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01-13-2011, 07:29 PM #1
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laserlighter
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DIY Laserpowermeter at the base of Physics

Here i want to share my experiment to estimate the average power of any laser.

You can measure weight, length, time and temperature but not the power of an optical device..

Then i have a solution for you.

All you need is a specified small piece of aluminium glued on the small sensor of a thermometer and a stopwatch.

Very important is to know the exact weight of the small piece of aluminium. You can use a scale or measure the dimensions to calculate the volume. With the density of 2,7 g/cm^3 you can get the weight.

In my case i use a little plate with exactly 1,000g

I used a black eding to blacken the surface but you can also use carbon black from a lighter. I estimate the absorption at about 90-95%.

All you have do do now is to shine your Laser for a specified time (about 10sec) on the black surface and watch the maximum temperature raise.

for example:

My Laser gives 8,0°C temperature raise in 10 seconds. (from 23,1°C to 31,1°C)

With that data and the specific heat capacity of aluminum (0,897 J/(g · K)) you can calculate your laserpower.

Formula:

[Mass in Gramm] * [Heat Capacity] * [Temperature Raise] / [Time in Seconds] / [Absobtion]= Laserpower in Watts

example:

1,00g * 0,897 J/g*K * 8°C / 10sec / 0,9 = 0,797W

The big advantage: You can calibrate the LPM yourself. I dont like to guess the power by using a plack piece of aluminum foil with nearly no heat capacity and a lot of unpredictable heat emission...

Have fun and (don't) watch out for the reflexion when measuring high power lasers

//EDIT: maybe its more an experiment than a tutorial...

Last edited by laserlighter; 01-13-2011 at 07:44 PM. Reason: Experiment or tutorial?

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01-15-2011, 10:34 AM #2
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laserlighter
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Re: DIY Laserpowermeter at the base of Physics

new idea:

we can measure the heat capacity of small coins to get a reference material.

for example the 1ct coins.

so everyone can use the same material and its easy to measure more exactly.

01-18-2011, 12:06 PM #3
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Re: DIY Laserpowermeter at the base of Physics

Very nice idea! I will try this the next days!
One possible improvement could be to use a IR thermometer for contact free measurements.
Using a coin as target is great. If someone with a laser powermeter could calibrate it to get a combined value for [Mass] * [Heat Capacity], evryone could build a low cost powermeter.

01-18-2011, 04:33 PM #4
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Re: DIY Laserpowermeter at the base of Physics

I tryed it with a IR thermometer. the problem is that they depend on the emission from the surface and thats one factor we can not measure (like the absobtion).

If you add the heat capacity of the thermometer sensor you can measure very exactly.

I tried to combine as many good measureable units as possible, so you calibrate the device yourself.

01-18-2011, 04:57 PM #5
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enigmahack
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Re: DIY Laserpowermeter at the base of Physics

Here's a question. I haven't bothered to work out the mathematics, nor could I... But I'm curious to know how a laser that is focused would generate more heat over the surface area vs a laser of equal power that is unfocused.

My thought is that a laser that is more focused (sharper beam, whatever) will generate more heat in a specific area. The same power laser with a broader beam will have less heat generated in general, but compounded by the fact that the larger surface area will dissipate the heat more quickly will also be a contributing factor to prevent a reasonable amount of accuracy.

What, if any, flaws are in my logic. I see how this could potentially work, but I think I see how it could potentially not.
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01-18-2011, 07:29 PM #6
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Re: DIY Laserpowermeter at the base of Physics

Quote:
 Originally Posted by enigmahack My thought is that a laser that is more focused (sharper beam, whatever) will generate more heat in a specific area. The same power laser with a broader beam will have less heat generated in general, but compounded by the fact that the larger surface area will dissipate the heat more quickly will also be a contributing factor to prevent a reasonable amount of accuracy.
That's absolutely right. If the spot would be too small the surface around the dot would get very hot and you would have losses caused by heat radiation.

BUT, aluminium is a very good heat conductor. We are talking about an area of some square mm. Thats absolutely no problem for the small pice of metall to spread the energy very fast.

01-18-2011, 07:40 PM #7
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Re: DIY Laserpowermeter at the base of Physics

So as long as the surface area remains the same when taking the measurement, we'll have a constant that can be used for approximating the mW or W based on heat?

Sounds neat to me
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01-18-2011, 07:44 PM #8
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Re: DIY Laserpowermeter at the base of Physics

Many multimeters come with a temp probe function. I smell a "DIY hobbiest" grade powermeter kit?

01-18-2011, 11:42 PM #9
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Re: DIY Laserpowermeter at the base of Physics

There are way too many variables to get some precision out of this, try to thermally isolate the plate and prevent air currents across the surface, that should help a bit. But even if it won't work that good it's good to learn from it, a few tries more and you have something more acurate.

Last edited by Bluefan; 01-18-2011 at 11:44 PM.

01-19-2011, 12:42 AM #10
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enigmahack
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Re: DIY Laserpowermeter at the base of Physics

I remember years ago the idea of using a LED as a form of LPM. Is that a possibility at all?

What about using something like the solar thing from like a calculator?
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01-19-2011, 12:46 AM #11
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Re: DIY Laserpowermeter at the base of Physics

Very good thinking I say!
Maybe using a small square of Silver (eventually copper :P), with some thermal paste to the thermometer, enclosed on a little box with a very small hole for the laser, isn't more adequate? maybe a light sensor to detect when the laser is on and start counting the time for more precise measures?

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01-19-2011, 10:57 AM #12
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Re: DIY Laserpowermeter at the base of Physics

that are good ideas

-Insulation increases the accuracy. (i used a piece of foam plastic)
-short time measuring increases the accuracy. (max 30sec)
-do not use a small burning spot for measuring. (min diameter should be 1mm for good heat spreading)
-use the maximum temperature which is shown on you thermometer. the heat needs maybe a second to spread or your thermometer needs some time to get the right value.

to estimate the heat loss you can heat the plate and stop the time it needs to get back to normal temperatur.
in my case i measured a heat loss of 65mW at 10°C over ambient.

01-19-2011, 11:43 AM #13
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Re: DIY Laserpowermeter at the base of Physics

Interesting proof of concept. Do you have a actual LPM to compare results? If you know the efficiency, you can account for any error and actually be accurate to within 10-20mW I'd guess.

I'd also recommend using a metal will a LOWER specific heat. With a lower specific heat the metal will take less wattage to increase in temperature by one degree. Aluminum has a high specific heat compared to most metals. Since the temp rise is higher the difference can be measured to a higher accuracy, thus the results will be more accurate.

Try copper, lead, or bismuth if you can. You should find that the same laser will heat the same mass to a higher temperature compared to aluminum. This is the same concept of a heatsink, who are also made with low specific heat metals.

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01-19-2011, 02:25 PM #14
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Re: DIY Laserpowermeter at the base of Physics

@Xplorer877
Maybe a smaller mass?
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01-19-2011, 04:13 PM #15
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Re: DIY Laserpowermeter at the base of Physics

Quote:
 Originally Posted by Leodahsan @Xplorer877 Maybe a smaller mass?
A smaler mass would lead to higer temperatures that's right.
But higher temperatures means higher losses of heat.
The same with materials with lower specific heat capacity
5-10°C should be enough for correct measuring.
But for lower powers you should give it a try

Remember the error calculation. It makes no sense to decrease one error from 1% to 0,5% when another one increases from 1% to 2%...

I think there is always a little tweaking around.

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