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Old 09-07-2007, 09:08 AM #1
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Default Looking for a quality IR filter for LPM-1 meter

I read in a post somewhere that even a few mW's of IR can give a false reading on the meter(something about the sensor not being thermal?) and that you should place a high quality IR filter in the beam path that has a known transmittance at 532nm so wattage can be determined on some of the cheaper lasers.

If one were to just move all the lasers being tested a measured distance from the unit, say one meter distance, would that be enough to reduce the IR energy to a level where it's not a factor?

If an IR filter is the best option, anyone know of a good source?


Thanks.



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Old 09-07-2007, 12:15 PM #2
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Default Re: Looking for a quality IR filter for LPM-1 mete

Check out SteveTexas' sticky on IR filters in a toothpaste cap...it has links to some EXCELLENT IR filters,....
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Old 09-07-2007, 03:28 PM #3
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Default Re: Looking for a quality IR filter for LPM-1 mete

I would assume that at 1m distance that the IR would be so diffused that ambient light would overpower it... but it's good to check for IR leakage using an IR filter, and for getting the most accurate results.

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Old 09-07-2007, 03:56 PM #4
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Default Re: Looking for a quality IR filter for LPM-1 mete

No IR filter is perfect and you will also lose some green in the process. However having a spare IR filters is good to have around. Like with a Spymode 150mW laser I one had, I tested it without a filters and got a 300mW+ reading. With a filter it was only ~20mW. That is because the silicon sensor in the LPM responds to IR differently than green. It actually weights IR a lot more, so if you have your meter weighted to green, even small amounts of IR will throw off the reading significantly.

If a quality 10x10mm IR filter from Coherent Inc. will work for you, I can get you one shipped for $15.
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Old 09-07-2007, 04:28 PM #5
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Default Re: Looking for a quality IR filter for LPM-1 mete

Quote:
Originally Posted by Kwizatz_Haderach
I read in a post somewhere that even a few mW's of IR can give a false reading on the meter(something about the sensor not being thermal?) and that you should place a high quality IR filter in the beam path that has a known transmittance at 532nm so wattage can be determined on some of the cheaper lasers.

If one were to just move all the lasers being tested a measured distance from the unit, say one meter distance, would that be enough to reduce the IR energy to a level where it's not a factor?

If an IR filter is the best option, anyone know of a good source?


Thanks.
That was in a post I made I believe.
Altough WL was once saying that moving the meter away from laser would in effect remove the IR as it is diffused, is not true. Even though the lens system is designed for 532nm, the IR is nearly as focused as a 532nm beam. I've confirmed this by two methods.
The 1st method was moving the thermal head 30 feet away ( the distance I use for my divergence testing) and testing power output of a laser I came across with IR leakage; the thermal power measurement was the same at both distances. This means that both wavelengths of energy were still hitting the sensor element which is a little larger than 1/2 inch in diameter.

The other method I used was with an astronomy filter I have called a hydrogen-alpha filter. It passes wavelengths above ~650nm at 90% and greater transmittance, so it lets all the IR pass but blocks the 532nm. I then use this filter at the same distance and project the beam onto it with an IR indicator card placed behind. The card then lit up with a circular beam only a little bit larger than 532nm beam was without the filter. This proves the IR beam is not diffuse or being quickly diffused in a short distance.

Here's a link to the lumicon site where these filters are sold.
http://www.lumicon.com/images/1.htm

Note: this filter was not an IR blocking filter, but an IR passing filter. I think they sell both.
For an IR filter, I would suggest Melles griot or some other high quality manufacturer who provides a spectrum plot of that particluar filter, not just the typical plot for the filter model as each filter or filter lot will differ in transmittance as the thinkness of the coating as well as the formula changes slightly for each lot. Also try to get one that is as flat as possible above 750nm and a high level of transmittance at 532nm (>90-93%) and high level of blockage in the IR range of >95%.

Jack
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Old 09-07-2007, 04:43 PM #6
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Default Re: Looking for a quality IR filter for LPM-1 mete

Well, the IR does diverge more than the green, but a gaussian beam profile puts most of the energy in the center anyway - so at distance you may filter out some of the less intense energy around the outside, but you still get a lot of IR leftover.
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Old 09-08-2007, 09:59 AM #7
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Default Re: Looking for a quality IR filter for LPM-1 mete

Thanks for the replies everyone. It seems the IR is not as diffused as I thought and I'm going to get an IR filter.

I'd like to take you up on your 10x10mm IR filter Pseudo. Even if it doesn't come with transmittance figures at 532nm it's well worth it and can give me a very close idea(I"m assuming ~85% or so Tr). Never used private messages on this forum yet but I'll try to send you one and see if it works. :P

Thanks again, Kwiz

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