The confocal principle is now reasonably well known. A light source (typically a laser) is focused to a diffraction-limited spot (Airy disk) at the specimen by the objective lens. Reflected light or emitted fluorescence is focused by the same lens to a spot at the detector. At this point a pinhole is placed, so that only the in-focus spot will pass to the detector - light from out-of-focus planes does not form a spot and so will be blocked by the pinhole. By scanning successive planes, a three-dimensional image of the sample can be created. Confocal microscopes also offer a modest increase in resolution, since the detected signal is the square of the point spread function of the objective lens. In the best case the improvement will be by a factor of 2, but this requires an infinitely small pinhole so the real gain will be somewhat less.