A homemade power meter would be a simple project. Calibration would be the issue. I wounder if something like a 25 watt light bulb could be used as a standard reference? How close to 25 watts are they? I was thinking if you placed a black card over the bulb with a small hole and measured exactly 1 foot away that may be enough to get a calibration.
Another idea is to find a cheap laser pointer that is consistant in output in every unit. The ones I have are pretty random.
Is there any other way to simply calibrate a meter?
measuring output power
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wler
measuring output power
Well, I am afraid there is no simple way, and there is a good reason why the commercial power meters are soo expensive.
Having been regularly defeated on ebay, I was also pondering about building one myself, and in fact have been already looking into it. I think calibrating a sensor with photodiode can only be done by comparison with a calibrated sensor, if you want to have an accuracy of better than say 50% or so. And this needs to be done for each wavelength separately, and even doesn't work for multi-line lasers.
There is one method, however, which would give an absolute calibration for free, namely using a thermistor with the "DC-substitution" method.
Roughly speaking the temperature is kept constant when illuminating the thermistor by appropriately reducing the DC power that heats it, and this reduction in DC power can be measured very accurately.
The optical accuracy primarily depends on how much
light is truly absorbed by the thermistor,
but the good thing is that the true laser power is guaranteed to be more than what you measure, which means one is always at the safe side.
I have no idea how well this would work with a home-made device, but I have already planned a circuit and will give it a try at some point.
Having been regularly defeated on ebay, I was also pondering about building one myself, and in fact have been already looking into it. I think calibrating a sensor with photodiode can only be done by comparison with a calibrated sensor, if you want to have an accuracy of better than say 50% or so. And this needs to be done for each wavelength separately, and even doesn't work for multi-line lasers.
There is one method, however, which would give an absolute calibration for free, namely using a thermistor with the "DC-substitution" method.
Roughly speaking the temperature is kept constant when illuminating the thermistor by appropriately reducing the DC power that heats it, and this reduction in DC power can be measured very accurately.
The optical accuracy primarily depends on how much
light is truly absorbed by the thermistor,
but the good thing is that the true laser power is guaranteed to be more than what you measure, which means one is always at the safe side.
I have no idea how well this would work with a home-made device, but I have already planned a circuit and will give it a try at some point.
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Tom B.
measuring output power
Interesting approach. Just as a reality check I tried lighting up a tiny 10K thermistor with a laser pointer while measuring its resistance. I was able to see a noticeable and repeatable drop - from 9.7K to 9.25K, so a reasonable signal can be expected. Naturally, the reading was sensitive to the angle and position of the spot - the thermistor body was shiny black epoxy. If the practical difficulties with coming up with a very repeatable way of coupling light energy to the thermistor can be overcome, something like this might indeed be useful for calibration.
-
wler
measuring output power
Yes, that's the difficult point: to get all the radiation in, and the thermal stability under control.
Dependence on the position of the beam, and uncertainties due to internal reflections/inhomogeneities is unfortunately unavoidable, but I think this is even worse with photodiodes.
Many commercial laser and microwave power meters in the range ~0.1-300mW work in this way, the advantage being
not sensitive to wavelength
(eg microwave power meters can typically cover 100khz-18Ghz in this way). Usually one has a wheatstone bridge or two and a reference thermistor in the same case for temperature compensation. Such a thing is sensitive to temperature differences of a fraction of a degree, and the accuracy can be below a percent.
For inspiration see chapter III and especially
Fig 3-2 on p 21 of:
http://literature.agilent.com/litweb/pdf/5965-6630E.pdf
(it's 2.5MB, watch out).
However they use very delicate constructions that a hobbyist won't be able to build.
Obviously the problem gets more difficult when trying to measure small powers, lets say less than a few mW.
I personally like to aim for something that would work until a few 10's or 100's mW. There are various sorts of thermistors available (I checked the data of the Epcos thermistors sold by Farnell), and the main part would be to play with a few of them to
see which works best and especially how it should be thermally mounted. The circuit itself would be rather simple, essentially given by the left part of the above-mentioned figure.
Anyway, a nice project but the expectations shouldn't be set very high...
On the other hand, a photodiode meter is very simple (one is described in Sam's FAQ which I have slightly modified and which works fine) and very sensitive and extremely linear at low intensities, but absolute measurements are next to impossible. Relative measurements are fine, for example determining reference/object beam ratios or optical densities.
However even these are not more accurate than say 10-20% because of dependence on the lighting, etc. That is, I was trying the measure the attenuation of some beam splitter, and one day the intensity ratio is consistently 55%, next day with more or less the same geometrical setup it is 60%, etc.
I don't think that one can do much better than that, but for hobby purposes it is anyway not that important.
Dependence on the position of the beam, and uncertainties due to internal reflections/inhomogeneities is unfortunately unavoidable, but I think this is even worse with photodiodes.
Many commercial laser and microwave power meters in the range ~0.1-300mW work in this way, the advantage being
not sensitive to wavelength
(eg microwave power meters can typically cover 100khz-18Ghz in this way). Usually one has a wheatstone bridge or two and a reference thermistor in the same case for temperature compensation. Such a thing is sensitive to temperature differences of a fraction of a degree, and the accuracy can be below a percent.
For inspiration see chapter III and especially
Fig 3-2 on p 21 of:
http://literature.agilent.com/litweb/pdf/5965-6630E.pdf
(it's 2.5MB, watch out).
However they use very delicate constructions that a hobbyist won't be able to build.
Obviously the problem gets more difficult when trying to measure small powers, lets say less than a few mW.
I personally like to aim for something that would work until a few 10's or 100's mW. There are various sorts of thermistors available (I checked the data of the Epcos thermistors sold by Farnell), and the main part would be to play with a few of them to
see which works best and especially how it should be thermally mounted. The circuit itself would be rather simple, essentially given by the left part of the above-mentioned figure.
Anyway, a nice project but the expectations shouldn't be set very high...
On the other hand, a photodiode meter is very simple (one is described in Sam's FAQ which I have slightly modified and which works fine) and very sensitive and extremely linear at low intensities, but absolute measurements are next to impossible. Relative measurements are fine, for example determining reference/object beam ratios or optical densities.
However even these are not more accurate than say 10-20% because of dependence on the lighting, etc. That is, I was trying the measure the attenuation of some beam splitter, and one day the intensity ratio is consistently 55%, next day with more or less the same geometrical setup it is 60%, etc.
I don't think that one can do much better than that, but for hobby purposes it is anyway not that important.
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Colin Kaminski
measuring output power
I was using my ohm meter and a thermistor to try to figure out a relationship between stability and temperature before I was using the TEC and found that the ohm meter caused the thermistor to heat up simply by passing current through it. What can be done about this resistive heating?
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wler
measuring output power
well let flow less current through it, and use instead
a sensitive micro-amp meter to measure the current.
a sensitive micro-amp meter to measure the current.