# Measuring laser output power



## Arilou (Oct 13, 2013)

I recently bought some cheap lasers and wanted to measure the power output. I don't know why all the laser power meters that I saw for sale were so expensive, because this really isn't that complicated.

I hooked up a peltier module to a voltmeter and shined various lasers at it. The voltage gives a relative measurement of the power of the laser.

http://imgur.com/Xe1RrMz

That is a thermoelectric module on an old CPU heatsink. It would probably work better if I painted it black, but this setup is rather sensitive as it is. It can detect the heat from my hand from about 10 cm away.

The only problem is I'm not sure how to calibrate this to calculate the actual power of the laser. I need something to use as a reference. Is there a laser that I can buy that would, for example, have exactly 5 mW output?


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## Eidetic (Oct 14, 2013)

Problem with that approach is that the detector will not be linear. You might be close for powers around that of the calibration laser, but there won't be much accuracy. Holographers do what you propose all the time. The reason laser power meters are "expensive" is because of their accuracy over large ranges, and consistency. Watch Ebay for old meters.


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## Arilou (Oct 14, 2013)

Eidetic said:


> Problem with that approach is that the detector will not be linear. You might be close for powers around that of the calibration laser, but there won't be much accuracy. Holographers do what you propose all the time. The reason laser power meters are "expensive" is because of their accuracy over large ranges, and consistency. Watch Ebay for old meters.



I checked this by using two lasers, and the response is almost exactly the sum of each by itself.

Heat dissipation is linear, so the temperature is proportional to power. The voltage is proportional to the temperature difference. It might be possible that there could be some nonlinearity in the current flow, but this doesn't seem to be the case. If it was, it would be possible to add a resistor across the peltier, which would have a linear response.

A bigger source of measurement error is that 5 milliwatts is a very small amount of power. My finger emits more than 5mW of heat. If I breathe on the sensor, that's more than 5mW. So I have to hold the laser some distance away to get an accurate reading.

Even the used meters are going for over $200 on ebay. That's an awful lot for something that can be built with less than $20 in parts. I put this together from stuff I had laying around. If I bought an old meter, how would I know if it was calibrated correctly?


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## Arilou (Nov 6, 2013)

I put a piece of black electrical tape on the peltier, and under that put a small 100 ohm resistor to use as a calibration heat source. This may slighty underestimate the power of the laser depending on how much light reflects from the black tape, but the numbers I'm getting from this look fairly realistic. eg, the "eye-safe" lasers are under 5mW, and the 50mW laser that I bought from dragonlasers measures about 70mW.


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## Eidetic (Nov 6, 2013)

You're right about the old meters on ebay, but I've seen them there for $50 and less. Before meters were built for lasers, pulsed energy was measured in "Gillettes", referring to how many blades it would poke through. Many of the oldest laser power meters are simple photocells with volt meter. It's all about your application, and how accurate you need to be.


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## Arilou (Nov 7, 2013)

Eidetic said:


> You're right about the old meters on ebay, but I've seen them there for $50 and less. Before meters were built for lasers, pulsed energy was measured in "Gillettes", referring to how many blades it would poke through. Many of the oldest laser power meters are simple photocells with volt meter. It's all about your application, and how accurate you need to be.



I'm getting some fairly reasonable numbers out of this setup. I bought some cheap laser pointers from ebay, and all of the sellers claimed they were under 5mW. A few were actually under 5mW, but some of these are surprisingly high power, around 80mW. Rather dangerous to be playing around with, though not enough to burn through a razor blade.

The measurement errors are mostly environmental. Direct sunlight is around 100 mW/cm2, but even the indirect light coming in the window measures a few mW at the detector. If the heat or air conditioning is on, it will affect it. The heat from my hand will also, if I get too close. It's not too hard to get a reasonable estimate for 50mW or more, but the numbers can be a bit off if you're not careful.

Photocell meters tend to have varying sensitivity depending on the wavelength. They are not affected by thermal IR like peltier meters, but may need to be adjusted depending on the wavelength of the laser you're measuring. And of course you still need to do it in a dark room to avoid measuring the ambient light.


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## Eidetic (Nov 7, 2013)

If some of those laser pointers were green, there may also be lots of IR light coming out of them as well as the green beam, affecting the measurement.


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## bshanahan14rulz (Nov 13, 2013)

http://www.radiantelectronics.org/ < these look great for $100 and for the features! Ain't as affordable as a completely DIY $20 setup, but it does come as a full product.


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