Scanner Information from
Graphic Quality Consultancy
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Scanner Information fromGraphic Quality Consultancy |
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HOW UNSHARP MASKING SHARPENS IMAGES!U.S.M. means "Unsharp Masking". A contradictory term as it is used to sharpen images! Most people in the industry today are unaware that this term dates back to the days when colour separations were made using process cameras. These used low contrast continuous tone photographic masks to correct gradation and colour. It was found that by making the masks a little soft, i.e. "unsharp," an apparent increase in sharpness could also be achieved. This was done by creating small fringes around areas of detail. Early drum scanners created USM by using an additional analysing aperture and corresponding (fourth) photomultiplier tube (PMT). Analogue electronics were used to process the image and apply the USM "fringes". There endeth the history lesson! Modern scanners and image editing applications use digital technology to create these USM fringes. Many modern high-end scanners, including Celsis CASC drum scanners and all Fujifilm flatbeds, create the USM with special software on the Mac computer, using the Mac's normal processor. Dedicated scanner USM software is usually more sophisticated than that in PhotoShop. The software writers know their particular scanner's imaging characteristics, while using different types of originals, enlargements, etc., and factor these in. Therefore whenever possible use the scanner software or hardware to generate USM. However when using "consumer" scanners and cameras, you may get more control with Photoshop. It is often not realised what a dramatic effect USM has on scanned images. Without it the average image from any scanner will be very "soft" or fuzzy. This is for a variety of reasons, including the speed of the drum's rotation, or the traverse of the flatbed's scanning array. Perhaps the main reason is the effect of the analyse aperture or "spot" reading an area of high contrast edge detail as a single "grey" pixel. How detail should appear 
 On the left is a representation of how sharp edge detail in the original should appear. On the right is a representation of the "edge" of the detail area. What happens to detail without USM 
 Above we see an image without Unsharp Masking. The scanner aperture records the edge detail as "grey" pixels. Instead of having a sharp transition from light to dark we have "mid tone" or "grey" pixels between the light and dark areas. What happens to detail when USM is applied 
Above we see how USM replaces the "fuzzy" grey pixels with a white (light) fringe and/or a black (dark) fringe. |