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Thread: New Budget Laser Power Meter in the Works - Future GB material. 1-800mW

  1. #31

    Default Re: New Budget Laser Power Meter in the Works - Future GB material. 1-800mW

    OK, the review is done and the Group Buy thread is posted!

    I have some good news, this meter seems to have turned out very nicely. A big thanks to Tony from Andover Holography for working with me concerning the specifications.

    The LPM-1


    It's a nice looking unit with an easy to use menu and silver buttons, but who cares how it looks? How does it WORK?

    First, you select the wavelength you want to measure from the menu, including 473, 532, 635, 660, and 808nm options. Once selected, the display is continuously updated at 2Hz with the measured value. The peak value is recorded and can be reset with the push of a button.

    The really slick feature for us experimenters that none of the other portable meters have is the serial datalogging option. Automatic power-down can be turned off in the menu for long datalogging sessions.

    Example datalogging plot made using the meter and excel:


    It's a very easy feature to use and enables the tweaker/experimenter to monitor the output of their laser over time. The datarate is 57600-8-n-1. Samples transmitted every 0.5 seconds.



    Now, I was skeptical about the accuracy claims at first when I read about this meter, is uses a monocrystalline solar cell as the sensor, and relies on I->V conversion for the measurement. After I got the temporary test unit I tested the linearity and accuracy by comparing its reading with that of my Scientech calorimeter laser meter. (I used a red diode from our group buy for the test) The results were impressive:


    Linearity test comparing calorimeter laser meter reading with the LPM-1:


    result: less than 2% error (within the accuracy of my control standard) within the specified range. So there's no problem there.


    Here's a list of features:
    · Autoranging measurement of light power in 4 ranges from ~100 uW to 100mW (800mW with ND8 option)
    · At least 5% accuracy over native range
    · Multiple sensor calibrations for different wavelengths :473,532,635,660, and 808nm selectable in a menu.
    · 16 character by 2 line alphanumeric display
    · 3 Pushbuttons provide control of measurements & configuration using a simple menu system.
    · Automatic 5-minute power down to extend battery life (can be turned off for datalogging)
    · RS232 datalogging capability
    · Peak hold remembers the peak reading in a series
    · 2Hz (2 samples/sec) update and logging rate, 400Hz internal sample rate
    · Sensor at top of meter for easy, safe readings
    · Rugged surface-mount component construction
    · Operated on a 9V battery
    Optional features:
    · Range extending ND8 neutral density filter included and calibrated
    · RS232 datalogging pre-built cable OR
    · RS232 datalogging DIY kit
    · Backlit display

    The kit schematics for the RS232 interface will be posted free for those who can build their own.


    So, check out the group buy thread if you're interested.
    Last edited by dr_lava; 02-15-2007 at 03:46 PM.



  2. #32
    Enlightened erckgillis's Avatar
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    Default Re: New Budget Laser Power Meter in the Works - Future GB material. 1-800mW

    Nice meter better than my DIY. I'll take one from GB!

    .You can make your own for free...most optical mice have sensitive laser photodiodes. Techniques for calibration for various wavelengths and conversion factors exist.

    see

    http://www.cat.ernet.in/others/lasinfo/p-meter.html

    make you own with a diode of the same laser color and use the PNP junction effect to measure resistance to current changes in diode as power changes.

    basic physics

    Good enough for free DIY'ers

    Someone can baseline a known "Lab certified" Laser and then measure on these rigs. From that or several other known lasers we can get fixed measures for conversion factors.

    E


    Here are some comments on these approachs:

    (From: Bill Sloman (sloman@sci.kun.nl)).

    The important thing to note is that a photo-diode actually detects photons, not power. Up to about 850nm, each photon actually reaching the diode junction generates one pair of charge carriers. A 425nm photon, carrying twice the energy of an 850nm photon generates the same pair of charge carriers, so the same current represents the absorption of twice the power.

    Since the 425 nm photon has rather less chance than the 850 nm photon of actually surviving the trip down to the diode junction, so the actual ratio is closer to 2.5:1.

    Above 850 nm, the photons haven't got quite enough energy to separate a pair of charge carriers, and can only separate those that are already somewhat excited. The proportion that are sufficiently excited depends on temperature. A electric field also helps, so biasing the diode increases it sensitivity to long wavelength photons. As the wavelength rises above 850nm the extra energy required to separate the charge carriers also rises, so the proportion of 'sufficiently excited' carriers declines quite rapidly.

    In principle one could build a wavelength correction into the power meter, but you would need to add a wavelength sensor to the power meter to make it a useful feature.

    The Centronics data book gives a typical spectral response for the 5T series diodes, which effectively gives you the inverse of the wavelength correction function, albeit with rather low precision. The alternative approach is to use a sensor which responds to the heating effect of the laser beam. These exist, but what you win on wavelength independent calibration, you lose on sensitivity and zero stability - in effect you have built a thermometer to measure the heating effect of your laser beam on a more or less thermally insulated target. Unless someone has done something very neat in this line, it doesn't strike me as a practical proposition for your application, granting your limited budget.
    Last edited by erckgillis; 02-15-2007 at 10:56 PM. Reason: graphic text

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