= Confocal microscope
A <confocal microscope> combines focused illumination with a spatially restricted detection path aligned to the same specimen location. In a scanning implementation, a <laser> focus moves across the specimen and a pinhole in a conjugate image plane rejects much of the emitted <light> originating away from focus.
A <photodetector> records the signal at each scan position to build the image. Repeating at different depths gives <confocal optical sectioning>. Closing the pinhole improves rejection but discards useful <photons>, so <signal-to-noise ratio> and section thickness trade against one another. Measuring a small fluorescent bead through focus tests the actual three-dimensional response rather than assuming the nominal pinhole setting fixes it.<Introductory Confocal Concepts by Nikon Instruments Inc.>[Figures 1–2 and discussion of optical sections]{ref}
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