Wetgate

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Wetgate (meaning: "wet passage") is a technique for restoring films, practiced by copier works . A wetgate can be used in copier machines as well as in film scanners.

background

Film material wears out with use, i. H. with each pass through a projector. This wear and tear includes, on the one hand, chemical aging caused by the light from the projector, which leads to color changes due to uneven fading of the dyes in the color material, and, on the other hand, mechanical damage due to scratches as a result of repeated playback. During restoration, films are copied or scanned onto new film material, whereby the aforementioned damage is compensated. The wetgate process reduces scratches.

principle

When copying, light shines through the film. The film strip is located between two glass plates with two layers of air (a few tenths of a millimeter thick) between the film strip and the glass plates. Scratches represent a change in the thickness of this air layer and thus a change in the transmission of light, which is visible in normal (dry) projection. In wetgate projection (copying), the gap between the film strip and the glass plate is filled with a liquid that has approximately the refractive index of the film material, usually tetrachlorethylene . This makes scratches on the blank side (carrier) practically invisible; on the layer side only as far as the layer is not damaged - if the layer is deeply damaged, a white (positive), black (negative) or colored (colored material) scratch remains visible.

functionality

Wetgate machines have a (mostly exchangeable) film stage with an inlet and outlet for the copying liquid, as well as suction connections. The copying liquid itself does not attack the film material, but residues can decompose over time to sometimes aggressive compounds, so the film material must be dried as well as possible after copying. This is done first of all by vacuum suctioning off the liquid residues, the actual drying then usually by means of warm air directed over the film strip. Since the usual copying fluids are poisonous and mostly also carcinogenic, various auxiliary devices are required for the extraction of the vapors and their recovery through cooling condensation, as well as the cleaning of the exhaust air through activated carbon filters. The company ARRI used for wetgate in their pickup ARRISCAN a special, relatively harmless liquid that makes these additional measures unnecessary

Since residual dirt particles or splinters of the photographic layer can get into the copying liquid during copying, despite the previous cleaning of the films, the liquid must be cleaned by one or more filters before being fed back to the wetgate