 Optical sectioning

Optical sectioning selects signal from a thin depth range of a specimen without physically cutting it. Repeating the measurement at successive focal depths gives a stack of sections from which three-dimensional organization can be reconstructed.

In a scanning confocal microscope, illumination is focused into the specimen and a detection pinhole rejects much of the fluorescence originating away from focus. A thick fluorescent specimen imaged with a wide-open and then a reduced pinhole demonstrates the rejected background; reducing the pinhole also discards useful photons, so section thickness and signal-to-noise ratio must be considered together.^[ref]