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Old 04-08-2009, 04:57 PM   #1
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Default Simple Laser Power Meter Using IR Thermometer

Introduction to the Simple Laser Power Meter

For $17 or less, you can easily make a Thermal Laser Power Meter (LPM) with an accuracy of about 15%. The low cost includes an inexpensive InfraRed (IR) Thermometer, which you can also use for other things like checking the temperature of your laser diode and its driver, or the cup of coffee on your desk.

While this DIY meter isn't in the same league as more expensive LPM's (which are more accurate and easier to use), if you're on a budget and willing to do a bit of work, this meter will tell you if that 100 mW pointer you bought is performing close to spec.

Advantages

- The parts are cheap to buy.

- Being thermal instead of photoelectric, it works with all beam colors including UV and IR.

- It works with low power 5 mW laser pointers on up to about 500 mW, and can be modified to work with higher power levels.

- It takes only 30 seconds to get a reading.

- It's portable. If you don't want to risk taking your bulky and expensive LPM with you to the field for troubleshooting, you can take this instead. Readings are repeatable within 3%, so you can tell if your laser power has changed.

- The best part? No soldering is required for this project! In fact, the only tools required are a scissors and a ruler. Once you have the parts, you should be able to put this together rather quickly.

What it is

As the pictures show, this laser power meter (also known as a thermal detector or bolometer) consists of a standardized laser target and an IR Thermometer which is used to read its temperature. Simple, no?

How to use the Simple Laser Power Meter

1. Turn on the IR Thermometer and set it to take continuous temperature readings in degrees Fahrenheit.

2. Place it on a table and give it a minute to stabilize. The target disk should be vertical, as shown. If the area is drafty you may want to put it in a clear plastic box as pictured, but usually this isn't necessary.

3. Write down the temperature reading, then shine the laser beam on the target for 30 seconds.

4. At the 30 second mark, record the temperature again.

5. Subtract the two temperature readings to obtain the temperature rise, then multiply that number by 3.13 to get mW. For example, a 10 deg F temperature rise means you have about 31 mW of laser power hitting the target. Note that the accuracy is somewhat less at lower power levels because of the thermometer's resolution.

If you want a bit more accuracy, subtract 2% from the result for each thousand feet above sea level you are, and add 1% for violet (Blu-ray) lasers.



Next: Parts List and Construction

[Note: The following attachment is the picture above. It's due to a vBulletin import bug.]
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Old 04-08-2009, 04:58 PM   #2
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Default Simple Laser Power Meter Using IR Thermometer

Parts List and Construction

Code:
$12.99 - IR thermometer.
      http://www.harborfreight.com/cpi/cta...emnumber=93983
      These sometimes go on sale in their local stores for $9.99, 
      or $7.99 with a coupon.  You can sign up to receive ads and 
      coupons by mail, or check the weekly "Extra Savings Coupons" 
      here: http://www.harborfreightusa.com
      (Some other IR Thermometers can be used instead.  See below.)
 $3.19 - Can of spray paint: Rust-Oleum Flat Black #1976830
  Free - Household Aluminum Foil.  Standard weight.
  Free - 4# Fishing Line or sewing thread.
  Free - Glue
  Free - Tape

    (Free assumes you already have some on hand or
    can beg some from the neighbors.)
Construction Summary

1) Make the Official Warske Standard Laser Target, Mark I:

- Spray paint both sides of a small piece of aluminum foil. Use one coat of paint on each side, not too thick, but thick enough to cover the foil. For best accuracy, use the specified paint, but any other Flat Black spray paint can be used with good results. (See below.)

- Cut out a half inch square from the painted foil using sizzors. Make sure it is exactly 0.5 inches on a side and if not, trim it or start with another piece. If it's not the right size it will affect your readings, so get it as close as you can. Alternatively you can make it a 0.564" diameter circle instead of a square. (See below.)

- Glue some 4 lb. nylon fishing line to an edge of the square. Use as little glue as possible. I used Superglue (Cyanoacrylate). If you don't have 4# nylon line, use some light weight sewing thread soaked in glue to stiffen it.

2) Attach the target to the IR Thermometer so that it is 1/16" in front of the aperature with the nylon line side facing the thermometer. I used part of a wooden popsicle stick for a spacer and taped it to the thermometer, but you can use whatever is handy. Or you can just suspend the target 1/16" in front of the thermometer using whatever is available (see picture in previous post).

Construction Details and Alternatives

You can skip everything that follows, but for those interested, here is a bit more information:

Paint
The Rust-Oleum Flat Black #1976830 spray paint is the cheapest I could easily find, which means you shouldn't have to pay much for it either (which is the whole point of this project). I bought it in a local Bi-Mart store, and it worked much better than a more expensive high temperature paint I tested.

Black paint converts light energy into heat. It does this with high efficiency across the visible spectrum, and into the UV and IR as well. This heat input raises the temperature of the laser target in a predictable way, which is then read by the IR Thermometer and converted into a laser power level.

The important feature of the paint you use is its absorptivity: how much of the laser light is absorbed. Whatever light the paint doesn't reflect is absorbed and converted to heat. Notice that if it reflected more green light than any other color, it would look green, not black. That is why black paint is quite uniform with respect to wavelength in the visible spectrum. Typical flat black paint reflects between 1% and 10%. If it reflected more than that it would look grey instead of black.

I've tested this particular paint, and found that it reflects 4% at 650 nm (red) and 5% at at 405 nm (violet or Blu-Ray). I haven't checked other wavelengths yet, but I expect the results to be similar.

The bottom line is: Use this paint for the best results, but you can use any flat black spray paint. It may throw your results off a little, but not by more than 6%.

Painting
In addition to using consistent paint, it is important to apply it in a consistent way. I used a single coat applied in about 10 or so quick passes. If it goes on too thin, it won't absorbe as much light energy. If it goes on too thick, it will form an insulator between the hot outside surface of the paint and the cooler aluminum surface and affect the thermal properties of the target.

The technique I recommend is to put a small piece of foil on a sheet of white typing paper which has a black magic markered "X" drawn on it (see photos). Then spray on only enough paint so the black magic marker line just disapears.

Alternative IR Thermometers
IR Thermometers "read" the temperature of a surface by sensing the far infrared energy they emmit.

You can use a different IR Thermometer than the one specified. I used an old Radio Shack model that is no longer available, and it worked just as well, although I had to use a rubber band to hold down the button to get continuous readings. If you use a different IR Thermometer, make sure it has a small enough aperature so that the laser target will block it completely. You don't want the thermmometer to be able to "see" around the edges and read some other temperature. The Cen-Tech #93983 has a .32" aperature diameter. An aperature larger than that will probably give you lower readings. In the picture of three IR Thermometers, the one on the right has too large an aperature (the small hole on the bottom is for the built in laser pointer).

The Cen-Tech #93983 sold by Harbor Freight is available online and in their stores. Currently it is $12.99 online here:
http://www.harborfreight.com/cpi/cta...emnumber=93983
It often goes on sale for $9.99 and even $7.99 with a coupon. To get coupons, you can go to their web sight and sign up to receive their ads by mail, and also click on "Extra Coupon Savings" here: http://www.harborfreightusa.com/usa/...8_RetailA.html

It is helpful to have a way to take continuous readings. This is because your body heat will affect the measurements, and it works better if you don't touch the thermometer just before taking a reading. Also, it allows you to put the whole unit in a clear plastic box to shield it from drafts, and you can still read the display through the plastic. Finally, the reading may jump around a bit, and with continuous readings you can pick an average reading to make it more accurate.

The Cen-Tech 93983 has a LOCK mode, which gives continuous readings for up to 60 min (say the instructions). If your thermometer doesn't have this mode, sometimes you can hold the "on" button down with a rubber band to get continuous readings.

[code]To put the Cen-Tech 93983 in LOCK mode:
power on (that is, press Meas key)
press mode key 3 times
LOCK icon flashes, press Meas key
It is now in LOCK mode.
to un-LOCK, press Meas key[code]

Circular Laser Target
The easiest target to make is a square, cut with scissors, but its a bit difficult to get the dimensions exact. An alternative shape is a circle, which takes more work but has a few advantages. As long as the surface area is the same, the shape (circle or square) doesn't affect the accuracy.

The circular shape is nice because, after all, laser spots tend to be more round than square. Also it is relatively easy to make a punch that will punch out the target very accurately. Once you have made the punch, it is easy to make as many accurate laser targets as you want.

For a punch, I use a piece of thin walled brass tubing that has been sharpened on one end (see picture), a block of wood (soft pine works), and a hammer. The correct size tubing has an inside diameter of 0.564" and it is available at hobby shops and some hardware stores as K & S Engineering Model # 142 Round Brass Tube 19/32 O.D. .014 WALL. Sand down the outside of the tubing at one end until the edge is sharp. Don't sand the inside, because you don't want to change the inside diamater.

Aluminum Foil
The thickness of the aluminum is not too critical. It's the surface area that is critical. The foil I used is the standard household variety (see picture) and is 0.00071 inches thick. I have also tested the aluminum from the side of a soft drink can, which was 6 times thicker. It gives the same readings but takes longer to heat up due to its larger thermal mass.

Nylon Fishing Line
The laser target needs a way to support it without interfering with its optical and thermal properties. I use a short piece of 4# nylon fishing line because it is very thin (doesn't conduct heat well) and plastic (also doesn't conduct heat well). If you don't have fishing line, a bit of sewing thread soaked in glue and allowed to dry should work just as well.

Glue
I used a very small amount of cyanoacrylate adhesive (Super Glue) to attach the nylon line to the target. Just about any other glue should work as well. Glue the thermometer side of the target, since it won't affect the readings there. Don't put it on the laser side, because it will affect the reflectivity of the paint and the accuracy of the reading.

Plastic Box
Optionally you can enclose the LPM in an open ended plastic box to keep drafts away from the laser target (see pictures). In most situations this shouldn't be necessary, so I didn't include it in the parts list. However if there is a fan going, doors opening, or people moving around close by, a box would be a good idea.

You can make the box out of cardboard or other material, but you do need to be able to see the temperature display inside. You can cut out a hole to view the display and leave it like that, or tape some clear plastic wrap over the hole for better protection.

The box in the pictures came from a local Michaels Crafts Store (http://www.michaels.com) and cost about $2.50. It came with a lid, so it also can be used to store the LPM when it isn't being used.



[Note: The following attachment is the picture above. It's due to a vBulletin import bug.]
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Old 04-10-2009, 06:42 PM   #3
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Default Re: Simple Laser Power Meter Using IR Thermometer

Interesting concept. I'm interested to see how accurate this is. Have you got another meter to compare it to?

Also another issue that I think should be brought up is that black paint while it's specturally flat for most colors is not for all wavelengths. So, your measurements might be off at certain wavelengths.
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Old 04-10-2009, 07:01 PM   #4
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Default Re: Simple Laser Power Meter Using IR Thermometer

Nice DIY project.... [smiley=thumbsup.gif]

Very well explained.... This looks easy enough to be built
by practically anyone.... Well done.. 8-)

Even at 15% accuracy it is still usable for comparisons of ones
own lasers...

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Old 04-10-2009, 07:05 PM   #5
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Default Re: Simple Laser Power Meter Using IR Thermometer

Nice project.
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Old 04-12-2009, 02:50 PM   #6
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Default Re: Simple Laser Power Meter Using IR Thermometer

very nice
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Old 04-12-2009, 04:24 PM   #7
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by Kenom
...I'm interested to see how accurate this is. *Have you got another meter to compare it to?
*
Not in the sense you mean. *Eventually it should get compared against a trusted LPM (either directly or with a reference laser), and I'll post the results on this thread. *In the meantime, I used some simple physics to get a power reading on one of my lasers, and used that as the reference. *I checked for linearity (using a photodiode) and repeatability. *I also tested 5 of the IR Thermometers against a thermocouple over a range of temperatures. *I'm pretty confident about the 15% accuracy, and eventually expect that number will be tightened up.

Quote:
Also another issue that I think should be brought up is that black paint while it's specturally flat for most colors is not for all wavelengths. *So, your measurements might be off at certain wavelengths.
You make a good point. *As I mentioned in the second post, if the paint reflects much more at a particular visible wavelength, your eye will see the color, and the paint won't look black. *But you are right in that it could be off by a few percent and still look black, and there is no guarantee about the non-visible wavelengths. *

Using a photodiode, you can test the reflectivity of the paint for each laser color available, including the infrared ones. *I built a little test jig for this which measures and records the reflected light at each angle. *If you use a bunch of math to compare this to the incident light, you can find what percent of the light is reflected and what percent is absorbed. *So far, I have only checked it for red and violet, and the results were within 1 percent. *

Kenom, lasersbee, Rangedunits, & jamilm9: *Thanks for the nice comments.

My hope is that this technique can be used by the many people who have posted questions like: "Without spending much money, how can I tell if my new laser is is really putting out 150 mW, or is just a dud."
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Old 04-12-2009, 04:31 PM   #8
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Default Re: Simple Laser Power Meter Using IR Thermometer

Your'e welcome.
.


I think jerry is getting a little mad, people won't buy his Silicon sensor after seeing this thread.

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Old 04-12-2009, 04:39 PM   #9
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by Rangedunits
Your'e welcome.
By the way, *use Reputation points .


I think jerry is getting a little mad, people won't buy his Silicon sensor after seeing this thread.

-greg
Hey Greg...
first... I am all for any DIY LPM that a member here comes up with... and I see
no reason for me to get mad about this or any other DIY LPM thread... If you read my comments
on any of those threads you will see that I even encourage the DIY LPMs... 8-)

BTW Greg... FYI.... it is considered bad manners to beg for Rep points... IMO... :-/

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Old 04-12-2009, 04:44 PM   #10
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Default Re: Simple Laser Power Meter Using IR Thermometer

Jerry, there obviously some mis-understanding here. I know your a LPM collector, im pretty sure that baby will get in your hands soon.
Btw, i edited my last quote..., begging for Rep wasn't what i was looking for, i wanted to empathize the fact they are there for something...; Help,fast payments, Etcs.


I will attempt to make my own.


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Old 04-12-2009, 04:52 PM   #11
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Default Re: Simple Laser Power Meter Using IR Thermometer

Hey Greg...
* *it seems to be an easy/inexpensive DIY LPM project... and you can use
the 120mW LPM Module you purchased from us to calibrate the DIY LPM build...

And yes... I have already ordered an IR Thermometer.... ;D

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Old 04-12-2009, 05:07 PM   #12
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by lasersbee
Hey Greg...
* *it seems to be an easy/inexpensive DIY LPM project... and you can use
the 120mW LPM Module you purchased from us to calibrate the DIY LPM build...

And yes... I have already ordered an IR Thermometer.... ;D


Jerry
I KNEW IT! ;D ;D ;D ;D



This thread definitely need more attention.

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Old 04-13-2009, 12:03 AM   #13
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Default Re: Simple Laser Power Meter Using IR Thermometer

It should get stickyed. send c0ld a pm.
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Old 04-13-2009, 12:05 AM   #14
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by jamilm9
It should get stickyed. send c0ld a pm.
I sent a pm about, the donation ive sent.. im waiting his reply to confirm he got it.

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Old 04-13-2009, 12:08 AM   #15
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Default Re: Simple Laser Power Meter Using IR Thermometer

Hey guys.... don't you think a few other members should build this IR based
DIY LPM and test it against a calibrated LPM... before this thread gets
stickied... *:-? :-? :-? :-?

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Old 04-13-2009, 01:31 AM   #16
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Default Re: Simple Laser Power Meter Using IR Thermometer

I am rather skeptical of the magic 3.13 you came up with to convert temperature to mW (And how did you come up with it anyways considering you don't have a real LPM?)

The heat dissipation of the black target will vary with its mass/size/thickness/material/shape. *Not only that, but the ambient room temperature (which will in turn affect convection) and any drafts will have a huge impact on the readings.

You have addressed this somewhat by specifying dimensions but I doubt that will be accurate enough. *For example, what happens if you move the piece of aluminum foil closer to or against the thermometer?


I think a real test you can do already is get someone else to make one or make a few yourself and see how repeatable measurements are across every thermometer. *Unfortunately, my gut feeling tells me that when different people build these using different materials and to different tolerances you are going to end up with some pretty major variations.


TL;DR I can believe that each individual thermometer will be quite accurate as long as you calibrate each one separately, but if you encourage a hundred people to scavenge enough junk together to build one of these, you are going to get a hundred different measurements without some way to calibrate them. Someone needs to take a good, hard look at that 3.13 mW/*f.
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Old 04-13-2009, 01:32 AM   #17
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by lasersbee
Hey guys.... don't you think a few other members should build this IR based
DIY LPM and test it against a calibrated LPM... before this thread gets
stickied... *:-? :-? :-? :-?

Jerry
I agree...
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Old 04-13-2009, 05:13 AM   #18
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Default Re: Simple Laser Power Meter Using IR Thermometer

Good questions, 691175002, so I took quite a bit of time with the reply...

Quote:
Originally Posted by 691175002
...rather skeptical of the magic 3.13
I doubt it is exactly 3.13, but that is the best number I have at the moment, and I believe it will give the 15% accuracy stated. *Someone with an accurate LPM will, I hope, double check this. *If it turns out that I am off by a few percent and the real magic number is, say, 3.3, then I will edit the first post to include the more accurate number. *I had anticipated doing that, and it is one of the reasons I think the initial 15% accuracy statement can be tightened up.

Quote:
... how did you come up with it anyways considering you don't have a real LPM?
As I mentioned earlier, I used some simple physics to come up with the number. *It may be worth detailing my method in a separate thread if anyone is interested, but basically it involves using a thermal conductor with an accurately known heat capacity and reflectivity. *By measuring the thermal time constant when laser power is applied, the actual amount of power can be derived. *An interesting feature of this is that the thermal resistance doesn't affect the final result (unlike the LPM of this thread).

As a sanity check, I've tested the SLD1239JL-54 laser, and the diodes from some LPC-815 and PHR-803T sleds. Their power vs current curves line up nicely with what has been reported on LPF, but that only gives me ballpark numbers, of course.

Quote:
The heat dissipation of the black target will vary with its mass/size/thickness/material/shape.
Generally, the thermal resistance (heat dissipation) of a vertical, thermally conductive surface such as aluminum, will vary most strongly with its surface area, while other variables are much less critical.

The mass and thickness (within limits) does not affect steady state dissipation, although it does affect thermal time constant. *As I mentioned in the construction section, "The thickness of the aluminum is not too critical. *It's the surface area that is critical. *The foil I used is the standard household variety... *I have also tested the aluminum from the side of a soft drink can, which was 6 times thicker. *It gives the same readings but takes longer to heat up due to its larger thermal mass."

The material specified is aluminum. *Certainly if someone decided to use plastic wrap for the target instead, the results would be different.

The shape is either round or square, and the sizes were adjusted to make the two read the same.

Quote:
Not only that, but the ambient room temperature (which will in turn affect convection) and any drafts will have a huge impact on the readings.
Ambient temperature within a normal range doesn't measurably affect the readings. *If you want to learn more about this topic, you can research how convection heat sinks work.

Strong drafts will affect the readings, and for greatest accuracy in the presence of drafts, it is recommended that the meter be shielded from them by putting it in a box such as the one pictured in the construction section. *In an area where there are no fans going, no open windows, no doors being swung, no people running past, you will get the same results whether you enclose the meter or not. *Note that drafts also affect many high end LPMs.

Quote:
You have addressed this somewhat by specifying dimensions but I doubt that will be accurate enough.
The accuracy of the dimensions is very important. *That is why I showed a picture of a digital micrometer measuring the square target, and I suggested the round target be cut with a specific piece of tubing which is manufactured to rather high tolerances (the different sizes of tubing sold actually nest together with a sliding fit).

For example, if the square target is .510" by .510" instead of exactly a half inch square, this will throw the readings off a little bit. *Doing the math,
(.510 * .510) / (.5 * .5) = 1.040
we see that the surface area is off by 4%, and the readings will be thrown off by roughly the same amount. *While 4% error here would be unfortunate, I think you will agree this is still a better method of measuring your laser's power than seeing whether it will light matches! *And the price is hard to beat. *This gets into the question: "How accurate is accurate enough."

My feeling is that most people can get much closer than .510" if they are careful and give it a couple of tries.

For perfectionists, I would recommend the tubing cutter method. *Note that, once you are set up with that, you can stamp these targets out both quickly and accurately. *Maybe someone would want to go into production and mail them out to folks for a nominal fee. *As an exercise, calculate the material cost per target given that about $6 buys you a roll of aluminum foil (75 square feet!) and a can of paint...

Quote:
For example, what happens if you move the piece of aluminum foil closer to or against the thermometer?
Good question. *I was going to run that experiment, but I thought I would provide my results-so-far when I saw that the IR Thermometer went on sale. *Would have been a shame to miss it.

There isn't any reason I know of to think that distance is especially critical, but if it is much farther the IR Thermometer starts to be able to see around the target (so it will average the ambient into the readings). *Much closer and the air flow around the target (thus the thermal resistance, which is critical) will be affected. *So it's best to keep the target about 1/16" away until someone runs the test (volunteers?).

Quote:
I think a real test you can do already is get someone else to make one
It looks like that will happen...

Quote:
or make a few yourself and see how repeatable measurements are across every thermometer.
I actually did quiet a lot of testing on this. *The answer is: incredibly repeatable.

Quote:
Unfortunately, my gut feeling tells me that when different people build these using different materials and to different tolerances you are going to end up with some pretty major variations.
Based on my tests, if it is done carefully and per the instructions, I think people will be within the 15%. *Of course, many *would call that a pretty major variation. *As I said at the beginning, "... this meter isn't in the same league as more expensive LPM's (which are more accurate and easier to use)..." *But it does cost less, and it is quite useful.

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Old 04-16-2009, 05:41 PM   #19
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Default Re: Simple Laser Power Meter Using IR Thermometer

Interesting idea to measure laser power this way!

A few thoughts and things:

Quote:
The important feature of the paint you use is its absorptivity: how much of the laser light is absorbed. Whatever light the paint doesn't reflect is absorbed and converted to heat.
Another important aspect is IR emissivity, which is not normally indicated for this type of paints, as it is of no importance for normal use. I does affect to what degree the contactless thermometer is able to measure the termperature however. An interesting idea would be to paint one site flat black, and the other side (facing the thermometer) in radiator paint.

As for the size of the sensor disk being an issue in accuracy: You could use a practically infinite sized disk as well, conductivity would still be limited and result in reproducible measurements. Infinite would be large enough to cause to heating on the outer edge, perhaps an inch or two would suffice.
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Old 04-16-2009, 06:58 PM   #20
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by Benm
Another important aspect is IR emissivity, which is not normally indicated for this type of paints, as it is of no importance for normal use. I does affect to what degree the contactless thermometer is able to measure the termperature however. An interesting idea would be to paint one site flat black, and the other side (facing the thermometer) in radiator paint.
Thanks for the ideas. *You are right about the effect of emissivity on measuring temperature. *The surprise is that the effect is small unless you are dealing with a highly reflective surface like a mirror. *In fact, the emissivity of black paint and white paint are quite similar. *Here is a table http://www.engineeringtoolbox.com/em...nts-d_447.html

The reason is that emissivity involves wavelengths in the far infrared.

If you get one of these thermometers and point it at a white wall or a black wall in the same room, the temperature reading you get will be the same within the resolution of the thermometer. *I was surprised by this myself.

The other interesting thing is that, for this application, the emissivity doesn't matter at all as long as it is consistent. *This is because, if we had a different emissivity than we do, it would just show up as a change in the conversion factor from temperature to watts. *That being the case, it is easiest just to use the same paint on both sides of the foil, even though they serve two completely different purposes.

Quote:
As for the size of the sensor disk being an issue in accuracy: You could use a practically infinite sized disk as well, conductivity would still be limited and result in reproducible measurements. Infinite would be large enough to cause to heating on the outer edge, perhaps an inch or two would suffice.
I'm not sure I quite understand your idea. *

If the disk is much larger, you get a temperature gradient across the disk because of the thin aluminum. *That is, if you have a focused laser spot heating the center of a very large, thin disk, the center will be higher temperature than the edges because of thermal resistance in the material and cooling at the edge due to convection and radiation. *That thermal resistance will depend on the thickness of the material, which we don't want, because we don't have good control over the thickness.

So the trick is to make the disk small enough so that the thinnest material (in this case standard weight aluminum foil) is still thick enough to minimize the temperature gradient. * The disk could probably be larger than I did make it, but it would need some careful testing to verify that.

Thanks for the good feedback, and let me know if I didn't understand.

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pseudolobster, thanks for the +1!
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Old 04-16-2009, 11:18 PM   #21
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Default Re: Simple Laser Power Meter Using IR Thermometer

You understood right - the infinite-size approach will only work with predictable results if you have material of know thickness and composition. I wonder how much that actually varies between rolls of tinfoil from any particular brand though - possibly less than the variation in accuracy in cutting out disks of certain sizes.

All the factors considerd i do doubt it would be possible to come up with a design that is, say, 10% accurate without calibraton against a know source.

One interesting thing to investigate is to actually create a simple to build known source. Since this design is thermal, a halogen bulb of a certain power and at a certain distance could provide a calibration source. For example, a 100W halogen bulb at 10 cm distance (4pi.r2=1245cm2) would rain down 7.596 mW/cm2 on the disk. Given a 0.5 inch diameter disk, that should result in a reading of 4.9 mW.

I've been working on a thermal sensor design with ~5x5mm square targets making that approach less feasible, but with something this large it could be practical.
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Old 04-17-2009, 12:56 AM   #22
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by Benm
Since this design is thermal, a halogen bulb of a certain power and at a certain distance could provide a calibration source. For example, a 100W halogen bulb at 10 cm distance (4pi.r2=1245cm2) would rain down 7.596 mW/cm2 on the disk. Given a 0.5 inch diameter disk, that should result in a reading of 4.9 mW.
I was thinking of doing something like that, but since I couldn't easily validate the reflectivity of the paint across the full spectrum into the far IR, I figured it wouldn't be all that meaningful.

But I was curious what the result would be, and by the time you ran the numbers, I couldn't resist. *I don't have a small high wattage halogen. *That would be nice since it gets a little closer to being a point source. *But I do have a clear, long life incandescent that clocked in at 90 W using a &quot;Kill A Watt&quot; meter, which itself is probably only accurate to 5%.

The filimant is distributed over an area, and the globe is large, both of which could make the results less accurate.

I set it up for 10 cm (plus or minus a few mm) from the laser target to the center of the filament.

My initial temperature was 63.5 F, and 30 seconds after I turned on the light I read a stable 101.9 F.

Using the procedure in the first post, I get
(101.9 - 63.5) * 3.13 = 120.2 mW

Well, 120.2 mW isn't even close to the 4.9 mW you calculated, so I checked the math:

Surface area of sphere at 10 cm = *4 * 3.14159 * 10^2 = 1256.636 cm^2 * Check.

The disk I'm using is actually 0.564&quot; in diameter, to give an area of
3.14159 * (0.564 * 2.54 / 2)^2 = 1.611815 cm^2

So the power hitting it from a 90 w incandescent should be about
1.611815 / 1256.636 * 90 * 1000 = 115.44 mW

That suggests the meter is reading high by 120.2 / 115.44 = 1.041
Which is 4.1% high.

That seems pretty good, but I think the error sources in my quick experiment could easily swamp that out, so I can't claim it as validation. *But it is interesting.

Good ideas, thanks!

Quote:
I've been working on a thermal sensor design with ~5x5mm square targets making that approach less feasible, but with something this large it could be practical.
Maybe your idea would work for it after all. *Lets see, your surface area is 1.6 / .25 = 6.4 times smaller, so you would get about 20 mW from your 100 W halogen.
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Old 04-17-2009, 07:19 AM   #23
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Default Re: Simple Laser Power Meter Using IR Thermometer

I wish I could give you another point, but it takes a week or two before I can give you another. Very impressive, and your logic is profound. You are on a similar level as Igor IMHO. I cant wait to see how it measures up to a LPM. If it measures up, you will have just given the average hobbyist a very useful, and affordable tool.

Defiantly sticky material. [smiley=thumbsup.gif]
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Old 04-17-2009, 01:15 PM   #24
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Default Re: Simple Laser Power Meter Using IR Thermometer

Quote:
Originally Posted by Blu-Ray_Junkie
I wish I could give you another point... I cant wait to see how it measures up to a LPM. If it measures up, you will have just given the average hobbyist a very useful and affordable tool.
Thanks for the previous point and the encouraging words. *I did cringe to see all those posts about &quot;How do I measure my laser's power w/o spending a bunch of money&quot; and not be able to offer a reasonable alternative.

It's worth pointing out that you can look at the usefulness of this device on two levels. *On one level it can be used for relative measurements, and on the second level, it can be used for absolute measurements.

For example, let's say you made one of these laser targets but you got the dimensions off by quite a bit. *It isn't going to work for absolute measurements until you calibrate it somehow. *That is, you can't use it to measure mW of laser power.

But you can still use it for making relative measurements. *It can tell you, for instance, that Laser A is 18% more powerful than Laser B. *Or that the power of Laser A has dropped by 4% since the last time you measured it.

Furthermore, since its response is quite flat across different wavelengths (compared to a silicon photodiode like the LaserCheck uses), it can compare lasers of different colors. *(I noted in the first post that with the specified paint, the Mark I target is 1% less sensitive to violet than to red.)

The LaserCheck is quite accurate for red lasers, but the violet lasers not so much. *

Part of the problem is that the Blu-Ray diodes are not all at the same wavelength. *Folks on this forum, such as daguin in his thread &quot;Strange Things About Blu-Ray&quot; http://www.laserpointerforums.com/fo...num=1219161671 have noted that violet diodes with the same output power can appear to be very different in brightness. *

The same is true with the LaserCheck: it sees the different wavelengths as different signal levels. *To get an accurate reading for a violet laser, you have to tell it the wavelength, but the problem is that you don't know the exact wavelength.

And even if you know the wavelength, there is a question about how accurate the LaserCheck and similar meters are for violet lasers because the photodiode response curve starts to tail off in that region of the spectrum.

That question, how do I calibrate for violet, was the genesis for the &quot;Power Meter Calibration and Comparison&quot; thread http://www.laserpointerforums.com/fo...m=1217029972/0, which was started by pullbangdead almost a year ago and is currently up to 710 posts!

If you have a red laser, a violet laser, a LaserCheck, and an uncalibrated thermal LPM, you can use the combination to get an accurate power reading on the violet laser:
Use the LaserCheck to read the power of the red laser, use the red laser to calibrate the thermal LPM, and then use that to get an accurate reading on the violet laser. *

This gets us to the second level I was talking about. *I still maintain that if constructed carefully with the Mark I target, the LPM described here should be accurate to 15% without further calibration (I know, I know, we still need some independent corroboration of this). *If calibrated against a LaserCheck using a red laser, it should be as accurate as the LaserCheck plus or minus another 3%. *So that is another possible solution to the violet diode problem.
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Old 04-17-2009, 01:44 PM   #25
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Default Re: Simple Laser Power Meter Using IR Thermometer

Thanks for correcing my calculations - the number felt low as i wrote it down, must have messed up somewhere in the process.

Quote:
Originally Posted by Warske
I was thinking of doing something like that, but since I couldn't easily validate the reflectivity of the paint across the full spectrum into the far IR, I figured it wouldn't be all that meaningful.
You should assume the paint is fine: You DO assume its emissivity is close enough to 1 in the IR to make contactless IR measurement viable, which implies it absorption of energy at similar wavelengths (i.e. heat from the lamp) must also be close to 1. Absorptivity and emissivity are exactly the same thing (albedo), we only use either term to indicate purpose (i.e. absorb or emit photons).

In theory it could be such that the albedo of the paint is high in the near-IR, but low in the visible and far-IR (thus yielding too low measurements), but i see no reason to assume that.



Quote:
Originally Posted by Warske
That suggests the meter is reading high by 120.2 / 115.44 = 1.041
Which is 4.1% high.

That seems pretty good, but I think the error sources in my quick experiment could easily swamp that out, so I can't claim it as validation. *But it is interesting.
4% is a very promising result - between the size of that lightbulb, not-fully-1 albedo of the paint coat and other disrupting factors such as convection around the sensor disk, it is better than i expected it to be (then again, you could just have been lucky here).

At least it is prove of principle, demonstrating that the sensor and measurement setup does work as expected and works independent of wavelength, even far into the IR - making it usable for 1064 or 808 as well.
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