Power Meter Calibration and Comparison

they are in the spreadsheet now.

Kenom said:

they are in the spreadsheet now.

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No, i meant Pseudo's numbers.. But i see now, there is no blu ray number from Pseudo..

sorry. the only information I got from psuedo was in a post about 5 pages back where he indicated what the red was on his meter and that the bluray was dead. He then shipped it back to me to be repaired. I did NOT do extensive testing with the bluray on my meter before shipping it back out. I did though verify taht the reading was the same on both of my meters to make sure that it was accurate on both. So my 360001 sensor was the same as the one daguin is now using. Why they are showing such a difference is beyond me. We shall see as it gets passed to others and how it compares.

pullbangdead said:

Average: 70mW
Standard deviation: 7.1mW
% standard deviation: 10%

Two data points, and we already have >10% standard deviation, wow.  

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Seeing, how the red measures "the same" on both, but the blu not, it's not the metr's amplifiers, that are calibrated incorrectly.. It's the coatings.... If the coatings actually were spectrally flat down to 400nm, the difference would be the same with both lasers..

I feared this would happen. But it is strange, that it happens on thermopiles with coatings specifically made for a certain wavelength range.

I recently made a meter using a simple DIY thermal sensor for the "thermopile", and matte black paint for the "broadband coating". (The rest of the meter - circuit - was the same as with my thermopile meter, but obviously a much different gain setting) The variation between red and blu was only 1%! (a blu ray shows 1% more than a red of the same power)


Or maybe the coatings are fine, but it's some kind of dust or smoke or other sediment ON the coatings, influencing the absorption and the readings. We know, that blu rays absorb better in many surfaces. But the broadband coating of the thermopile should absorb all wavelengths more evenly than that.. Crap ON the coating could change it tho... And it could change it more for the blu than for red...

Does anyone have data on the Scientech meter's official wavelength range?

I used to have a manual for that sensor but lost it. If I remember correctly it was rated down to 400nm.

According to Coherent, the PM10-19C thermopile sensor (used in the knimrod meter) should be flat from 190 to 11000 nm.

Yeah, those thermopiles seem to be good... Mine is less than 2% off from Electron's meter... I have the exact value, and the corrected reading could be used to get an indirect reference for the blu ray laser...

Zom-B said:

According to Coherent, the PM10-19C thermopile sensor (used in the knimrod meter) should be flat from 190 to 11000 nm.

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Are you really sure it's the model number you quoted? Because it only has 6 screws on the front of it and a BNC connector, whereas the PM10-C has 8 screws on the face of it with a 25 Pin connector.

Is there a label on the back of the unit your are getting that info from? There are dozens of those Sensors from Coherent that look similar, but have different characteristics........such as a UV coating instead of a Broadband coating, and that could make a big difference when measuring BluRays.

IgorT said:

Does anyone have data on the Scientech meter's official wavelength range?

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The 36-0001's "Spectral response" range is 250nm - 35um (micrometers)

Scientech claims 3% accuracy for this calorimeter.

Peace,
dave

The other possibility of course is, that the laser changed in power. That, we might not know for sure untill it returns to Kenom...

The reason i'm thinking this is because i remembered, that it did not let itself be set higher than 75mW. That should not have happened from a new pot. Every pot goes down to zero..... That's not normal operation. Things like that always have a reason.


Kenom: Did you check the PSU output voltage, when fixing the laser?

Dave: Could you measure the PSU output voltage?

Another thing to check on the PM OEM Sensors is the Output impedance is close to 2.5k ohms.

I have some of those and I had one of them re-calibrated by Coherent and the Output impedance is dead on @ 2.5k ohms.

IgorT said:

The other possibility of course is, that the laser changed in power. That, we might not know for sure untill it returns to Kenom...
Kenom: Did you check the PSU output voltage, when fixing the laser?

Dave: Could you [highlight]measure the PSU output voltage?[/highlight]

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9.21V

Peace,
dave

Since there is a precision multiturn pot inside the heads, for calibrations, i think this may vary slightly from one thermopile to another because each may require a slightly different calibration, that's why the pot is there....

Not sure tho. I should measure mine again, i forgot what it was.....

daguin said:

9.21V
Peace,
dave

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Thank you Dave.. That's a relief!

You probably are very correct Igor, Coherent defines its as "Output impedance is close to 2.5k ohms" on their specs.

When you turn the pot, the impedance shifts. It's impossible to get both a dead--on impedance and high precision with this setup.