= Conjugate pinholes in confocal microscopy
Two planes are optically conjugate when an <optical lens> system images one onto the other. In <confocal microscopy>, an illumination pinhole is imaged onto the selected specimen point, and that specimen point is imaged onto the detection pinhole. Thus the two <apertures> select corresponding positions in different planes; they need not occupy the same physical plane. Modern <laser> instruments can supply the illumination point with an <optical fiber> or a suitable focused <laser> mode instead of a separate metal illumination pinhole. <Confocal microscopy (chapter manuscript) by Brad Amos and others>[Amos, McConnell and Wilson, Confocal microscopy, manuscript pp. 5, 7–8: conjugate foci, Fig. 3, fiber-delivered illumination]{ref}
A fluorescent emitter at the selected <optical focus> forms a compact <point spread function> at the detection <aperture>. An emitter displaced axially produces a broadened spot there through <defocus>; a smaller fraction of its collected <light> passes. The emitter still exists and still emits <photons>. Rejected <photons> strike the opaque mask or other parts of the instrument instead of contributing to the accepted detector channel. Neither a finite pinhole nor <diffraction> makes the accepted out-of-focus contribution exactly zero.
\Image[/confocal-conjugate-pinhole-ray-geometry.png]
{title=Conjugate illumination and detection pinholes in <confocal microscopy>}
{id=confocal-conjugate-pinhole-ray-geometry}
{description=Original unfolded <geometrical optics> schematic, not a scale drawing of an instrument. The top panel images an illumination pinhole onto the specimen; the lower panel images the selected specimen point onto the detection pinhole. Dashed orange <light rays> from an axially displaced point strike the mask, while an axial orange <light ray> passes. Real spots have finite <point spread functions> through <diffraction>; the drawing's perfect green intersection is only a ray construction. A <fluorescence> instrument may fold both paths through the same objective <optical lens> using a wavelength-selective <beam splitter>.}
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