[Elphel-support] optical filters for AR use?

[Felix Schill]

What you are looking for is called a bandpass filter, and they are
available in many variants for almost any wavelength you can imagine.
Probably the most comprehensive source for optics I know of is edmund
optics - they have quite a range:
http://www.edmundoptics.com/onlinecatalog/browse.cfm?categoryid=41
(not necessarily the cheapest in general though). Maybe do a search
locally for optical filters. Not sure if you can get matching thread
sizes for your lens, but you might be able to stick an unmounted filter
onto the back of the lens or something.
Ideally you would want a monochrome sensor for this sort of thing, as
the RGB filters will affect the wavelengths you're after (i.e. if you're
after IR then mostly the red pixels will respond). It should work to
some extent as well with a colour sensor if the IR filter is removed.

I haven't tried with the Elphel, but have used other colour video
cameras (e.g. the GoPro helmet camera) without IR filter. Makes the
world look quite different, and there seems to be a lot of IR around in
outdoor scenes. Unfortunately there will also be a lot of UV around if
it's sunny (and your camera won't be very sensitive to UV), so it might
be difficult to find a wavelength that gives you good contrast against
everything else.
The best shot is probably to go for a very narrow-band IR lightsource
and a well-matching filter for exactly that wavelength.
I assume you want to use active markers (LEDs and such) for non-visible
night-time use, or alternatively, an IR lightsource on the camera, and
small retroreflective markers. As they only reflect light back to the
source, they would be quite invisible unless you have a head torch or
something. Of course your markers could also have a bandpass filter that
blocks visible light, ideally cheap plastic filters...

I hope that helped. Good luck.

Felix


Ákos Maróy wrote:
> Hi,
> 
> I'm new to Elphel cameras, but I've been looking for a solution for a
> specific task for some time now.
> 
> I need to track markers in outdoor environments, which I can place on
> specific objects. The markers ideally should not be obviously visible to
> the naive observer, and should work ideally both in day-light and night
> time, with artificial human-visible lights around. (spotlights,
> streetlights, beamers, etc.)
> 
> I was contemplating on using either UV or IR light, and cameras with
> optical filters, but these seem to be quite difficult to come buy. I see
> that CCDs ordered with Elphel cameras have the option of having the IR
> filter removed (or not installed) - which is a good start.
> 
> but, I wonder, are there optical filters available to allow only UV or
> IR light to enter the optics of the camera? or, with the IR filter
> removed, can one filter for IR wavelengths by software?
> 
> any hints or suggestions welcome.
> 
> 
> Akos
> 
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