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I'm wondering if I can find a cheaper source of ND filters that work in the near infra-red (out to ~800nm.) We have been using Kodak Wratten Gel filters. (ND 0.70) We originally purchased large sheets, but they now come in only smaller (and more expensive) sizes. I looked into the Lee ND filters (polyester) But these don't block light out past ~700nm. Any other sources would be appreciated.

For what it's worth, the use case here is to attenuate a diode laser beam, without disturbing the polarization; the laser can be tuned from ~770-795 nm.

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    Why do need these filters? – JenSCDC Sep 29 '14 at 19:17
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It really boils down to two options.

One option is to use a lowpass filter, and let it just 'work' for the IR range. Put a blue filter on there and it probably has fairly low transmission out in the 750-800nm range. I wouldn't be able to tell you how much without specialized equipment (go to your local chemistry or physics department and ask them).

The other option is to go with a scientific filter that does have the characteristics you are after (and would have the details for the characteristics for the 750-800nm range. One such example is Edmund Optics and their absorptive neutral density filter.

Filters

And there you can see that you can get filters that do a good job of blocking the 700-750nm range, and they will likely have information on the range beyond that. While I don't see anything in the type that would work for specifically what you are after (50mm diameter filter - you likely want a square filter - these aren't going to have camera lens threads on them).

  • Hi @MichaleT, thanks for the answer. The idea of using a color filter is interesting. (Though browsing through a catalog they mostly look to be long pass, all long wavelengths go through.) I'm already using the Kodak Wratten (filter #96) past there specs. I'm getting about 33% attenuation from an OD 0.7 filter. – George Herold Sep 29 '14 at 19:47
  • @GeorgeHerold You wouldn't need to use a scientific filter for that... though the thing to do would be to look at the other filters. For example edmundoptics.com/optics/optical-filters/color-dichroic-filters/… - the light blue LB-120 has a 20% transmission from 700nm to somewhere in the 800s (where it slopes up to about 40% transmission by 1000nm). The green and blue have even lower transmission in the 700-1000nm range, though its not completely opaque. – user13451 Sep 29 '14 at 20:09
  • I think the operative phrase is cheap. The tiffen pieces of plastic are now about $5 per square inch.(It was ~$2. in ~2004) (you should check out Thor labs for optics.) – George Herold Sep 30 '14 at 0:13
  • @GeorgeHerold maybe some welder's glass then? The problem is you are not asking for something that is typically used in recreational photography (more into the scientific and industrial applications... almost with specialty art) where the supply and demand curves are not working in your favor. – user13451 Sep 30 '14 at 0:15
  • Yeah, I like to say I live on the trailing edge of technology. – George Herold Sep 30 '14 at 0:29

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