How do you calibrate your lasers so that the numbers on your laser screen actually mean something useful?
Well, there is a process which you need to follow. The video shows this process in detail.
So, you need to measure the laser energy for each of your ‘input’ settings using an energy meter designed for Q-switched pulses. I usually record three measurements for each input setting, and then average them.
Then you fire the laser at some burn paper (or a dark photo in a magazine will do). You then measure the diameter of these spots. I generally fire three spots at two or three different energies, just to see if there is much variation.
You tend to find that the higher energies will deliver slightly larger spots, just because the laser-generation process is more efficient inside the crystal.
Knowing the energies and spot diameters for each input setting, you can then calculate the fluence (energy density = energy / spot size area) for each setting.
Then you plot the calculated fluences (energy densities) against the input setting, from your laser (as in the above graph).
This is then your calibration chart!
To find any output fluence you simply look along the x-axis – which corresponds to the ‘energy’ setting on your laser screen – and look up till you meet a curve – either 532nm or 1064 nm (for an Nd:YAG laser) – and then read off the corresponding fluence on the y-axis.
So now the numbers on your laser screen make sense – real fluences!
I tend to stay below 6 J/cm^2 for tattoo treatments (until you are tackling the real deep pigment when you will need higher fluences). Hence the green dotted line on the above chart – everything below that line is in the ‘Safe Zone’. In other words, in this example, the laser can be set up to around 1300 mJ, and you know you are operating safely.
Note that this chart was constructed on the 1st July 2020 – calibration charts are time-dependent! Since the flashlamp will degrade with time, you really should get your system re-calibrated after around 150,000 shot (or so). These curves will ‘drop’ with time.
You should also note that the 532nm output is really a mixture of 532nm and 1064nm since frequency-doubling crystals are not 100% efficient. Typically, this can be in the ratio of 70% 532nm to 30% 1064nm.
Calibration charts are important since they tell you what your real laser outputs are. The ‘mJ’ numbers on your screen are quite meaningless! Unless you measure the actual output and its corresponding spot diameter, you cannot know the real numbers.
Mike & PA.