Energy and power: The difference between a $5000 and a $80000 machine.

We will discuss the treatment parameters, energy and power, together since they are closely related. Keep in mind that this article is written from a perspective of lasers causing a photothermal reaction. With low-level lasers and LED devices the energy and power are greatly reduced and are less important parameters to achieve optimal results. For lasers creating a photoacoustic reaction, the energy and power is even more important than for a photothermal reaction. Let us start with defining what energy is and what power is.

Energy is the ability to create change. You can think of it as the fuel in a car. A full tank of petrol in my car gives me the ability to drive for eight hundred kilometres. It does not mean that I will drive for eight hundred kilometres. Nor that I will drive the eight hundred kilometres over eight hours or twelve hours. It only means that the ability is present to go from point A to point B which are eight hundred kilometres apart. Energy is measured in Joule.

Power is the rate at which the change or work takes place. The more power my car has, the less time I need to make my eight hundred kilometre journey. This is what determines whether my trip takes 8 hours or 12 hours. Power is measured in Watt.

The relationship between energy and power is:
Power (watt) = the change in energy (joule) divided by the change in time (second).
This relationship can thus also be transformed to:
Energy (joule) = power (watt) multiplied with time (second).

This is all great to know, but how does it apply to a hair removal treatment? To have an effective photothermal treatment (a treatment where you achieve a result by converting the light to heat and the heat causes the result; hair removal is one of the best known examples of this type of treatments) you need a certain quantity of energy to generate the heat and then you need to keep the heat on for a minimum period of time. This period of time is determined by the thermal relaxation time of your target.

The Thermal Relaxation Time (TRT), is the time needed for 50% of the heat energy to be conducted away from the target molecule. The TRT for the hair root is between 10 ms and 50 ms depending on various factors. For hair removal you will need a pulse length of more than 10 ms. Less than 10 ms and you will cause damage to the hair, but not the surrounding tissue and the dermal papilla causes a new hair to grow.

Because the TRT for each individual hair is not known and you want to have an effective treatment, you decide to work with a pulse length of 50 ms or more. Your success now depends on your capability to provide the hair root with a certain quantity of energy (light) for 50 ms or more.
To transfer 25 joules over 50 ms requires a power of: 25 J / 0.05 s = 500 W. (These amounts are only for this calculation example and do not reflect actual amounts needed during a treatment.)

As the hair become thinner and the melanin content of the thin hair is less than that of the original thick hair, you need more energy to create the same heat, thus the energy and power go up the TRT remains the same.

In your day to day live as a laser therapist, you will not really deal much with the concepts described in this article on their own. You will mainly deal with fluence, which is energy over a given area. We will discuss this concept in another article. The less expensive machines tend to show energy as an adjustable parameter instead of fluence. This creates the illusion that the machine has a higher capability than it really has. More about it in the article over fluence.

Energy and power is of utmost importance when it comes to choosing a machine to buy. Count yourself lucky if you manage to get a complete technical specification sheet out of the representative trying to sell you a machine. Usually they will either highlight energy or power. The less expensive the machine, the more the focus will be on power.

It is easy to state a high power output on paper. The manufacturer will use the shortest pulse length possible on the system and the maximum energy to calculate the power for marketing purpose. Thus, 10 joules divide by 0.005 s (5 ms) gives 2000 W. Therefore the machine has a lot of power, because it can convey 10 J of energy in 5 ms. The first impulse for most people will be that this is a strong machine when they only see the 2000 W.
But this is useless to you if you want to do hair removal, because you need a pulse length of more than 10 ms (in practice over 50 ms) and you need sufficient energy to create enough heat. When you then look at a machine that is suitable for hair removal and you take 30 J and divide it by 0.05 s (50 ms) you get 600 W.
This is the catch for many people. When you only look at the power of a machine, the better machine will seem on paper like the lesser of the two. You need to look at both parameters and more importantly the relation between the two in a specific machine.

The difference between a $5000 and a $80000 machine is the ability to generate enough energy for long enough to provide effective results even if you have a limited amount of the target molecule to work with. When you factor spot size (energy divided by area, fluence) into the equation, this becomes even more evident. Not only do you have to provide the correct quantity of energy over a minimum period of time, you also need to do it over a large enough surface to make a treatment practical.