

How far does light penetrate into the skin….?


When light falls onto any object, some of it is reflected (which is why we can see it!!). However, some of the light energy will ‘enter’ the object and start a merry dance with the atoms and molecules in there. Some of the light will be absorbed – in physics, this means that the electrons in those atoms will become ‘excited’.

This means that the electron effectively ‘takes’ the photon’s energy and jumps into a higher orbit (from the ‘ground state’ to the ‘excited state’) around the nucleus. This is the ‘absorption’ part – the energy of the photon is transferred into the atom, via the electron – see image below.

The electron will sit in the higher orbit for a very short time, but at some point, the electron will drop back into its original orbit. When it does this, the energy which the electron took from the photon of light will be released – in some case it will cause a vibration which we feel as heat, in other cases it will generate a new photon – this is a ‘scattered’ photon, which is discussed in the next section.

We wrote an imaginary ‘ride on a photon’ to see what happens to it here.


Scattering is a very important optical event which occurs in the skin. As soon as light enters the skin it is very likely to be scattered, many times. From the above image, you can see that scattering is actually a photon o flight emerging from an atom after an electron drops back into its ‘natural’ orbit.

The real-world effect of scattering is that the spot diameter of the original laser/IPL beam expands as the depth increases. This means that the fluence also drops with depth! 

So, at some depth the effect you are trying to induce in your targets will not happen, simply because the fluence falls below the required threshold, due to scattering. 


Anisotropy is an important optical issue in the skin. This describes just how much a beam of light will ‘spread out’ due to scattering. So, blue light has a ‘wide’ anisotropy angle meaning that it spreads out very widely, which prevents it from penetrating far into the skin. Red light, however, has a ‘forward scattering anisotropy’ which means that more of its energy can penetrate into the deeper dermis compared with blue (or yellow or green) light.

The amount of anisotropy a light beam will experience depends on its wavelength. Clinically, this is important because it tells us that blue should never be used to treat deep targets! It also helps to explain why green and yellow light are also limited in terms of depth.

As can be seen above, blue light scatters widely, while red light scatters in a ‘forward’ direction.

Thermal Relaxation Time

The thermal relaxation time of any object is how quickly, or slowly, it cools. Technically, it is defined as the time taken from the maximum temperature to fall to 50% of its maximum value.

The TRT is a cornerstone of the theory of Selective Photothermolysis (SP). It ‘defined’ the pulsewidths used in many modern lasers for skin conditions.

The idea is that we want to heat up the desired targets, without heating the surroundings too much – thereby preventing adjacent damage.So, when the theory of SP was created, the originators chose their pulsewidth of their laser according to the TRT of the blood vessels they were trying to shrink.

A close up of a map

Description automatically generated

This image is taken from our paper on this subject. Click it to read our paper.

However, while this was a good idea, it was flawed. While they were considering minimising thermal damage to the surrounding tissues, they didn’t properly consider the actual damage needed to ensure that the blood vessels were sufficiently damaged!

This limited the damage to those blood vessels and, consequently, many did not respond properly. Then along came IPLs with their much longer pulsewidths. In many cases, the IPLs could destroy unwanted blood vessels that the earlier lasers could not!

This was simply due to the longer pulses that IPLs could generate, which are needed to effectively ‘cook’ the larger targets (vessel, hair etc).

So, while the idea behind TRTs is fine, we must always consider how much energy and time are needed to ensure the ultimate destruction of the target we are trying to remove.

After all, you don’t bake a cake based on its ‘cooling’ time!!

Photo by Elli on Pexels.com

Repetition Rate

The number of shots fired per second is known as the ‘repetition rate’ It is usually measure in Hertz (Hz) and is just the ‘speed’ you choose to fire the shots at.

It doesn’t make a lot of difference to the clinical outcome, in most cases, as long as the laser/IPL spot is constantly being moved across the skin surface. However, if you keep the laser/IPL tip at the same spot, then a high repetition rate will device a lot more energy to that area than a slower rep rate, which may lead to tissue damage!

So, setting your device to 10 Hz means it will fire 10 pulses of energy every second (which is quite fast!!).

Fluence versus Depth

The fluence (and intensity) from a laser or IPL device drops as it penetrates into the skin. In fact, its value drops quite rapidly, meaning that very little energy reaches the deepest parts of the dermis.

You can see from the image below that most of the energy is absorbed in the top layer – in fact, around 63% of the incident energy is absorbed in the topmost layer, leaving only around 37% left for the deeper parts.

This explains why it is always more difficult to target deep blood vessel, hair follicles or tattoo ink, with sufficient energy to do the job required!

Hopefully this will clear up some doubts about these expressions We will discuss these in more depth in our upcoming book, which will be released soon…

But, if you have any queries about the above, please let us know by leaving a comment here.

You can obtain a pdf version of this here.

Mike & PA.

Published by mikejmurphy

I started in medical lasers in 1986! I have been involved in clinical research, medical laser and IPL development, sales and marketing across Europe and South East Asia, business start-ups, theoretical research into laser-tissue interactions, training course development, writing articles for peer-reviewed journals and industry magazines plus I play bass guitar in a rock 'n' roll band too!!

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