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FrozenGate by Avery

Very simple DIY laser power meter with thermistor

Mikos

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Jul 28, 2008
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Hello,

I will need to measure real output power of fake 200 mW green laser pointer I have ordered on eBay, so I have been searching for some simple and cheap approximate laser power meter.

My first idea was simple photodiode + ammeter. I am sure it would work (and it could be even pretty accurate and have fast responses), but the problem is that I can't calibrate it (I don't have any other laser sources with precisely known output power). So I have searched the net and found Laser Power Meters chapter in Sam's Laser FAQ. And there is one brilliant idea:

Diode detectors are a pain to calibrate unless you have a light source of known energy at the same wavelength you're trying to measure. A method which resolves (mostly) the calibration problem is to use a small thermistor. Epoxy a 1/4 watt resistor to one side and coat the other surface with lamp black. Put thermal insulation around all of it except the smoked side. Apply about 1/4 watt of power to the resistor and let it come to equilibrium and measure the resistance of the thermistor. Then focus the beam of the laser on the smoked thermistor and reduce the power to the resistor to keep the thermistor resistance at the same value. The laser power should be equal to how much the resistor power was reduced. It's very cheap, fairly accurate, uses your DMM for the readings, and will measure CW or average power of small pulsed lasers.

What do you think about it? It can be made really easily and it would be extremely cheap. It would have slow responses (so you can measure only average power, not peak power), but it can be relatively accurate (at least for my needs, I assume error about +- 5 mW). Do you have any experiences with this design?
 





Re: Very simple DIY laser power meter with thermis

This is project many many have tried. As far as the photodiode goes, my favorite quote on the topic so far is: "Some people use this to make a crude laser power meter out of $8 worth of parts - note that this is about the worst calibrated and least linear light meter it is possible to make without some arrangement involving measuring the growth of a vegetable"

You have to have a known source to calibrate against and then perhaps you can make a laser power magnitude estimator with a +/-1,000% margin of error. Sorry, but if it were quite that easy, we would all have them already. Your best bet would probably be to find a local university, or contact an LPF member with a power meter to test it for you.

The problem with the thermistor is that you're assuming you have a 100% perfectly absorptive coating, and all the heat is transferred efficiently and effectively. Such is not the case, and while its a pretty interesting idea, it'll still give you an insane margin of error. Perhaps you can make a simple laser power 'estimator' that way, but don't count on a meter.
 
Re: Very simple DIY laser power meter with thermis

Yes, I know that I won't have perfectly absorptive coating, but it should be good enough for my needs. Even if the error will be +- 10 mW (I really doubt that it can give worse error than that in 30 - 200 mW range), it is still acceptable for my purposes. And also one can assume that real output power would be greater than reading from this laser "estimator" (because imperfect absorption you have mentioned).
 
Re: Very simple DIY laser power meter with thermis

I could measure it for you if you want.. it's about $2 to ship back to you so that'd be all you'd need to reimburse for.
 
Re: Very simple DIY laser power meter with thermis

sk8er4514 said:
I could measure it for you if you want.. it's about $2 to ship back to you so that'd be all you'd need to reimburse for.

Thank you for your offer. But are you sure that shipping costs are only about $2? I am from Czech Republic and I am afraid that shipping costs to/from USA are much greater (about $15 for smaller packages).

I will try this DIY laser power "estimator" and if it wouldn't work, than I will see what to do next...

But thanks you anyway.
 
Re: Very simple DIY laser power meter with thermis

Btw. is there anybody who tried this DIY thermistor-based laser power meter (or "estimator" if you want)?
 
Re: Very simple DIY laser power meter with thermis

It's worth a try; however, for any serious designs you'll want a real meter to do comparisons with since any DIY meter will need to be calibrated. From the thermistors I used way back in my labs, they have a logarithmic response, though there might be linear ones as well. Good luck!
 
Re: Very simple DIY laser power meter with thermis

Bionic-Badger said:
It's worth a try; however, for any serious designs you'll want a real meter to do comparisons with since any DIY meter will need to be calibrated.  From the thermistors I used way back in my labs, they have a logarithmic response, though there might be linear ones as well.  Good luck!

I will try it and see if it works. Btw. this power meter should't need calibration (at least in theory) - it is "calibrated" by heat generated with that power resistor (glued to thermistor). But of course there are other factors like unperfect absorption, heat dissipation, etc. so for accurate measurements calibration would be still needed (but for approximate measurements it should be IMHO ok).
 
Re: Very simple DIY laser power meter with thermis

I worked on something similar for a pulsed yag laser. I used a carbon disk from a D size carbon-zinc battery. I pulled out the anode and cut a thin disk from the carbon rod. I glued a thermocouple to the back and hit the front of the target with a known energy source to calibrate it over 0.1 J to 5 J. For calibration I basically measured temperature rise above ambient versus incident energy. Then I hit the target with the pulsed yag and measured the temperature rise and back calculated the joules per pulse. With the higher pulse energies I would vaporize some of the surface so I knew my measurement was underpredicting the energy.

I your case you may be able to glue a 1/4 watt metal film resistor on the back of the carbon disk along with a thermocouple or thermister. Insulate the back of the assembly with some styrofoam. Apply rated power to the resistor for different times to calculate joules (power x time) for different applied energies and measure the temperature rise. You now have energy versus temperature rise. Remember to let the temperature reach steady state. Then you can aim your laser at the target for exactly 10 seconds, measure the total energy, then divide by 10 to get the average power. Or maybe this is no better than the concept you proposed.
 
Re: Very simple DIY laser power meter with thermis

Mikos said:
Thank you for your offer. But are you sure that shipping costs are only about $2? I am from Czech Republic and I am afraid that shipping costs to/from USA are much greater (about $15 for smaller packages).

I will try this DIY laser power "estimator" and if it wouldn't work, than I will see what to do next...

But thanks you anyway.

The laser power estimator will definitelly give you a reading. But that reading will be useless for you, as you won't have anything to compare it to.

The only thing you can do with it is measure how much the power is changing, in percentage..


BTW, i'm from EU (Slovenia), and have a meter. If you want it measured, i would gladly do it for you, just to see it, cos i know which laser you are talking about.... :)
 
Re: Very simple DIY laser power meter with thermis

IgorT said:
The laser power estimator will definitelly give you a reading. But that reading will be useless for you, as you won't have anything to compare it to.

The only thing you can do with it is measure how much the power is changing, in percentage..

I don't think it will be useless. And it will definitely give me absolute readings, not percentage.

First I measure power dissipated by resistor. It is U^2 / R (and R is constant, in my case 300 Ohm). The only thing I need is voltmeter. I am using 9 V power supply, so the power dissipated by resistor is 270 mW (let's call it P1).

I let it come to equilibrium and measure resistance of thermistor (let's call it R1).

Then I fire laser on the thermistor. Electrical resistance of the thermistor now goes down as the power of the laser is added to the thermodynamic system (resistor + thermistor) and temperature goes up. I let it come to equilibrium again.

Then I slowly lower voltage on the resistor (with potentiometer) until resistance of the thermistor isn't same as R1.

Now I measure voltage on the resistor. Let's say it is 7 V, so the power dissipated by resistor is 163.3 mW (let's call it P2).

The power of the laser must be P1 - P2 = 106.7 mW. Real power of the laser will be greater than that because absorption of laser light is imperfect (there will be some losses, but nothing big, lamp black is pretty good at absorption of light in visible spectrum). I think that error in 10 - 250 mW range would be lower than 5 mW.

I didn't try it yet, but I have everything I need and I will try it as soon as I will have some free time.
 
Re: Very simple DIY laser power meter with thermis

Mikos said:
I don't think it will be useless. And it will definitely give me absolute readings, not percentage.

I think you misunderstood me.. I did not say it will give you a percentual reading. Of course it will give you an absolute reading. I just said that that this reading will be completelly useless for anything but relative comparisons. Like for example, "laser A is doing 20% of laser B's power". Or for example "laser A drops to 32% power after 15 seconds"...

That's what i meant with percentage.


Mikos said:
Real power of the laser will be greater than that because absorption of laser light is imperfect (there will be some losses, but nothing big, lamp black is pretty good at absorption of light in visible spectrum). I think that error in 10 - 250 mW range would be lower than 5 mW.

Umm.. In the 250mW range, 5mW would mean 2.5% error. People here use thermopile meters, and the readings vary by much more than that from one to another.

Even expensive professional LPMs have an uncertainty factor of 3%. If you could get 2.5% accuracy with a thermistor without any calibration, everyone here would be doing it, and meters wouldn't cost a fortune..

There are ways of making a pretty accurate meter without a thermopile. But they still require calibration with a real meter.


It might work in theory, but there are too many assumptions involved. A thermistor is not a theoretical black body, no matter how you paint it.

Let us know what you get, then i can still measure it for you, and you can compare.
 
Re: Very simple DIY laser power meter with thermis

I thought that proffesional thermopile laser power meters have much better accuracy in this range. Ok, you are right, then I assume I will have greater error. But still I think that it would be enough for approximate estimation. I will try it and let you know.
 
hi
I was told to try using an ammeter and a green LED (not clear lens) and firing the laser at it and taking a reading in microamps and divide that by 2.8 to get power in milliwatts so
I have a true 100mW green laser from DX,it's exelent,very bright well focused and less than £20 ...so I tried this and the result was 330uA/2.8 = 117mW ! is this a flook or does this work as I have no standard as a guide?
 
hi
You just dug up a 2 year old thread in your first post.

I doubt your technique will give a very accurate reading. The only way to get a real reading near to the "true" value for a laser's power is using a laser power meter or similar, which uses either a thermopile, TEC or calibrated sensor.
 
Hey CB.... at least he's doing research and reading...;)
A refreshing change from the noobs with their hands out
for info...

@ lasermouse...

welcome.gif
to the forum... I see you found the [Search] button...:gj:
Enjoy your stay...:cool:

Jerry
 





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