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 _______________________________________________ Support-list mailing list [email protected] http://support.elphel.com/mailman/listinfo/support-list_support.elphel.com
