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FrozenGate by Avery

5% difference in readings between two LPMs

snoutke

snoutke

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Nov 1, 2010
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Hi,

This question is addressed mainly to forum members MarioMaster and LaserBee. But of course other members who feel they have the knowledge and experience are invited to comment as well.

I have both LPMs, LaserBee USB and the RadiantAlpha.

While doing measurements, I noticed a systematically reading difference between the two LPMs. The Alpha readings are 5% lower (in average) than the LaserBee readings. Assuming both LPMs are calibrated, my question is whether this is normal, and if so, which LPM should I believe?

Also, it seems that according the RadiantAlpha, my lasers are more stable, meaning, they fluctuate less compared to the LaserBee. So again, where is the truth here? Do my lasers fluctuate or not?

And one more thing: The RadiantAlpha has this zero knob. For some reason, it never reaches zero, even if turned to the minimum. It will always show a reading of .002 or .001. Maybe this is because wheather diferences between my country and the original country it was calibrated in, i.e., the wheather in my country is warmer and therefore even in the minimum zeroing it shows .002 or .001?

I'll be glad to hear your imput about these issues.
:thanks:
 





Sounds like normal calibration variance. Unless you had an LPM which has been known to be recently calibrated by a professional company like Coherent, then it's hard to say which one is closer.

If your Alpha will not settle at .000 make sure that there are no ambient sources of heat like sunlight, air vents, etc. If the meter still does not zero, you could adjust the internal zero calibration by adjusting the pot marked 101 on the pcb. (this will not affect the power calibration)
 
Last edited:
MarioMaster said:
If your Alpha will not settle at .000 make sure that there are no ambient sources of heat like sunlight, air vents, etc. If the meter still does not zero, you could adjust the internal zero calibration by adjusting the pot marked 101 on the pcb. (this will not affect the power calibration)
Click to expand...

Thanks for your prompt response.

Instead of adjusting the pot marked 101 on the pcb, can I just subtract 1mW or 2mW accordingly from the readings? Will it still be considered as accurate readings?
 
Yeah... like MM said 5% is well within LPM tolerances....
and even more so for Hobbyist LPMs..
The LaserBee A and the Alpha would probably be closer
since they both use a basic amplifier.

The only difference I can see between the readings of
the LaserBee USB and the Alpha might be that the LB-USB
uses a micro-controller and we are able to compensate for
our coating's non linearity in the Firmware.

That being said... you would need to check each of your
hobbyist LPMs against a recently calibrated Professional
LPM to see which one is closest.


Jerry
 
Last edited:
snoutke said:
Thanks for your prompt response.

Instead of adjusting the pot marked 101 on the pcb, can I just subtract 1mW or 2mW accordingly from the readings? Will it still be considered as accurate readings?
Click to expand...

Yes, this will still provide accurate readings.
 
MarioMaster said:
If the meter still does not zero, you could adjust the internal zero calibration by adjusting the pot marked 101 on the pcb. (this will not affect the power calibration)
Click to expand...

Is it a component colored blue, marked as W101 with the number 60 under it?

And if so, do I adjust it by rotating the tiny screw that is on it?
 
What about the other question about response timing ?
Is the Laserbee faster responding and that shows a slight fluctuation or could that be the amplifier adding artifacts to the reading ?
:thinking:
 
Fact is that the Laserbee response time is faster than the Alpha.

I can see that both when I measure a laser and when I let the sensor cool off.

It takes longer for the Alpha sensor to reach zero after measurement than the Laserbee.

What are the implications of that?
 
The 15mm X 15mm Sensor we chose for the LaserBee A and the
LaserBee 2.5W USB was chosen after buying ~30 different TECs
and meticulously testing their surface linearity and response times.
Then chosing the one brand/model that gave us the fastest response
time to use in the new budget LPMs.

You must remember that the lower cost Alpha and the LaserBee
LPMs that use a 15mm X 15mm sensor will have a slower response
time (due to thermal mass of the sensor) than our other LaserBee
models that use our proprietary 8mm X 8mm sensors that have a
response time at ~10 seconds.


Jerry
 
Having never seen either in person it sounds like there might be more thermal mass in the Alpha. This would give a smoother response and a much better result for marketing or sales of the tested unit.
The Laserbee would be more real time with it's quick fluctuations possibly revealing short term issues. (Like temperature effects building up in drivers or such.) A better tool for the builder who is needing the extra seconds when pushing a unit to it's break-down point.
Just my thinking on it though.
As I said, I've never even seen what these things look like.
:undecided:
 
It also has to do with the spacing between plates.... the number
of P/N thermocouple junctions.... the thickness of the plates
and the broadband coating on the plate surface...
All these things can increase/decrease response time...


Jerry
 
Indeed having a fast response time is desirable, Radiant already has a system in place for our next generation meters that have vastly improved response time.
 
I have both too and i think they are both accurate but the laserbee rock because it can make accurate graphic too :na: .

For people don't have always a computer i think the best for price is the radiant :)
 
Now that I'm testing my Alpha and comparing it to the LaserBee, I can report that the Alpha sensor is more sensitive than the the LaserBee sensor. In fact, it's so sensitive that if I approach my Alpha from distance and sit beside it (to start doing a measurement, for instance), it immediately effects the reading by adding 1 or 2mW, so that I have to sit still and not move if I want to make an accurate measurement. Even if I only move my hand, it can change the reaeding by 1mW.
 
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