Simple Laser Power Meter Using IR Thermometer

I checked the wiki and IMHO your formula is right. We just miss some detail. Some important detail :thinking:

Hey wait .. it must be the air. This is radiation cooling only. Air will cool down the target, that is what is not included in this formula.

hmm, you must be right about air cooling. so in addition we'd have to calculate power of the heat lost due to conduction with the surrounding air. this might be doable, we know the temp of the air, it's the same as the starting temp of the target. i'll crack open a physics textbook and see what i can work out

would you mind making another target with the same dimensions as Warske? so 0.5" on the side, and see what kinda numbers you get with that?

since you're talking about metric, im guessing you're not in the USA. what kinda foil and paint are you using?

I used common aluminum foil you use in the kitchen. No idea how thick it is. As for paint I used black marker. It's far from pure matt black, but IMHO it absorbs more then 90% of the light.
That air cooling might be a pain to compute. As soon as the target warms, the air will start to move around it. It will be reproducible if you use the box as you did, but too complex to compute.
Let's try this: let's have target horizontal, not vertical, and let's have the thermometer under it. That should limit the air flow considerably, and the temperature of the target should get higher. Not that it helps the matter of measuring power anyhow, but it should be interesting experiment.
I think this is workable way of measuring laser power, but it will have to be calibrated, rather then computed.
I did not do any more test yet.

Neat idea. I will have to be building one of these soon...

Hey guys,

i m really interested in this LPM but I want to know how this "magic" number 3.13 was deduced. I hadn't had the time to look into the equations yet but I will do that.

@comradmax
I think you are on the right way but one thing I noticed in your reasoning is the following: when I take you're equation then the laser power is proportional to the end temperature to the power of four. The original equation by Warske gives a linear dependence between end temp and the power. I'm really wondering what kind of approximations Warske used...

Warske must have found the 3.13 constant empirically. If you had a laser of known power measured with another LPM, then you'd just have to adjust the temp value to suit your needs. I dont know if his method scaled linearly, but i suspect it's only true for values in a close range to what he measured with. In other words, the exact size of the target, paint used, range of laser powers ~1W, the error that HIS thermometer has, air effects, etc. Once you leave this "safe" region, then all bets are off!

i got my IR thermometer today! i'll be making this and trying to experiment with variations :san:

Made one with a 0.5" square of painted foil, and measured my 2 cheap pointers: 405 from ebay and 532 from swimminsurfer256. I get 33mW for Violet and 40mW Green, which seems consistent with what LPM-holding members have measured

I noticed that after the initial 30s the temp reaches a plateau, but fluctuates. I recorded temp at each 30s and averaged them, did this 3 times for each laser, then took the highest.

Many thanks to Warske for posting this:san: I am getting the parts together for my first laser build, planning to use a LOC from the LPC-815 sled. With the thermal DIY meter I can now check the power of my beam, this is great!

Hi all.
I purchased one of these from DX I think. I'll post the link in the test results when completed.
This is a very fun thing to experiment with. I do have some opinions on it though.
I needed one because the last 2 yrs we have been doing various remodleings to the house and during the winter the house has cool drafts I cannot find. So, It now gives me an excuse to buy one.

Is it a LPM.....well yes. My repeatability results are amazing for what it is. Is it accurate? Who knows! It makes a good cheap comparator, but has a lot of room for error even if a calibrated laser is used to create your value. Would I sell or post the results implying its the accurate output...sadly no. I'll explain in my results post. I'm testing 5 diff lasers, 5 tests each on both types of target.

If you want to post accurate numbers for sale or whatever you would be better off to get something like the HLPM II 200mW LPM Module + Shipping $54.99 from laserbee.

Vaporizer said:

Hi all.
I purchased one of these from DX I think. I'll post the link in the test results when completed.
This is a very fun thing to experiment with. I do have some opinions on it though.
I needed one because the last 2 yrs we have been doing various remodleings to the house and during the winter the house has cool drafts I cannot find. So, It now gives me an excuse to buy one.

Is it a LPM.....well yes. My repeatability results are amazing for what it is. Is it accurate? Who knows! It makes a good cheap comparator, but has a lot of room for error even if a calibrated laser is used to create your value. Would I sell or post the results implying its the accurate output...sadly no. I'll explain in my results post. I'm testing 5 diff lasers, 5 tests each on both types of target.

If you want to post accurate numbers for sale or whatever you would be better off to get something like the HLPM II 200mW LPM Module + Shipping $54.99 from laserbee.

Click to expand...

That is $54.99 only to LPF Members...
The HLPM II costs more on eBay and even more on our
Website at its retail price..

I'll be waiting for your results on your IR LPM project..:beer:


Jerry

You can contact us at any time on our Website: J.BAUER Electronics

I would love to have real LPM .. and 55 is lovely price .. but 200mW is not enough these days. Today you need 2W .. and who knows what tomorrow. It would be nice to be able to predictably lower the power of laser to let's say 1/10. Then this LPM would be useful up to 2W.

DrSid said:

It would be nice to be able to predictably lower the power of laser to let's say 1/10.

Click to expand...

Would the level of attenuation through laser goggles scale linearly with output power?

If you read the HLPM II Thread... you will get some info on how to
read higher powers of Lasers... I was discussed there...
The link is in my Sig...

Jerry

You can contact us at any time on our Website: J.BAUER Electronics

Ah, must have read that before, prolly where I picked up on the idea of using filters for attenuation in the first place Thanx!

Took longer to get back to this than I planned, but here ya go!

Ok, the first part of the test is done! I'm amazed. I made both targets, round & square. This post is for the the round targets only. I had made them and getting them exact sizes needed I had to respray another piece of foil for the square targets. Error possibility #1, diff paint thickness's. The round were much more closely related visually from the description of the visible beam and other listed characteristics on this forum. Lack of paint on the square ones or to much? I have no calibrated laser nor a true LPM. So what's the point? Well, if care is taken, you can get comparison results for changes made, between lasers, lens swapping gain or loss, even a fair portion of how much IR your green laser is putting out as a %.

Now lets be clear, the numbers I'm posting are the results of the multiplied temperature diff at the start, after 30 seconds lasing the center of the target, and then multiplied by 3.13. They seem uncannily close to what I was expecting. I wouldn't use these numbers for a laser sale or even in my sig. Maybe with an "~", but that still seems like a stretch with no true reference. I would rather use the ma its running. Fun? Yea it was fun to do and has some purpose for self comparison.

Here is the IR Digital Thermometer I used.
DealExtreme: $16.20 Mini 1.3" LCD Digital InfraRed Thermometer with LED White Light (-50'~260'C/2*AAA)
Aluminum foil .002 (.05mm) Came with a sandwich.( washed and rolled flat) lol
Paint - Krylon Flat Black - Indoor/Outdoor (10 min dry) (I had resprayed my security doors)

The target was affixed to cover the detector opening leaving 1/8" clearance so no heat was trapped. It was edge glued to balsa wood. Balsa wood conducts very little heat. The lasers were set 4" from the target as my "standard" distance.
Draft free area and equipment was allowed to stabilize between tests for 15 min. Caution was given to breathing on the target. Target was coated on both sides with paint. A timed exposure of 30 seconds for each test reading the endpoint temperature. During the test I realized that the temperature dropped when I exhaled....ok, the target is only .002 thick + paint so there is little mass for heat storage.
I did notice that even the highest reading occurred in abt 20 seconds and little to no change for the last 8-10 seconds.

The lasers tested were as follows:
1. <5mw Red 660nm pointer - ebay
2. 5mw overspec pointer Green 532nm - swimminsurfer256
3. 5mw 405nm overspec pointer - ebay "itemquickship"
4. 200mw Keychain Green 532nm - FocalPrice
5. 200mw Green 532nm module - Deal Extreme

Fresh batteries before each test. 5 tests were ran on each with ambient temperatures from 72* - 81*. I did each laser at the same temp. This way some lasers were not tested at 72* and some at 81*.

Here is the average of 5 tests of each laser: (Same order as above)
----------------------------------------------------------Average Output
1. <5mw Red 660nm pointer - ebay------------------------------3.2mw
2. 5mw overspec pointer Green 532nm - swimminsurfer256-------36.9mw
3. 5mw 405nm overspec pointer - ebay-------------------------52.3mw
4. 200mw Keychain Green 532nm - FocalPrice------------------106.2mw
5. 200mw Green 532nm module - Deal Extreme-----------------153.4mw

If you read the descriptions of what nm/mw looks like, burns, burns with a focusing lens, visible in these conditions......they are close to what I was expecting and as ppl describe.
I ordered 4 IR filters from snoctony. It's the small green glass in the picture. I decided to test the 5mw overspec 532nm pointer filtered. Low end pointers are notorious for power coming from IR(808nm and up). Not that they are bad, it's a get what you pay for thing. If it's putting out a lot of IR and your goggles aren't rated for IR, only 532nm, 405nm and maybe 445nm, you could get a good IR eye hit and no blink response at this wavelength. Not a good thing at all.
So, with a filter added the 5mw 532nm Green pointer which read 36.9mw it now read 10.4mw! 2/3's IR. Wow! I did do a visual beam test and filtered or unfiltered it looked the same, but it sure heated the target less! I also had a CD/DVD burner diode(IR) handy. Totally unmounted, no lens I decided to try it, but I had to position the diode abt 1/8" from the target and used clips to give it some heat sink for the 30 seconds.
The result was a whopping 98mw! This is one that normally has a dull faint red glow you think is dead. Now you can see why not to look into it! Was it really 98mw? Again, who can say with what I had to use for the test. I can assure it raised the target temperature ~30* in 30 seconds unfocused. Actually abt 20 seconds. With a focusing lens, an eye hit from this will be a disaster for your eyes without proper goggles.
:wave: Are we paying attention?

Changing targets made from the same sheet, stamped with the same tubing at the same time all have slightly diff readings. So even if used for a comparator, if the target is changed you can't use it against any old readings. Just more possible errors.

Again, do not take the mw values I've listed as real. They are the OP's 3.13 multiplied by the target temperature change in 30 seconds. Nothing more.

It was fun and I hope you enjoyed the post and here are a few pics!

I'm going for this $100 LPM (as soon as they restock).

http://laserpointerforums.com/f41/i...ctronics-radiant-alpha-power-meter-56689.html

Will do some test then.

I have 2 non-contact thermometers here at home (see below). I will try this tonight and report back. Very interesting.

One thing nobody has raised yet is this, even of the final calculated result is not very accurate, this should provide good A/B testing for comparing various lasers...

I wonder how this would hold up to a 1W 445 though....