Photocell with secondary electron multiplier, laboratory sample

The spherical photocell converts the light reflected by the object into electrical current. The photomultiplier amplifies the weak photocurrent. The electrons are conducted into a system of metal plates, where they trigger further electrons, and this number snowballs from plate to plate, amplifying the signal. The more sensitive the photocell, the better the image resolution, because a shorter exposure time produces a sufficiently strong signal.