USM

Definition: UnSharp Mask: technique for improving apparent sharpness of image. * NB: it is the mask that is unsharp; the result is an image made more sharp. * (a) Digitally, USM applies a Laplacian filter to remove low frequency i.e. big detail and to pass only high frequencies or sharp detail. * Also known, in print industry, as peaking. (b) A mask made from an original negative with an spacer or diffuser in between so that the mask is unsharp. The mask and original are then printed in register: the blur caused by the unsharpness accentuates the boundary between dark and light in the image in the following way: where the image is light, the blurred mask lightens it further; where the image is dark, the blurred mask darkens it. At edges, this increases the apparent contrast between light and dark, causing the edge to look sharper. (c) In drum scanning, the difference between small and large aperture signals are added to the large aperture signal.

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