The confocal scanning optical microscope (CSOM) has the major advantages over the standard microscope that it has extremely good range resolution and cross-sectioning capability, somewhat better transverse resolution, and is well adapted to quantitative measurements. The basic reason for the cross-sectioning capability is that the objective lens is illuminated from a collimated beam, as shown in Fig. la, which focuses the beam to a small spot on the object. The light reflected from the object then returns back through the objective lens and a beamsplitter, and is focused to illuminate a pinhole in front of the detector. The beam is defocused by moving the object out of the focal plane. Light at the pinhole is defocused and very little light gets back to the detector. Typically, with a large aperture lens, the 3 dB points of the response are of the order of 500 nm apart.