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Thread: At home method to test power density

  1. #1
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    Default At home method to test power density

    You will not be able to determine the spectra response from this, but you can determine power density in an approximate manner. I have not verified this method, but have compared it to the acnelamp result. At a later date, I should be able to compare this with test data using an optical power meter.

    There are 3 steps.

    1. Determine the power density at the source.
    2. Determine the power density at the target.
    3. Compare your result to published data.

    Step 1.
    http://heelspurs.com/led.html has a method to determine the power density. This is only approximate and not verified. I believe it is based on the conservation of energy. You can read how it was done, but here are few derivatives that I took.
    - Obtain two foam coffee cups. For the first one use a caliper to measure exactly 2cm. You can scribe a line on the foam cup very easily and it will be fairly accurate.
    - Then scribe another line about 2.5-2.75mm and cut the cup along this line.
    - I first used a black marker to cover and relatively large sheet of alum foil black. Then carefully fold the entire piece inside of the cup, bottom and the walls, making sure not to leave air pockets between the cup and foil. You can cut the foil a bit to now see the scribe at 2mm.
    - Make sure the top is very flat by turning it upside down and working it some.
    - I placed this cup inside another cup that was cut down to make sure the original cup's top edge was still exposed. This provides additional insulation. Remember, you are trying to use conservation of energy so any leakage will affect the result.
    - I placed the LED array on top of the cup and then covered the area with saran wrap to ensure that you would not get airflow between the unit and cup.
    - Other than that, its basically like heelspurs.

    Now, one important thing.
    My results were coming out ridiculous at first. But a simple calculation will show you that the equation he uses is not correct. He says that he got 100mW/cm^2, but if you insert this into his equation, you would find it would need a delta of 7142 deg C, obviously erroneous. So I manually derived the equation and found that the error was that it should read 1W/cm^2 = 2 cm x 4.2 x C / 600.

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    So after you have obtained "C", the power density is 14mW/cm^2 * C.

    For my experiment with the acnelamp, "C" was equal to 2.0 and therefore I expected to have a result of 28mW/cm^2.

    Now let us look at the acnelamp response:
    http://www.acnelamp.com/images/acnel...bletop_red.png

    What was just measured was actually the total power/cm^2. This should be compared to the area under the curve (or integral) of the spectrum response. Extrapolating the curves in an approximate basis I got .5bh = .5 (674-632)1950 = 4200 microW/cm^2 or 42mW/cm2.

    I repeated the test twice and got the same result. The measured result was 33% less than the stated values, but many approximations and sources of error have been made along the way. I found this to be an acceptable result considering the level of testing.

  3. #3
    Senior Member Twickle Purple's Avatar
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    Are you able to also include the pics that Scott over at Heelspurs has showing this test, or has he taken those down now?

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    Step 2.

    Now that you have the power density at the source, you will want to estimate the result at the target. You can use a simple square root approximation, but this could give you some error for a large array. It really is based off of a single point. So, first measure the area of the array that you are using. Then shine the light on a piece of cardboard, or whatever, at the distance you want the unit from the target. Measure this area. Since the light will diffuse at the edges, I used the "ring" that was halfway between where the light was strong and where it died out. Now divide these two areas (should be less than 1.0) and multiply that value by the vale you measured in step 1. That is the target power density, at least in an approximate sense.

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    Step 3.

    Now you want to compare the result to research data. This is a tricky thing since we do not even know what is good for rosaceans. But here is what you can do now. You wish to know the total energy/cm^2. So multiply the result you found in step 2 (make sure this time to use the answer in W/cm^2) by the amount of time (in seconds) that you wish to use the unit. This will give you the number of Joules / cm^2.

    According to some researchers, the number of Joules/cm^2 is what you are after so if your power density is low you can compensate by bringing the unit closer or increasing the amount of time of exposure.

    Now compare your result to other researchers. I have some data, but want to do more research before I fill this box in. Everyone who obtained research papers can share thoughts on what the proper energy ranges might be.

  6. #6
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    Another method.

    IowaDavid stated that one might try to rent a power meter from a camera shop. This sounds like an excellent idea. In that case, proceed directly to step 3 after you have obtained the results. But really...where is the fun in that? haha.

    So two methods are presented that would enable one to determine the power density at the target source. It would be great if we could create a database from the different units.

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