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Thread: Lasercheck Readings

  1. #21
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    Just tried using my prism, it's a load of poop.
    It will make a nice ornament but it isn't any use as an optic, the faces are slightly 'orange peeled' so I can't get a sharp beam out of it .

    I did have another thought though, why not set the lasercheck to 808 and then 1064 and take the readings and subtract them from the 473 reading!.
    I only read a couple of mW on each, is that good?

    Jim

  2. #22
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    Jimbo;

    I see where you're going, but it won't work that way. Let me try to explain:

    The problem is that the sensor for the lasercheck wand is a broadband sensor. (It is sensitive to all wavelengths.) When you change the wavelength setting, all you are doing is changing the calibration curve in the electronics. You are not installing filters in front of the sensor; it remains unchanged. Also, remember that the sensor will react differently depending on the wavelength of light that strikes it. (The response curve with respect to wavelength is not linear.)

    As an example, let's say that 100 mw of 650 nm light striking the sensor produces a current of .1 milliamps. Now, if you hit the same sensor with 100 mw of 488 nm light, you might end up with a current of .3 milliamps. See the problem? This is why you have to change the wavelength each time you measure a different laser. If you don't, the power reading will be wrong. (Sometimes it will be *very* wrong, because of the non-linearity of the sensor mentioned above.)

    Remember that the lasercheck is designed to measure a *single* wavelength only. The calibration curves are designed (and calibrated) for single wavelengths only. So when you try to measure a beam that has multiple wavelengths, you will NOT get an accurate reading, because no matter what you set the wavelength to, some of the light striking the sensor is going to be of a different wavelength and thus the calibration will be off.

    In your example, by setting the lasercheck to 808 nm, all you are doing is measuring the total power of the beam and then using the 808 nm calibration curve. You are not filtering out the 532 nm light; it is still striking the sensor and contributing to the sensor's total output. (The same thing applies to your other example when you set the lasercheck to 1064 nm.)

    The only accurate way to use a lasercheck to measure total power in a multi-line beam is to first separate the beams into individual wavelengths and then measure each wavelength separately. Unfortunately, since your prism is obviously messed up, that isn't an option for you right now.

    I should point out that for lasers that have a narrow band of output wavelengths (say, a multi-line argon that is only making 5 lines from 512 nm through 456 nm), it is possible to average the difference between the lines and set the lasercheck to a middle setting (like 500 nm, or even 490 nm) and take a reading that way to get a "ball park" estimate of the total power. But you have to remember that this reading is NOT accurate, and may actually be off by 30 to 50%. But it's better than nothing.

    However, in the case of a dpss laser, you're dealing with a spectrum from 1064 nm all the way to 532 nm, which is way too wide a band to just use an average setting like that. The other factor is that the percentages of total power at specific wavelengths is far more closely defined with an Argon laser vs a dpss laser. A multi-line argon will make roughly 47% of the total power at 514 nm, and another 35% of the total at 488nm, so splitting the difference at 500 nm is a pretty close estimate. DPSS lasers, on the other hand, can use from 3 to 10 times their optical output in 808nm pump input, so depending on the quality of the optics you could have very little or a whole lot of 808nm in the output beam. The same goes (to a lesser extent) with the 1064 nm IR in the output. The ratio's are based on the quality of the optical coatings and the efficiency of the crystals, not to mention the presence or absence of a final IR filter, and will be different from laser to laser.

    Does that help explain it some?

    Adam

    Edit: oops! I just noticed that you were talking about a 473 nm blue laser, not a 532 nm green laser. ops: So you're not going to have 1064 nm IR leaking, but rather 946 nm IR... Still, the same problem exists in that you don't know the efficiency of the optics, plus you have a super-wide band of wavelengths, so you can't just average them and expect to get an accurate reading with a single measurement using your Lasercheck.

  3. #23
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    Yeh, a great explanation and a totaly logical one too.

    It looks like I will have to buy a new prism and make sure it is suitable first.

    Jim

  4. #24
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    Apr 2006
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    There's an easy way of blocking the visible part of your laser. Use a piece of exposed photographic film, like the beginning of a roll of negatives, you know, where the negative is all black. Use several layers if necessary. The film will block most of your blue or green light, yet let the IR light through. The problem is, it'll let both 808 and 1064nm through, so you can't have an accurate reading for each wavelength, but at least you'll know if your laser is producing IR. Another potential problem with this approach is that the laser, if powerfull enough, may melt the plastic and render this procedure useless.

    In case someone doesn't have a laser check, you can use a digital camera with night vision capability, like the handycams. Put it in nightshot mode, put several layers of the film in front of the laser and point it towards a wall, where the camera might see the IR beam. Don't point your laser towards the camera, as you may burn some pixels on the CCD.

    Just my $.02 on this subject
    Remember the future?, That'd today, as you imagined it yesterday.

  5. #25
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    Mar 2006
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    If you have a small TEC (Peltier) and a large block of aluminium, and a red diode laser or other good single-line source of at least 50 mW, preferable more than 100 mW, you can get round the problem of multiline measurement, even without calibration of the heat sensor.

    Stick the TEC cold side onto the block, paint the other side black with something you can remove later (with acetone) if you want to recover the TEC for other uses. If you have some carbon powder (soot, activated charcoal), load some of that into the paint to make a deep matt black. Use something as a cowl to shade the TEC from ambient heat. A toilet roll tube will do, don't let all that old Blue Peter technology go to waste. Connect a millivolt meter to the TEC wires. Let it settle. Note the reading once it is steady, ignore the fact that it might not be at zero; scale and offset error don't matter, only stability is important here.

    Measure the diode output with the Lasercheck, then point the beam at the blackened TEC. As soon as the new value of millivolts is steady, note it after subtracting whatever reading was there before. Now point the multiline beam at the TEC and measure it. The proportions are linear, so you can divide the new millivolt reading (after subtracting initial offset) by the red diode's reading to get a ratio value, then multiply that by the actual red diode power to get the unknown multiline power value. This will be accurate to the same spec as the Lasercheck's single-line accuracy. What error there is in the TEC method will be a lot smaller, so long as you had stability during every measurement, and so long as your black TEC was really black.

    Another advantage to the TEC method is the wider surface area. If you are trying to catch small divergence after passing through a prism to separate beams, you can still catch a whole beam spot after much greater distances than the Lasercheck will let you use. You'll have losses in the prism, but apart from that, the Lasercheck accuracy will be the base accuracy for the whole show.

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