![]() ![]() But this diffusing has to be considered when you want exact reproduction: the thickness of the black parts is usually ever so slightly reduced to accomodate for this, otherwise the image would smear out a bit on the exposed plate. ![]() Retracting the diffusor towards the end of the exposure time will "sharpen" the wanted image a bit. The reason is simple: it diffuses light, so that you don't get an image of the borders of the cut film snippets onto the plate. During exposure, most of the time a semi-transparent foil is placed above the plate+film+glas stack, but on the last minute or so this is retracted. The exposure unit has a high-power UV source, about 1 to 1.5 meters above the plate surface. Normally there is large, empty carrier film, on which the different typeset or raster-image films are mounted (using clear spray adhesive). Just look at, for example, how the printing plates in offset printing are exposed. Probably not really important here, but be aware that using diffused light, as well as as semi-transparent films, will also cast slight shadows. Using quartz glass is preferred, if possible. Oh, and about the glas: be aware that most normal glasses tend to filter UV light to some extent, requiring a longer exposure time. The lines will get thicker (and the spacing thinner) in the areas between the tubes, and will be the same widths directly above the tubes. Stack a second film on top of it, and the result will be less even. Using a single film, where the printed side is in contact with the photosensitive coating of the PCB, will produce a quite even result. For example, 10 mil lines spaced 10 mil apart, running in the same direction as the UV tubes. You can test this easily by exposing the PCB with a simple pattern of thin lines. This in turn will cast shadows at those places where the light hits at an angle. Since the paper/film is not infinitely thin, the second sheet will always be at a distance (the thickness of the first sheet) from the photosensitive coating. Everywhere else the light will hit at an angle. There you will have only four places where the light goes straight from the tube to the PCB: right above the tubes. Take a bog standard exposure box that uses four UV tubes, for example. However, pretty much everyone at home uses a small (read: thin) exposure box or similar, where the light sources are rather close to the board. If you use two films stacked on top of each other, this will cast shadows, unless you use a real point source for the UV light that is really far away. There should always be a glass there anyways, and some pressure applied (or a vacuum used) so that the film is in contact with the PCB. ![]()
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