# To measure the energy density of a (brief) laser pulse

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I have the problem of making "at home" or almost, a measure of a laser's pulse energy for unit area of the target: those kinds lasts for ~ tens of milliseconds, up to some hundreds of ms and I should be able to verify that this energy "density" doesn't go beyond 40 J/cm^2 for a single pulse (required by local laws concerning aesthetics lasers).

What could be a not very expensive way to do it?
Thanks

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sophiecentaur
Gold Member
On a practical level, if you can measure the DC Energy Input for the pulse, you could put an upper limit to the Laser Energy out. That should satisfy any regulations. For your personal information, you could read around to find the likely efficiency.
I guess it could be possible to do it photographically if you scanned the laser across a white screen (rotating mirror perhaps). Measuring the luminosity of a stripe (using RGB values on your camera image) and then calibrate against a known illuminant power on the screen.

tech99
anorlunda
Staff Emeritus
I think @sophiecentaur 's idea is most practical. Measure the DC current and DC voltage from the battery. If that is less than 40 mw (and it probably is), you need go no deeper.

tech99
Gold Member
I think @sophiecentaur 's idea is most practical. Measure the DC current and DC voltage from the battery. If that is less than 40 mw (and it probably is), you need go no deeper.
We need to find the area of the spot as we require J/cm^2.

anorlunda
Staff Emeritus
We need to find the area of the spot as we require J/cm^2.
If you shine the spot on a ruler scale and photograph it, you can measure spot size.

But J/cm^2 can't be the correct units. J/s*cm^2 which is watts/cm^2 makes more sense. Just measure the DC power, and the spot size.

sophiecentaur
Gold Member
We need to find the area of the spot as we require J/cm^2.
Photograph it through a dense neutral filter. You can graph the brightness profile (RGB again) and find the half power width
If you shine the spot on a ruler scale and photograph it, you can measure spot size.

But J/cm^2 can't be the correct units. J/s*cm^2 which is watts/cm^2 makes more sense. Just measure the DC power, and the spot size.
I agree that J/cm2 means nothing about how dangerous the pulsed beam is. But neither is W/cm2 either - on its own. If the 'Law' was written by anyone with sense, they will work to an exposure model of the retina (?) which includes total energy PLUS a time factor to account for the actual damage done. The OP needs to find out the specification for the legal requirement. A pulse, if short enough could be much higher power than a continuous beam so working to the continuous figure might give a very unimpressive installation. More info needed here, I think.

But J/cm^2 can't be the correct units. J/s*cm^2 which is watts/cm^2 makes more sense.
The first who understands me in months!
I keep making critics to my colleagues, but the law says exactly this.

However there is "some" reason: these are pulsed kind of lasers, specific for hairs removal, they shot several single pulses lasting a maximum of 300 ms every pulse.
For another kind of laser used in aesthetics (continuous beam) for other treatments, the law puts a constraint on the power per unit area, as you write.

Thanks to all for the answeres.

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Photograph it through a dense neutral filter. You can graph the brightness profile (RGB again) and find the half power width
Good idea, but do these kinds of filters actually exist? Are they really neutral up to what we need? (I ask because don't know).
I agree that J/cm2 means nothing about how dangerous the pulsed beam is. But neither is W/cm2 either - on its own. If the 'Law' was written by anyone with sense, they will work to an exposure model of the retina (?)
... yes and it also relates to possible skin/tissutal damages.
which includes total energy PLUS a time factor to account for the actual damage done. The OP needs to find out the specification for the legal requirement. A pulse, if short enough could be much higher power than a continuous beam so working to the continuous figure might give a very unimpressive installation. More info needed here, I think.
Max 40 J/cm^2. Max pulse duration 300 ms. Defocalized. Spot area greater than 10 mm. Wavelenght in the range 800 - 1200 nm.

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sophiecentaur
Gold Member
I keep making critics to my colleagues, but the law says exactly this.
If it is how you say then the law is bad and you should ask 'higher up' for clarification / advice. You need to cover your tail to avoiding becoming a test case in court and also just to avoid hurting the subject of the treatment, even if it turns out to be 'legal'.
However there is "some" reason: these are pulsed kind of lasers,
I suggest that the "reason" is that the total energy delivery needs to be spread over a time so that the dissipation rate of the heat prevents excessive local temperatures.

anorlunda
Staff Emeritus
OK, now it is clearer what you're doing. So does the law apply to the energy of a single pulse? That's more difficult to measure.

@sophiecentaur 's photographic approach may be the best for a single pulse.

Your restriction of "at home" suggests that you have no tools more sophisticated than an analog multimeter and a cell phone.

I can also suggest contacting manufacturers of these laser devices and ask them how they calibrate the energy density of their products. They may be able to make a specific suggestion for you. Perhaps they could send you a series of cell phone camera images of a single pulse, each for a specific energy level. That would make it easy. Capture a cell phone picture of a single pulse, and compare the picture with a series of calibrated photos.

If it is how you say then the law is bad and you should ask 'higher up' for clarification / advice. You need to cover your tail to avoiding becoming a test case in court and also just to avoid hurting the subject of the treatment, even if it turns out to be 'legal'.
Thanks.
I suggest that the "reason" is that the total energy delivery needs to be spread over a time so that the dissipation rate of the heat prevents excessive local temperatures.
Good intuition.

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sophiecentaur
Gold Member
Good idea, but do these kinds of filters actually exist? Are they really neutral up to what we need?
Regular (high power?) laser safety goggles would surely be adequate for protection of the optics and the transmission loss could be calibrated at the laser wavelength.
It's an interesting measurement project.

anorlunda
Staff Emeritus
I just though of another approach. Can you require that the laser devices be certified by the manufacturer to not exceed the legal energy density? Then the law can be rewritten to require a certified device, rather than asking someone local to measure it.

sophiecentaur
sophiecentaur
Gold Member
I just though of another approach. Can you require that the laser devices be certified by the manufacturer to not exceed the legal energy density? Then the law can be rewritten to require a certified device, rather than asking someone local to measure it.
That sounds like an ideal solution. Lasers are still scary enough for you to bring in health and safety and demand an answer.
I was thinking of other ways of attenuating a laser beam so that it could be measured with a camera. I bought a small hand held laser recently (a bit more powerful than a classroom pointer) and it came with a set of holographic filters to fit on the front and produce pretty patterns. One of them splits the beam up into several hundred individual spots. Just one spot would be easy to deal with.

I just though of another approach. Can you require that the laser devices be certified by the manufacturer to not exceed the legal energy density? Then the law can be rewritten to require a certified device, rather than asking someone local to measure it.
We are not bound to make a measurement, but the problem is we have to prevent people from using those devices when the certification doesn't relate to that value or when there isn't a certification at all. And even when they certifies it respects all values, we cannot always trust them (already happened!)

Consider that they want, because aesthetics shops ask them, to sell powerful lasers, which are effective in removing hairs quickly and they sometimes try to intentionally elude local laws! Furthermore, often these firms have old surgical lasers they don't want to throw away and which they simply disempower for aesthetics purpose. These for hair removal are always "class 4" lasers!
Sometimes it's... a jungle.

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anorlunda
Staff Emeritus
Sometimes it's... a jungle.
That reminds me of Mr. Monk

I sympathize. Doesn't the local hospital have the same problem with their devices? My guess is that you trust the hospital but not the street vendors.

In #10, I suggested contacting the manufacturer of such devices for help. Did you try that?

You could also contact a larger richer city and see what they do. If they have fancy instruments to measure energy density, they probably don't use them every day and might be willing to let you borrow it.

One more source to try is NIST https://www.nist.gov/ or the state equivalent of NIST if you have one.

I'm sure there are simple ways, like exposure on film, or discoloring a piece of paper, but they are useless unless they are calibrated. Such calibrations are what a state laboratory or NIST could provide.

How much research did you do before posting? I found:
http://www.ophiropt.com/user_files/laser/beam_profilers/tutorial_from_sensor_to_display.pdf
http://www.laserfocusworld.com/arti...meters/a-heated-topic-laser-calorimeters.html
https://www.researchgate.net/post/How_can_I_calculate_the_intensity_power_density_for_pulse_laser

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sophiecentaur
Gold Member
Furthermore, often these firms have old surgical lasers
That sounds dodgy. Whoever is 'responsible' for supervising this sort of activity would certainly be liable to prosecution by the Health and Safety Executive in UK. Not only that but any insurance could be void, which also could be a headache.

OK, now it is clearer what you're doing. So does the law apply to the energy of a single pulse? That's more difficult to measure.
Good question! The law doesn't specify it. However the law refers to
technical standards: CEI-EN-60825 and CEI-EN-60825-1 which are not very easy to interpret, in some part they talk of "energy (per unit area) per single pulse" and somwhere else of "energy (per unit area) for a single pulse or train of pulses".
Your restriction of "at home" suggests that you have no tools more sophisticated than an analog multimeter and a cell phone.
Yes.

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lightarrow

That reminds me of Mr. Monk
I sympathize. Doesn't the local hospital have the same problem with their devices?
My guess is that you trust the hospital but not the street vendors.
No, the problem is that surgical lasers are obviously more powerful than those for aesthetics shops, so they don't always worry about modifing them in order to put a constraint on the output energy/power, etc, in a correct way...
In #10, I suggested contacting the manufacturer of such devices for help. Did you try that?
My duty is to prevent aesthetics shops from using *at the moment* lasers (or other devices) which could cause a damage in clients. It's /their duty/ to contact vendors, *after* I have prevented them from using the device. And, I repeat, in a case we found certifications that the laser cannot go beyond 40 J/cm^2, but making the device work, we could arrive, in the display, up to > 100 J/cm^2!

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That sounds dodgy. Whoever is 'responsible' for supervising this sort of activity would certainly be liable to prosecution by the Health and Safety Executive in UK. Not only that but any insurance could be void, which also could be a headache.
Whish we had the same laws in Italy...

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anorlunda
Staff Emeritus
My duty is to prevent aesthetics shops from using *at the moment* lasers (or other devices) which could cause a damage in clients. It's /their duty/ to contact vendors, *after* I have prevented them from using the device.
No, you misunderstand. My suggestion was that you contact manufacturers asking for help on how to do your measurements.

Did you check out those sources I provided in #16?

No, you misunderstand. My suggestion was that you contact manufacturers asking for help on how to do your measurements.
Ah, ok, sorry.
No, they don't tell us these sort of things (especially could be used against them
).
Did you check out those sources I provided in #16?
Not yet, but I'll do it as soon as I'll have some more minuts.

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sophiecentaur
Gold Member
Wiki has this information which discusses both energy, power and wavelength. The two coloured graphs show that all three are factors.
Whish we had the same laws in Italy...
lightarrow
I thought that the regs were EU based.
There are many different classes of laser but I have only found broad guidelines about conditions where the use of each class is acceptable. They seem to be more prepared to commit for CW operation - not surprisingly.
Frankly, the problem of measurement is totally trivial compared with the problem of specifying what would be acceptable and safe. Something that is on your side would be the fact that you are coming with the question after a long history of laser use for all sorts of applications so there is bound to be something written, somewhere, which can tell you what you really need and what spec you should be using. The naked numbers you have been supplied with are probably not enough for you to feel safe and justified.
You are dealing with lasers that are powerful enough to damage tissue (it's their purpose here) so, in addition to the laser spec, the conditions for use are very important (but that is only stating what you already know ).

I thought that the regs were EU based.
Yes but vendors are allowed to sell them; it's not allowed to use them in improper way. For example you can use a class 4 laser (with specific characteristics, with specific workplace, training, etc.) for hair removal but not for other kinds of aesthetics treatments. Another problem is that beauticians are not properly formed, they know very little about these technical things and they trust what vendors make them to believe...
There are many different classes of laser but I have only found broad guidelines about conditions where the use of each class is acceptable. They seem to be more prepared to commit for CW operation - not surprisingly.
Frankly, the problem of measurement is totally trivial compared with the problem of specifying what would be acceptable and safe. Something that is on your side would be the fact that you are coming with the question after a long history of laser use for all sorts of applications so there is bound to be something written, somewhere, which can tell you what you really need and what spec you should be using. The naked numbers you have been supplied with are probably not enough for you to feel safe and justified.
You are dealing with lasers that are powerful enough to damage tissue (it's their purpose here) so, in addition to the laser spec, the conditions for use are very important (but that is only stating what you already know ).
Yes, conditions of use, workplace requirements are all another story, but in my work it's immediately clear if these are correct or not, according to the law. The real problem is to find a cheap way to make measurement's, even if we are not forced to do them (what is written in a certification makes the vendor responsible for having written it, but we cannot directly do anything against them, only against the beautician; then she can charge the vendor and take him in a court... ).

We have the possibility to send those devices to specific laboratories, but in that case we should justify the beautician's lack of profit for all the time she can't use it, if the results are negative (= it's compliant to the laws).
So, if possible, I would prefer to have a simple way to decide if the device is certainly out of specifications.

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sophiecentaur