A type of display used for computer monitors, HDTV, smartboards, and large advertising displays, They are relatively low-power devices based on liquid crystal and light-emitting diode (LED) technologies, and differ from older cathode ray tube displays in having minimal depth. Typically, the front layer of the display is composed of an array of cells of thin liquid crystals, with each cell corresponding to a single screen pixel, and each controllable individually by a pair of transparent electrodes that can be used to modify the optical properties of the cell. A backlight and coloured filters are also necessary because the liquid crystals merely form a transmissive layer and do not actually emit light. A number of different backlighting technologies have been used, with early liquid-crystal display (LCDs) using isolated cold cathode fluorescent lamps and an intervening diffusing layer to spread the light, while newer LED displays instead use lower-energy LED light sources in different configurations, depending on price and quality requirements. Direct LED systems use an array of LEDs to provide a grid of light sources, allowing localized dimming of regions of the screen and correspondingly improved black levels and uniformity of illumination. Edge LED systems use fewer LEDs, illuminating the screen only from the edges, with a corresponding drop in both the uniformity and controllability of the illumination, but are cheaper and thinner than full-array, local-dimming displays. In practice, the white-light sources for the backlight may be achieved by colour mixing the light from three small red, blue, and green LEDs placed as a cluster using surface mount technology; much of the light is lost when passed through colour filters to produce the individual red, green, and blue pixels. However, developments in quantum dot technology are leading to displays in which the quantum dots can be tuned to deliver the correct frequencies of light without the use of filters, providing a wider range of colours with lower energy requirements. Organic light-emitting diode (OLED) displays were the first to completely dispense with the need for a backlight, replacing the passive layer of liquid crystals by a large array of organic light-emitting diodes. In active-matrix organic light-emitting diode (AMOLED) displays these are individually controllable in an active-matrix format.