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Step 2Remove the IR reflective coating
Looking at the coated side of the hot mirror, it seemed to be thin and delicate, scratching if gently touched. Not so. It's an absolute pig to remove. I decided that I would use a slighty abrasive paint renovator cream made by Auto Glym to try and polish it off. This is a petroleum based cream containing fine aluminium oxide powder. It's used to remove the oxidised layer of paint from car body work prior to waxing. I chose it as I believed it would not scratch the glass surface.
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.
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.
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