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

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Andy Resnick
Science Advisor
Education Advisor
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).
Is this an ANSI Z-136 type exposure limit?

If this is a regulatory requirement, you must measure the pulse energy with "approved" procedures and equipment. Because you don't appear to require detailed information about the pulse shape or duration (but you must check this!), a simple calorimeter measurement is likely sufficient. There are many available devices on the market, but you may have to ensure it is calibrated, etc.

anorlunda
Staff Emeritus
I think it could be done very simply. Use the thermal paper of the kind used in cash register receipt printers. A single pulse should make a dark spot on the paper proportional to the energy. What you need is a laminated pocket card showing calibrated spots created by lasers at different energies. You could simply compare the spot in the shop with those on the card. The calibration card could be created by a national standards board such as nist.gov That is why I suggested that you ask those sources; perhaps something similar has already been done.

I even found a laser calorimeter on amazon for $130. I don't think you're going to get very far on your own with a$10 multimeter.

sophiecentaur
Science Advisor
Gold Member
I even found a laser calorimeter on amazon for $130. @lightarrow would have to buy one to find out if it actually works but it could be a way forward. I don't like the thermal paper idea because it would not be likely to have a linear response and how could one calibrate it? perhaps something similar has already been done. I'm sure it will have been done. It's just a matter of locating the info. Perhaps there are conventions of cosmetic operatives and beauticians where documents are presented. I started searching on Google but there was a vast amount of stuff with no solid information in it. Not too surprising. Is this an ANSI Z-136 type exposure limit? For what I can understand with a quick look, yes, that is the American equivalent of the European technical norm IEC 60825, concerning laser safety procedures/equipments and laser devices specifications. These technical norms are always for a fee (excepting old versions). If this is a regulatory requirement, you must measure the pulse energy with "approved" procedures and equipment. Certainly. This is done in the labs I was saying, obviously I can't contest the improper working of the device only on the basis of rough evaluations, but this evaluation could be useful for us to decide to send it to specialized labs or not. Because you don't appear to require detailed information about the pulse shape or duration (but you must check this!), a simple calorimeter measurement is likely sufficient. There are many available devices on the market, but you may have to ensure it is calibrated, etc. I saw the documents Anorlunda linked, very interesting. Just a question: I have to assume that when they say, for example, something like 500 mJ/cm^2 at 10 microseconds pulse (I don't remember exactly now, it's just an example) as peak energy density measurable, it means that if the pulse lasts 10 ms, the peak energy density which can be measured by the instrument it's 1000 times, that is 500 J/cm^2? Thanks. -- lightarrow Last edited: I think it could be done very simply. Use the thermal paper of the kind used in cash register receipt printers. A single pulse should make a dark spot on the paper proportional to the energy. What you need is a laminated pocket card showing calibrated spots created by lasers at different energies. You could simply compare the spot in the shop with those on the card. The calibration card could be created by a national standards board such as nist.gov That is why I suggested that you ask those sources; perhaps something similar has already been done. Aha! Good idea. I'll see if it works (in addition, it could depend considerably on the laser wavelenght and on the reflectance of that specific piece of paper). I even found a laser calorimeter on amazon for$130. I don't think you're going to get very far on your own with a $10 multimeter. 130$ can be acceptable...
Thanks

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lightarrow