The emulsion side of a photopositive transparency is placed in intimate contact with the sensitized face of the aluminum. Ultraviolet light in the form of an arc, a UV lamp or sunlight may be used for exposure. Length of time for exposure is usually determined experimentally and varies with the amount and type of diazo compounds used in the formulation. A good indication that sufficient exposure has been achieved is the apparent bleaching of the yellow diazo color in the exposed areas.
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