(Heat / Energy / Analysis / Temperature)
21 July Update: Added new graphics of actual testing and temperature-power plots. Introduction:
I wrote an article for laserpointerforums and the critics were kind to me, so I thought I would rewrite the article a weebit differently and make an Instructable. Here is the original article if anyone wishes to read it: http://laserpointerforums.com/f42/h-e-t-light-coherent-64606.html
One problem that hobbyists face is determining the power of their laser(s). Sometime we just want to know if we got what we paid for and sometimes we have two of the same and want to know which is the most powerful. With lasers, the brightest is not always the most powerful since the eye is sensitive to different wavelengths in a non-uniform manner. For review of how the eye sees laser light you can jump over to Wikipedia and do some reading: http://en.wikipedia.org/wiki/Laser_pointer
I strongly encourage everyone to read the above link; specifically the HAZARDS section. When doing any laser experimenting, wear your laser goggles that are specific to the wavelength (color) blocking for the specific laser. If you are into extreme lasers, you may consider professional goggles that block multiple wavelengths. If you invite others to witness your laser experiments or play, ensure that they have goggles, too.
Now, let's move on to what we are here for - a discussion on Laser Energy. Part One - Understanding Laser Energy & Power
The most common terminology for consumers regarding Laser Power is milliwatts, mW (proper label mW but often shown as mw.) Common dollar-store red laser pointers are under 5mW. Direct from Wikipedia:
"In the U.S., lasers are classified by the American National Standards Institute and Food and Drug Administration (FDA). Visible laser pointers (400–700 nm) operating at less than 1 mW power are Class 2 or II, and visible laser pointers operating with 1–5 mW power are Class 3A or IIIa. Class 3B or IIIb lasers (operating between 5–500 mW) and Class 4 or IV lasers (operating above 500 mW) cannot be legally promoted as laser pointers."
Laser Energy is different from Laser Power. If you absolutely have a burning desire to know the specifics, read
but I will summarize to note that Power is Energy over Time; therefore, Energy is a timeless dimension which represents the optical force of the laser beam relative to the beam radius. Sometimes I hear optical energy being referred to as beam intensity but one must be careful in the use of this terminology: http://en.wikipedia.org/wiki/Gaussian_beam
What if you just want to burn something!
The pros do their laser power measurements using relatively expensive Laser Power Meters: these start at the "hobby" level of about $100 and go up into the thousand$ of dollars for lab quality instruments. Generally, the more expensive, the "closer" the meter is designed to report similar to the National Institute of Standards and Technology (NIST). Before 1988 NIST was called the National Bureau of Standards, NBS. NIST spent nearly a billion dollars back in 2009 doing "NIST" stuff and employing in excess of 2900 employees. Your tax dollars at work. But, it is important work since without standards, how would anyone know what Watts were what?
There are two broad methods of determine laser power in the reasonable dollar marketplace: one is the optical diode and the other is the thermopile. With the optical diode, the laser beam causes a current to flow and the current moves a meter, or is amplified or conditioned and measured with electronics. The physics of using a diode are well understood and the accuracy is fair but the problem most annoying is that this type of detector must be "adjusted" to the wavelength (color) of the laser. So, expect to flip a switch, enter a menu, or do some other manual effort to inform the Laser Power Meter (LPM) that you are using an Inferred laser, Red laser, Green laser, Blue laser, etc. The thermopile has been around for well over 100 years and is also well understood. Essentially, the laser beam is absorbed by a black target which heats part of the thermopile while another part is kept unheated (fixed cold or at room ambient - essentially a stable temperature.) An electric current is produced and measured by a meter or other electronics in a manner similar to the way the diode current is measured and displayed.
Is there another way? Yes, we can use the laser to heat an object above room temperature and record the temperature rise. That temperature rise will then be related to the power of the laser. Remember those old high school physic and chemistry classes designed to teach us about hot and cold water, specific heat, and how a calorimeter worked? Another old device: http://en.wikipedia.org/wiki/Calorimeter
If we work with a uniform object and we have access to a reasonably accurate thermometer and a laser power meter (LPM) to calibrate our uniform object, we can then use the thermometer and a few conversion charts or a little math and derive the power of our laser without needing the LPM anymore. (Most colleges have LPM in the Physics department and some controlled access may be granted to the community by the department head. Some laser clubs have get-togethers where a LPM is brought into the meeting for the purpose of calibration or power rating unknown lasers.) In fact, if we were really good at manufacturing our uniform laser targets to exactly the same specifications each time, then we could just photocopy the charts or put the formula out on the Internet for everyone to use. Of course, uniform in this context would essentially represent "identical", at least to a couple of percent. Part Two: I am not the first (to try this simple feat)
Back a few years ago, a fellow laser enthusiast published an article on Laserpointerforums on building a cheap homemade LPM. His idea was to use some common aluminum foil from the kitchen, some black paint, and a cheap handheld under $20 digital IR thermometer and some very simple math. Wow... a lot of forum noise ensued since our good experimenter was unable or unwilling to divulge the elaborate physics and math used to derive the simple solution. I will end my comments of this brave pioneer and just say, "Rest in Peace..." I assume he went on to do great things and just kept them to himself afterwards.
If there is a lesson to be learned about forums, it is if you post something controversial, you better be able to prove it. 'nuff said.
I took a different approach, an approach much like Edison and others who experimented and experimented and experimented until they understood their experiments. Yes, there are a lots of "experimented" in that last sentence, but essentially this is how much of the old science was conducted. Einstein and others did their experimenting at the formula level instead of in the lab. Personally, I find that somewhat boring... I really like my laboratory, there is a kind of peace in my little fortress of solitude. Part Three - Experimenting, the quest for making a laser target
I published an Instructable not long ago about making a recording (logging) thermometer. This is a highly accurate device which is accurate to 0.02C degrees and is factory calibrated. Part cost is under $30.
Now, armed with this sensitive device, I set out to create a laser power meter. I needed a target and there are a few criteria which must be met:
- Target should be simple to create and duplicate and cheap too!
- Target must absorb various laser wavelengths uniformly - that is, act like a black-body object
- Target should emit the absorbed energy uniformly (emissivity near 1.0)
- Target should be stable and allow repeated testing to produce similar results
- Target should be thermally stable from less than 5mW to at least 500mW (testing to 750mW)
I tried the aluminum foil target that was mentioned a few years back. It made me crazy; certifiably crazy. I used scissors, hobby knife, another hobby knife with a new blade, even a real straight-edged razor blade all to no avail. Aluminum foil is strange stuff... I wonder what marvelous machine is used to put that stuff smoothly on a roll? Like a paper road map, once unfolded, it never goes back unwrinkled. I tried a can of black Rust-Oleum(R) and I tried plasti-kote(R) and they both did not work as I had hoped, but the BBQ paint from plasti-kote(R) did best and it was repeatable but getting a uniform coating was difficult and keeping the paint off myself was impossible. Eventually, I found another product which I could brush on and used it for the final testing that I published in the laserpointerforums and which I will repeat here. I used glass cover-slips designed for covering glass microscope slides for the base target material. The results are fantastic, in my opinion, and cheap. Part Four - Smoke Test
There was some smoke at high laser powers using the BBQ paint; I expect that I had not let the product cure long enough- like a kid, I get anxious. The test run with my Arctic Laser and the aluminum foil coated with BBQ paint went over 500ºF before I destroyed the target; which was not my goal, although it was kind of neat... after I took the battery out of the smoke detector. Analysis; the foil was simply too thin to have sufficient mass to be thermally stable with 750mW of pure optical energy from my Wicked Laser.
Subsequent testing with the glass slips and carbon black lacquer coating yielded excellent results and a more moderate temperature range of around 225ºF for the 750mW blue laser. Wow! I repeated it and repeated it with the same readings. I used a real LPM to test the power and it was right on at 750mW. I tested each laser, first with the LPM, then with the "logging" digital thermometer, and again with the LPM to ensure that I had not consumed too much battery power and caused a false reading.
Part Five - Results 07-21-2011 Update:
I have added new graphics showing the temperature rise and power meter measurements. Theses tests were made utilizing a new target design, so please do not directly compare the "picture" results with the results in the chart below - thermal rise from the laser energy is based on many characteristics: convection, conduction, and radiation. With the new targets, I increased the conduction by design to ensure a lower thermal rise-time and ultimately a lower temperature. I was able to do this because of the extreme accuracy of the IR sensor used (0.02K degree). Note that for 'damping' reasons, the software in the PICAXE limits the serial output to 0.1K - However, the end-user can change this software easily since the source is freely available.
For this article, I ran 7 tests with various lasers.
Power nm Color Measured mW (LPM)
5mw 650 Red 5
30mw 405 Violet 28
30mw 532 Green 39
50mw 532 Green 50
200mw 660 Red 197
200mw 808 IR 217
750mw 445 Blue 750
Putting all of this into Excel and graphing, I got the results shown in the attached images. I ran the equation out to 3 terms, but using the one "calibrated" test target, I can now easily check laser power by simply measuring the temperature rise of the black glass target with my highly accurate logging thermometer and I can do this for a variety of common laser wavelengths knowing that my results will be accurate. Plus, I have all of the results saved in my PC for later use; for example, I can always go back and retest a specific laser if I think that the diode has been degraded.
There are three ways that thermal energy is dissipated: convection, conduction, and radiation. Because there are components of convection and conduction and because these can change with drafts, air density (elevation), and other variables, it is important to understand that the tests have some variables that are environmental in nature and representative results require identical conditions to the conditions at calibration. However, the largest component of heat dissipation in our tests is the radiation component, so reasonable accuracy should be achieved - adequate for hobby use.
For your safety:
Please do not use any shiny material around laser beams. I know my picture shows shiny aluminum foil, but I was wearing my laser protection eye wear and I have since redesigned the front area to be all-black with no reflections.