>-----Original Message-----
>From: [email protected]
>[mailto:[email protected]]
>Sent: Wednesday, August 14, 2002 2:03 AM
>To: [email protected]
>Subject: EMI Coatings for Windows
>
>
>
>Can any one help?
>
>Have been asked to find an EMC Coating that can be used on a window.
>Window being used for Low Light Camera, Colour Camera and
>1.54micron Laser.
>Coatings considered so far cause problems with attenuation of
>the Laser.
>
>Companies approached so far Chomerics and Pilkingtons.
>
>Anybody covered this ground previously?
>
>Regards
>Andy
>
>Andrew Price
>Principal Development Engineer (EMC Specialist)
>BAE SYSTEMS Avionics
>A125
>Christopher Martin Road
>Basildon, Essex
>SS14 3EL
>
>tel: +44 (0) 1268 883308
>email: [email protected]
>
Andy:
Another place to look at is OCLI, at:
http://www.ocli.com/
But first, what is the window used for? Is it in your lab wall or
penetration port, or is it used in your product?
I have an OCLI window, about 6" x 10", built into one of my penetration
ports. Even with such a large optical area, I have no measurable RF leakage.
I regularly test equipment that uses IR LED's as data links. I can usually
accept the IR loss through the window, but there have been some problems. I
solved this by letting the laser shoot through a 1/8" diameter drilled hole
in the aluminum plate port cover. Initial alignment is a bit tricky, but
it's a zero-loss solution. You could also use a honeycomb structure, with no
glass at all. The field of view is somewhat restricted, but again, zero
loss.
If the window is for your product, you have more constraints. But I was
recently surprised by a shielding solution that worked out very well for an
IR laser detector cluster. The detector has to work out to a range of about
2 miles, and the detector chip puts out only about 40 uV (into a 15 MHz
bandwidth, high Z pre-amp). The detector was mounted about 1 inch behind the
plane of a thick metal housing, with a 2" diameter hole for a non-conductive
lens. The equipment was failing radiated immunity at a level of only a
couple of V/M, way down at 1 MHz!
A conductive window was deemed too disruptive to the already finalized
design (that's a familiar story), and a full aperture copper screen caused
too much IR loss. We experimented with copper screening across only the
detector chip surface (about 1 square inch). We found that the detector chip
could be shielded very adequately by creating a grounded conductive
perimeter (picture frame) around the detector chip, and then stringing about
4 equally spaced, very thin, single copper wires across the detector chip
face. We only lost a few percent of the IR power, and the shielding solution
was "good enough" to move the immunity threshold up to >150 V/M.
Conductive windows on your product are expensive (and the edge termination
also adds cost and complexity).
Regards,
Ed
Ed Price
[email protected]
Electromagnetic Compatibility Lab
Cubic Defense Systems
San Diego, CA USA
858-505-2780 (Voice)
858-505-1583 (Fax)
Military & Avionics EMC Is Our Specialty
Shake-Bake-Shock - Metrology - Reliability Analysis
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