Optical sectioning is the simultaneous illumination and viewing of only a thin region of a specimen. An illuminated slit is imaged at the plane of interest and is swept laterally by the action of an oscillating mirror. The light returning from the specimen reflects from a second facet of the oscillating mirror and forms a stationary image of the illuminated slit. At this stationary image a second slit is placed, which passes light from the desired plane and rejects scattered light from other depths within the specimen. Light passing through the second slit is reflected from the third facet of the oscillating mirror and is focused to the final image plane. The image is reconstructed as the image of the second slit sweeps across the image plane. An important ophthalmological application is the examination of the endothelial cell layer of the cornea, either by contact or noncontact techniques. Optimization for image illuminance and resolution is discussed.
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