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Simple Laser Power Meter Using IR Thermometer

Warske

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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.

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Next: Parts List and Construction

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Warske

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Parts List and Construction

Code:
$12.99 - IR thermometer.
      [url]http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93983[/url]
      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: [url]http://www.harborfreightusa.com[/url]
      (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/ctaf/displayitem.taf?Itemnumber=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/emails/display.do?fname=18_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]

[u]Circular Laser Target[/u]
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.

[u]Aluminum Foil[/u]
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.

[u]Nylon Fishing Line[/u]
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.

[u]Glue[/u]
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.

[u]Plastic Box[/u]
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 ([url]http://www.michaels.com[/url]) 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.

[IMG]http://laserpointerforums.com/laser_pointer_forums_3/forum/attachment.php?attachmentid=20807&d=0[/IMG]

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Kenom

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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.
 

lasersbee

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

Very well explained.... This looks easy enough to be built
by practically anyone.... Well done.. :cool:

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

Jerry

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

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Kenom said:
...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.

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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Your'e welcome.
;).


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

-greg
 

lasersbee

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Rangedunits said:
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... :cool:

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

Jerry

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


-Greg
 

lasersbee

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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

You can contact us at any time on our Website: J.BAUER Electronics
 
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lasersbee said:
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... ;)
[highlight]
And yes... I have already ordered an IR Thermometer.... ;D[/highlight]

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



This thread definitely need more attention.

-Greg
 

lasersbee

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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

You can contact us at any time on our Website: J.BAUER Electronics
 
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691175002

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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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