Schematic of the operating principle of the confocal microscope. (Left) A conventional, or wide-field, microscope. The specimen is illuminated over an extended region by a light source and condenser. Light rays arising from three points in the specimen are shown. The dashed lines emanate from two points in the focal plane, one centrally located (darker dashed lines), the other off axis (lighter dashed lines). The third point is on axis but located below the plane of focus (dotted lines); it gives a blurred image at the detector. The detector forms an image from the sum of all the simultaneously arriving light rays. (Right) A confocal microscope. Two pinhole apertures have been introduced. The upper aperture allows only the focused light rays from the on-axis, in-focus point of the specimen to pass to the detector. The lower aperture restricts the illumination so that it is focused on the point seen by the upper pinhole aperture.