[Elphel-support] optical filters for AR use?

[Ákos Maróy]

Felix,

Thanks for the detailed info.

> 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.

thanks for the pointer. indeed, at between 100 - 400 Euros, these
filters aren't particularly cheap.

> 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.

so the IR filter would not have an effect on the monochrome sensor?

> 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.

yes, actually this is the other end of the problem: to find a wavelength
that is not widely present otherwise. but for this reason, I wonder, is
there no way to filter the light spectrum digitally at first, so as to
see what wavelengths are typically present / not present in the scene /
setup I'm aiming for?

> 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...

yes, basically this is what I'm aiming for. as for the 'cheap plastic
filters' - where / how would I find those?

> I hope that helped. Good luck.

very much so - thank you!


Akos