I had read about how the filter - essentially a thick piece of glass, should be replaced to maintain the autofocus ability. Finding a replacement piece of glass the correct thickness isn't easy and dealing with optical companies is difficult and expensive for such a small piece of glass. I had thought of using microscope slides, the glass from photographic glass slide mounts or even an old filter (glass or optical acrylic). This involves cutting and layering the glass to the correct thickness.
This is too much work for something that may not be successful anyway. So this is what I did instead.....
Step 1: Open Up Your Camera
I modified a FujiFilm MX2700. Looking at the reflected surface on one side of the glass showed that it appeared to have a yellowy reflective coating which was optically transparent when viewed straight-on. The other side looked like a normal optical coating. So why not just remove the IR reflective coating and place the glass back into the camera with a thin piece of dark filter to cut visible light and allow IR through?
Note that this is for the MX2700, your camera may have a different arrangement. The important thing is to find and remove the IR blocking coating once you've resigned yourself to modifying the camera and otherwise having to throw away the glass filter anyway.
Step 2: Remove the IR Reflective Coating
At first I used a small amount of cream on a piece of kitchen paper to gently rub the coating. I expected it to start coming off very easily. Ten minutes later the surface was beginning to look a bit scratchy and a series of concentric rings from the centre were forming. These were presumably the layers of coating from several depositions. I'm assuming it's coated using some sort of vacuum molecular beam epitaxy like process!!?!?
The coating was holding out very well so I decided that a plastic based kitchen scourer might be a bit more pursuasive. After another half an hour, I wasn't getting far and began to start thinking that a replacement piece of glass might be necessary. I decided that a change in technique might help. I used an old Cokin type A acrylic filter as a flat surface and rubbed the hot mirror over it in a circular motion adding more paint renovator cream every couple of minutes. This seemed to work very well and after a few minutes the coating gave way to a nice shiny glass surface. It should look like window glass. I think shiny flat plastic sheet like acrylic is probably the most ideal surface to rub on. Rub in a circular motion and add more cream every few minutes. Any scratches you see should be the coating and not the glass. Clean the finished filter with window cleaner. Try not to scratch the optical surface on the other side of the filter - if it has one. Otherwise, you may as well remove this as well.
So don't be afraid, the glass won't scratch as long as you stick to using the aluminium oxide based renovator cream that I used. It's far cheaper and easier than trying to get optical glass of the correct thickness cut.
You will also need to get a visible light blocking filter that allows IR through. These are the Wratten 25, 87 and 89 filters often talked about. These can often be obtained from eBay. I wanted the filter inside the camera, so it had to be very thin. If your in the UK try SRB Film services (they do ship abroad) who sell a thin Lee polyester sheet filter for infra-red photography. Cut the filter to size and place on the front of the hot mirror. Attach it using whatever you need to, but try and keep it flat. On a FujiFilm MX2700, the hot mirror fits into a plastic holder and the IR filter can sit in this, with the modified hot mirror on top.
If you can only obtain a normal filter that screws onto the front of a lens, then you'll need to attach it in some way to the front of your reassembled camera. There are various systems available for using filters and things for digital cameras with non-removable lenses, so have a look around.
Step 3: Finished Product
So why not give this a go with an old digital camera? If you have to throw the hot mirror away anyway then there's nothing to lose.