![]() Reread the section on exposing the screen for more information. Light will pass through the maximum density areas of the film positive given enough time, thus exposing the emulsion. Widespread over-exposure can also happen if you allow the photo emulsion to receive too much exposure. Reread the section on making positives for more information. It is possible to use two copies sandwiched together, as long as the clarity of the minimum density areas are not affected. If you printed or copied directly onto the transparency, try increasing the darkness of the image. If you made the film positive by hand double siding you should use a clear film positive with an opaque ink. The size of mesh openings and thread width, stated in microns, is readily available from all mesh manufacturers. The finest detail that one should attempt to print with a specific mesh count will be equal to the width of one mesh opening and two threads. Also, assuring that details are sufficient for the mesh count used will achieve this goal. Utilizing the correct exposure time will lessen the loss. Thin lines will be decreased or be completely obliterated. This two minute difference in exposure can cause the UV light to completely pass through the opaque areas of the film positive.Īll light will bend and thus an over-exposed stencil will result in the light bending around the edges of the details in the film positive. This amounts to a 36.5% difference in light transmission, thus a 5 minute exposure using film would increase to 6.825 minutes using vellum positive. Clear film positives allow 91-percent of the UV light to reach the stencil, where vellum allows only 54.5-percent of the light to pass. Wide spread over-exposure is likely to happen if your positive material is not transparent enough to allow the UV light to harden the photo emulsion before the UV light has penetrated the maximum density areas of the film positive. The contrast between the clear and opaque area of the film positive must be extreme for the film positive / UV light / photo emulsion combination to work well. the clear areas, of the film positive than the maximum density or solid areas of the film positive. One should always be more concerned with the minimum density, i.e. The use of semi-opaque positives such as vellum materials will require a longer exposure time, and thus the UV light may penetrate the opaque areas of the film positives much quicker. This is often caused by the use of film positives that are not made of optically clear film. ![]() Extreme over-exposure can "burn" through the opaque areas of the film positive rendering the stencil a solid block of hardened emulsion.Ĭommon Causes of Over-Exposure Īn excess of UV light reaching the stencil causes over-exposure. This over-exposure happens when photo emulsion stencil making when the exposure is longer than it should be and the UV light begins to creep around the edges of the positive areas of your art, thus decreasing fine lines or completely obliterating them. As UV light reaches the photo emulsion it becomes crosslinked, rendering it difficult or impossible to wash out during the development stage. When photo emulsion is exposed to UV light, it polymerizes or will crosslink, making it more difficult to wash out. Over-exposure causes a loss of detail in your image area.
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