22 2 DISPLAY SYSTEMS Figure 2.7 The basic design of plasma flat-panel display. When an electric current passes between the transparent electrodes (top) the gas in the addressed cell will glow (left center). black. (This is not true in an LCD pixel element, where it is not pos- sible to completely block the light.) On the other hand, the same technology that allows a plasma pixel to be completely shut off requires that its only option for "on" is to be completely on. In other words, the only way to create gray levels is to make each pixel flash very quickly. As a result, there are signifi- cant demands on the electronics controlling the plasma display. In an LCD display, it is easy to create any grayscale level by letting the liq- uid crystal twist the light into the appropriate polarization angle. In a plasma screen, the grayscale is harder to achieve. Another problem often discussed about plasma displays is the risk of burn-in effects. One way to avoid them is to avoid content that is too static. Organic Light-Emitting Diodes Displays Organic light-emitting diodes (OLEDs) are used only in very small display devices. But OLEDs are definitely one of the more exciting technologies for future display systems. OLED technology can be used for larger screens as well. They have low power consumption