Subject: Re: help: reducing filters' interreflections

From: [email-masked] (David Williams)
Date: Mon, 22 May 95 23:14:09 GMT

In Article <[email-masked]>, [email-masked]
(Choong) wrote:
>Can anyone advise me on how to reduce/eliminate interreflections between
>interference filters and ND filters (reflective) when used with
>a camera?
>
>I am using narrow band interference filters and ND filters(reflective)
>infront of a CCD camera and they are causing lots of overlapped secondary/
>tertiary... images from the interreflections.
>
>Any advice/help is much appreciated.  Thanks.
>
>-Sing
>[email-masked]

The ND filters are probably metal (inconel) deposited onto glass, so they
shouldn't be very sensitive to angle.  I would put the ND filter in front
and tilt it until the ghost image was out of the field of view.  The
interference filter will shift wavelength if it is tilted, so you probably
don't want to tilt that one.

Another choice might be to use absorption filters like Wratten #96 ND's
(available at fine camera stores).  They are good in the visible (400-700
nm) and show increased transmission in the near infared but might still be
usable in the 700-900 nm range.

You might also be able to replace the video camera lenses' iris with a large
pinhole then eliminate the ND filters altogether.  The iris diameter needed
can be calculated beforehand knowing area of the opening is proportional to
its light transmittance.  If the hole is small enough, it becomes a pinhole
camera and the lens (which could also be causing ghost images) can be removed.

Good Luck

David Williams
Delco Electronics
[email-masked]

---

From: [email-masked] (Ian Ashdown)
Date: 26 May 1995 15:08:02 GMT

>[email-masked]
If you are using a lens with a reasonably narrow field of view, you
could tilt one or more of your filters at 45 degrees to the lens axis.
The reflected light can then be directed to a light trap rather than
back into the filter in front of it. Of course, this requires a larger
filter to prevent vignetting. (If you have a 90-degree wide-angle lens,
forget it -- the filter would have to be infinitely long!)

As for an efficient light trap, the ideal device is apparently a glass
tube shaped like a cow's horn that is painted matte black on the inside.
In fact, I recall reading a paper on the topic -- the best performance
the researcher's obtained was from a cow horn painted matte black.

I have always wanted to submit a purchase requisition for ten thousand
cow horns and a gallon of matte black paint to line the walls of my
photometric laboratory. It may not work, but it would be fun to see
what Purchasing would do with the request!