Frank Petronio's suggestion of a DSLR scanner reminded me of my own ideas on the subject, and on the basis that I had enough materials and equipment lying around already, and that it wasn't going to cost me anything, I took it on. Eventually, I did have to buy something, since my idea of a lo-fi pellicle was unworkable. So a tiny piece of two way mirror blew the entire budget for contingencies, at €15.
I made this at the end of January, and made some tests with it, enough to show that it had some problems that needed attention. Then other things got in the way, and it's been lying around idle ever since, until now.
I designed it as an optical tube (like an optical bench, but hollow) so that components could be positioned with some accuracy, for repeatable results. I tried different combinations of components, but the internal layout and concept remained the same- using a flash unit for exposure in combination with a beamsplitter to allow the use of a separate focusing light. The big idea I had was to use a reflective film as the primary mirror (Prism? Reflector?)- the type used for road signs, that uses glass microspheres to reflect your headlights directly back at you. By putting the light source in the reflected position of the camera lens using an angled mirror, illumination can be increased at the outer edge of the field of view, even compared to a collimated source. And to counteract the falloff in the lens itself, a centre filter was made by stippling a pattern on the reflective film, to match the pattern observed on a test exposure. Falloff isn't much- according to the histogram, centre luminance varies from the edge luminance by a maximum of 15/255. I've yet to make a conclusive test of this setup, but that's the theory... It's also possible to get quite close to even illumination in post processing, without using a centre filter.
This reflective film was the source of the problem in January- it has a very glossy surface, and it acted like a plane mirror, giving a huge hotspot. This can be fixed in a number of ways, by covering the surface with magic tape, which also provided a good surface to add the stippling, or by introducing a diffusing layer between flash and mirror.
The base uses a large porcelain tile as a positioning stage; the film is taped to a piece of glass that glides over a card layer attached to the tile- the card layer allows for different masks to be cut, depending on coverage required.
The positioning system is very basic. A T square that is positioned using a pair of interlocking racks controls the y axis; a similar arrangement attached to the glass plate controls the x axis. When I started, I might have thought that some kind of automated positioning system would be necessary, but now that I've used it, I find that making the exposures is by far, the least time consuming aspect of the exercise.