Having settled on DI instead of FTIR for the time being, I went ahead and built a bigger, but still fairly small prototype table that i could use to develop the software (as opposed to a full-size one, since i dont have that much room in my office. The full size will come later when I have the software working well, and have written a few apps for it).
So, here is the second prototype (sans camera, i haven’t mounted it in there yet, but it goes just in front of the mirror. I had it sitting on some books for the testing I have done up to this point, i took it out to take IR photos of single malt scotch for my previous post, and I forgot to put it back in for these, so you will have to use your imagination)
The Table is only 640mm high, and fits under my desk. I may even put wheels on it to make it easier to move out of the way when I am not using it.
The surface is just tracing paper, and the plexiglass is cut to fit one sheet exactly.
The plexi is actually two layers of 3mm (since it was easily purchased form the local hardware store). I haven’t yet got around to buying a new sheet of 6mm or better. The double layer works fine for DI, i dont think it would work very well for FTIR tho. If you press hard, there is still a bit of bowing in the center, I dont really recommend the double layer, but it works if you dont have a single sheet of the right thickness.
The two IR illuminators, 840nm, which is just in the right range for the fire-i camera. they are 12v, powered by my 5/12v experimenting supply. Eventually i will just by a 12V wall wart for them. Altho they have a pretty purple color here, (thanks to my digital camera) to the naked eye you can only see a very dim red glow when they are on (since they are so close to the visible spectrum at 840nm). I have put some of the tracing paper into the glass covers to make them more diffuse, I also taped over the photosensitive resistor so that they are always on (they were originally for nighttime illumination for CCTV cameras) They are actually a bit close to the surface here, and they give a very spotlight-like coverage (even with the diffuser) which isnt exactly what you want. it makes the illumination very uneven, which causes the threshold filter to lose the blobs in the dark areas even though they are perfectly visible. This seems to be the big problem with the DI setup, nice even light coverage. I spent a great deal of time tweaking it before i came to the conclusion that i should be able to deal with the uneven coverage in software. Like a variable threshold. My thought is that if i put something that has roughly the same IR reflectance and my fingers (the cardboard back of the tracing pad is pretty close) and use that to cover the whole surface, then i can take a snapshot of that and use that to analyze the coverage of the IR lights. Once i have a lighting brightness map, i can use that to scale the threshold values accordingly and hopefully be able to easily pick out the blobs i want even as they pass from light to dark. More on that later, once i have tried it out.
The projector is a ViewSonic PJ556d (which has a latency of about 1/20th of a second according to my tests, which is a bit slower than i want, but still passable). You can also see my super-duper-pro-plywood-scrap projector mount and adjustable rotation mirror. As crappy as it looks, it work fairly well, I can move the mirror back and forth to adjust the image on the surface. The surface is not 4×3 (obviously) and at the distance I have right now, my projection is just wide enough to cover the whole surface (the whole exposed surface i should say, since there is a 43mm border all around from the wood i used.) but i have spill on the top and bottom, which i use the mirror adjustment to put where i want (generally so I can see the menu bar on top). The projector has a native rez of 1024×768, so that is also nice. It can go as high as 1280 × 1024 (or maybe even higher) but that seems silly, since it is just scaling the image, and looks terrible. I am sticking to 1024×768.
As for control, I am using my powerbook g4 12″, which also has a native rez of 1024×768, so i can easily run it in mirror mode when i am testing, or run as a second monitor so i can see all the config stuff on the powerbook and still get the ‘real’ image on the surface.
