How about the thickness? Would it be possible to replace the IR-block filter with an actual optical glass visible-block filter? How thick could a glass filter be? Some commercial glass filters are 3mm thick. Would the other dimensions have to change to allow a filter that thick?

I think it could be done - my main problem with glass stuff (mirrors, filters) is that I haven't been able to find a way to cut them to size without shattering them, or leaving awful ragged edges. The rubber gasket seems to afford a couple millimeters of space, though.

This guy seems to have worked out a process for cutting filters:

"Cutting your own custom glass can be done either with a standard glass cutter, by scribing and breaking, or using a diamond cutting disk on a light power tool (dremel). I found that using a glasscutter was very effective and also quite precise on 3mm thick glass, but when trying 1mm thick clear filter it resulted in splinters only…" more at: http://ir-photo.net/ir_mod.html

I'm not sure how 3mm thick glass would fit in a small camera like that. I would love to know how much it would improve the resolution of the photos compared to a film filter. It might be possible to make several little rectangles from one standard filter. Any body have an old Wratten 87 filter you're not using?

I wonder if the IR filter is the limiting factor - my instinct is that low light level is more of an issue, and that the graininess is from ISO -- but i'm not really an expert on photographic tech. Another issue may be the auto-focus mechanism. I was wondering if it uses only one color channel to focus, as it might be marginally faster. Then blocking certain color sensors more than others could cause problems for the focus. Also to keep in mind is that infrared focuses at a different setting then normal light due to chromatic distortion.

Maybe the color-focus theory could be tested by putting a red light in the foreground, a green light in the middle, and a blue light about 20 meters away, and seeing what the camera "likes" to focus on? But i don't really know if it'd help us improve the images. Interesting to explore though.

Many people do this (and companies like MaxMax and Lifepixel do it for a fee in a cleanroom).

The two tricks are getting the exact right refractive index and thickness so that the camera can still focus (or even better, still autofocus), and do it without trapping dust between the replacement glass and the sensor.

If you can live without working autofocus, pretty much any cut glass filter will do.