The principle behind SED is simple. The screen employs the same technique that CRTs have used for a century --shooting electrons at phosphors embedded in the screen, which then emit light. However, instead of using a single large electron gun (or three for an RGB colour screen) firing electrons through a vacuum tube, as in a CRT, an SED screen has a tiny electron emitter placed directly behind each pixel on the screen.
The trick has been designing an equivalent of an electron gun that is hundreds of times smaller and doesn’t consume huge amounts of power. Toshiba and Canon have worked together and come up with an innovative solution.
An SED screen has two plates of glass separated by a vacuum, one plate coated in red, green and blue phosphor sub-pixels, while the other is coated with a matrix of electron emitters. On the side with the electron emitters is a channel only a few nanometres wide -- with one side of the channel positively charged and the other negatively charged. When a small voltage of around 10V is applied across this channel electrons flow from one side to the other. Some get scattered rather than flowing smoothly to the other side of the channel. At this point, another set of electrodes take over. These electrodes run at a much higher voltage, in the order of 10kV, and they propel the scattered electrons towards the phosphor elements on the other plate of glass.
The result is like having a miniature electron gun on each and every coloured sub-pixel, giving pixel precision in drawing the screen, much as an LCD or plasma screen does. There’s no need to deflect a single electron beam across the whole screen, so screen sizes can be considerably larger than CRTs. Canon suggests screens of 40in (102cm) or higher that are only several centimetres thick.
Furthermore, because a SED screen doesn’t have a backlight, like a LCD, when the emitters are not firing, there’s virtually no light emitted. This means a SED display has superior black levels. This could result in unprecedented contrast levels of many thousands to one -- surpassing the contrast of even the best CRTs and high-end plasma screens. Because each pixel has its own electron emitter which can be individually switched on or off, pixel refresh times are also very high -- in the order of 1ms -- which makes motion blur practically non existent.
SED resolutions are as high as 720p today, with 1080p screens being slated for availability at launch. Toshiba and Canon have already demoed screens around 30 inches (76cm) in size, with screens above 50 inches (127cm) in size coming around launch time. The million-dollar question is: when will SED screens actually launch?