document: Autofocus system for microscope A technique is developed for microscope autofocusing , which is called the eccentric light beam approach with high resolution , wide focusing range , and compact construction . The principle is described . The theoretical formula of the eccentric light beam approach deduced can be applied not only to an object lens whose objective plane is just at the focal plane , but also to an object lens whose objective plane is not at the focal plane . The experimental setup uses a semiconductor laser device as the light source . The laser beam that enters into the microscope is eccentric with the main light axis . A defocused signal is acquired by a symmetrical silicon photocell for the change of the reflected light position caused by differential amplification and processed by a microprocessor . Then the electric signal is power-amplified and drives a dc motor , which moves a fine working platform to an automatic focus of the microscope . The result of the experiments shows a + or-0 .1 - mu m precision of autofocusing for a range of + or-500 - mu m defocusing . The system has high reliability and can meet the requirements of various accurate micro measurement systems
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