field can correspond to a potential on the order of tens of volts. On the order of 3%, a few of the electrons impact with slit material on the far side and are scattered out of the emitter surface. A second field, applied externally, accelerates these scattered electrons towards the screen. Production of this field requires kilovolt potentials, but is a constant field requiring no switching, so the electronics that produce it are pretty simple. Each emitter is aligned behind a colored phosphor dot. The accelerated electrons strike the dot and cause it to give off light in a fashion identical to a conventional CRT. Since each dot on the screen is lit by a single emitter, there is no need to steer or direct the beam as there is in a CRT. The quantum tunneling effect, which emits electrons across the slits, is highly non-linear, and the emission process tends to be fully on or off for any given voltage. This allows the selection of particular emitters by powering a single horizontal row on the screen and then powering all the needed vertical columns simultaneously, thereby powering the selected emitters. The half-power received by the rest of the emitters on the row is too small to cause emission, even when combined with voltage leaking from active emitters beside them. This allows SED displays to work without an active matrix of thin-film transistors that LCDs and similar displays require to precisely select every sub-pixel, and further reduces the complexity of the emitter array. However, this also means that changes in voltage cannot be used to control the brightness of the resulting pixels. Instead, the emitters are rapidly turned on and off using pulse-width modulation, so that the total brightness of a spot at any given time can be controlled. SED screens consist of two glass sheets separated by a few millimeters, the rear layer supporting the emitters and the front the phosphors. The front is easily prepared using methods similar to existing CRT systems; the phosphors are painted onto the screen using a variety of silkscreen or similar technologies and then covered with a thin layer of aluminum to make the screen visibly opaque and provide an electrical return path for the electrons once they strike the screen. In the SED, this layer also serves as the front electrode that accelerates the electrons toward the screen, held at a constant