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A test target is a printed or carved pattern used to determine the accuracy of an optical or electro-optical piece of equipment. Many scientific and consumer instruments rely on optics; devices such as cameras, microscopes, spectrometers, and light meters rely on lenses that must be carefully calibrated. Test targets are used to ensure that such devices are functioning as intended, by testing their optical resolutions. They are used to guarantee the reliability of equipment prior to ordinary use. Without test targets, there would be no reasonable way to determine whether the optical sensors in devices are degenerating over time.
A simple way to think about a test target is to compare it to a traditional eye exam. The target itself contains patterns in a specified resolution; like a patient reading an eye chart with increasingly smaller letters, the optical device being tested must correctly scan the pattern on the test target, determining the boundaries between one pattern and the next. Once the optical device can no longer accurately determine the separation point between patterns, it has reached its "limit of resolution." If the limit of resolution meets or exceeds the stated resolution limit for the device, the device is said to be operating correctly.
In order to correctly test the resolution of an optical device using a test target, the target itself must be of a higher resolution than the device can read. Setting a limit higher than the device should be able to reach makes it possible to know whether it is meeting or exceeding expectations of accurate perception. By attempting to force the device to read a resolution it should not be able to detect, a user can determine whether the device is calibrated correctly. Test targets are employed in factory settings to ensure that new devices meet quality control standards, but they are also used by those who rely on optical instruments regularly, to keep them in good working order.
A test target can come in a variety of formats. Some are printed on paper while others are on glass, quartz, or white opal. The patterns themselves also come in various types; multi-bar patterns and star patterns are among the common layouts used. Each type of pattern typically has a specific use. For example, five-bar microscopy patterns are primarily used to determine the output image of cameras and lenses.