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Extend the LPM range with a filter.

Blord

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I have a basic LPM from Bluefan and this meter has a range of 1mW-1999mW.
I was thinking how to use it with all these 2+ Watt laser popping up.
Then in a moment I thought about the filters used in astronomy. Astronomy is my other hobby and I have telescopes and accessories laying around.
The filters I used are high-quality color filters made of optical glass. I use the blue filter no. 80A. I tested the transmission of this filter with a 445nm laser.
Unfiltered the LPM reads 1538mW and filtered the power was lower at 1330mW. The transmission was 86% with this filter.

Now the real test with a 2+ Watt laser. It measures 1796mW with the filter on. I calculate the output is 1796/ 86%= 2088mW !
So I extend the reading of this LPM beyond the 2 Watt.
Let me know what you think or if there is a flaw in the situation. :thanks:




There is a seconde weaker dot next to the sensor. It appears that the dot is reflected back to the polished heatsink and then back thru the filter on the LPM heatsink. So the actual reading should be higher. :)
 



cilegray

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I have a basic LPM from Bluefan and this meter has a range of 1mW-1999mW.
I was thinking how to use it with all these 2+ Watt laser popping up.
Then in a moment I thought about the filters used in astronomy. Astronomy is my other hobby and I have telescopes and accessories laying around.
The filters I used are high-quality color filters made of optical glass. I use the blue filter no. 80A. I tested the transmission of this filter with a 445nm laser.
Unfiltered the LPM reads 1538mW and filtered the power was lower at 1330mW. The transmission was 86% with this filter.

Now the real test with a 2+ Watt laser. It measures 1796mW with the filter on. I calculate the output is 1796/ 86%= 2088mW !
So I extend the reading of this LPM beyond the 2 Watt.
Let me know what you think or if there is a flaw in the situation. :thanks:




There is a seconde weaker dot next to the sensor. It appears that the dot is reflected back to the polished heatsink and then back thru the filter on the LPM heatsink. So the actual reading should be higher. :)
Just be careful not to reflect the dot back into the diode.

And I think that 'dot' that's being reflected back and back again, is your 14% that's blocked.... Seems to me like your lens is reflecting back some of your beam, not absorbing it. It should still work just fine at expanding your lpm's range, just as long as the wavelenght doesn't vary too much on your different diodes, it should keep reflecting the same amnt of light back.
 
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Blord

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The filter is only letting 86% thru and some part is blocked and the rest is reflected. But that doesn't matter, it is the 86% that matter.
I did try with the red filter but that thing block almost 100% :)
 

Benm

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There is nothing wrong with using a filter to scale the range of a power meter, as long as the filter isnt damaged by the laser beam - don't try this with plastic filters, ever.

The most useful filter do do this is a neutral density filter though, since you do not need to measure or investigate the attenuation factor for a given wavelength. You could just insert a ND2 filter in the path to reduce the power by half, or a ND10 to reduce it tenfold, for example.

You could try those yellow, green and red filters to see if you can extend the range further, unless you're affraid they will be damaged... 2 watts is a lot to handle for a filter that was not designed to withstand and power at all.
 

joeyss

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You could always diverge the dot..size dosen't matter as long as it all hits the lpm and all gets through the filter.
 

Blord

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The reason to use the blue filter is to minimize the stress on the filter. This filter only take a little of the laser output and therefore less power is absorbed by the filter.
 

Cyparagon

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I'd suggest using a diffraction grating instead, or perhaps just use the reflection from a pane of glass. That way there is no need to dissipate heat in the optic.
 

MadEye

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The most useful filter do do this is a neutral density filter though, since you do not need to measure or investigate the attenuation factor for a given wavelength. You could just insert a ND2 filter in the path to reduce the power by half, or a ND10 to reduce it tenfold, for example.
This is true, but ND filters dont work that way. They let light through by 1/(10^ND)... So ND1 already is 1/10 while ND2 filters let only 1/100 through... It works the same way as OD with laser glasses...

edit: Ok thats not true that way, sorry. Looks like ND4 is really 1/4... But Ive never came across ND filters which did not give their transmittance in optical density... (But Im not a Photograph... :p)
 
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lasersbee

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This is true, but ND filters dont work that way. They let light through by 1/(10^ND)... So ND1 already is 1/10 while ND2 filters let only 1/100 through... It works the same way as OD with laser glasses...

edit: Ok thats not true that way, sorry. Looks like ND4 is really 1/4... But Ive never came across ND filters which did not give their transmittance in optical density... (But Im not a Photograph... :p)
If you are using Photographic ND Filters you will need to
test the filter's ability to reduce a Laser's beam depending
on the wavelength of the Laser in question.

I've posted about it numerous times that those cheap
photographic ND filters do not reduce the power of a
Laser the same at all Laser wavelengths...


Jerry

You can contact us at any time on our Website: J.BAUER Electronics
 
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MadEye

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I use an OD4 ND Filter (I thought it was also an ND4 filter till today...) from thorlabs and its really nice... The OD4 rating is for something like 633nm, but you can see in the datasheet it only varies a tiny bit.

They also have filters in OD 0.1, 0.2 ... steps, so if someone really wants to extend his LPM range, I can really recommend it. If you buy them unmounted they aren't even that expensive and you can build some custom frame to put them in front of your LPM
 
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lasersbee

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If you supply links to your Thorlab ND filters...
I'll supply the link again to the Cheap Glass Photographic
filters...:cool:


Jerry

You can contact us at any time on our Website: J.BAUER Electronics
 
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MadEye

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There are many different, absorbative and reflective... ;)

Thorlabs ND Filters

I used THIS ones, just already mounted... If you get the small ones its about 17€ (cant see the $ prices...)
 

lasersbee

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There are many different, absorbative and reflective... ;)

Thorlabs ND Filters

I used THIS ones, just already mounted... If you get the small ones its about 17€ (cant see the $ prices...)
If you look at the Graphs for your specific ND filter you
will notice that the absorption % is not perfectly linear
over different wavelengths...

You would still need to test the filtering ratio for the wavelength
of Laser being used with the ND filter...:)


Jerry

You can contact us at any time on our Website: J.BAUER Electronics
 
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MadEye

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There is no material which filters all wavelenghts by same percantage... But if you look at the dimension of the difference it shouldnt make that much difference... But of course you can make a chart of common wavelengths... ;)
 

lasersbee

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All I'm saying as I've said many times before is that you will
need to test the properties of your particular ND filter with
each different Laser Wavelength you are testing if you want
an accurate LPM reading....


Jerry

You can contact us at any time on our Website: J.BAUER Electronics
 
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