Originally Posted by guruguhan
You will have to make your own, and calibrating it is not easy.
In principle, the filter is just a round disk (I used phenolic plastic) about 5-6" in diameter which you place several inches under the center of the bulb of the HID lamp. It is held in place by a very small diameter rod or bar that you attach to an adjacent wall. Depending on design, you may also place small holes in the disk to even out the light source.
In essence, the filter eliminate the light that comes from directly above so that the print is exposed with reflected light. The theory works, but getting it just right takes a long time so in practice it is best to just increase the distance of the bulb from the printing frame and even the light out this way. Exposure times will be about the same both ways.