Surface-conduction Electron-emitter Display - Comparison

Comparison

Further information: Liquid crystal display television

The primary large-screen television technology being deployed in the 2000s is the liquid crystal display televisions. SEDs are aimed at the same market segment.

LCDs do not directly produce light, and have to be back-lit using cold cathode fluorescent lamps (CCFLs) or high-power LEDs. The light is first passed through a polarizer, which cuts out half of the light. It then passes through the LCD layer, which selectively reduces the output for each sub-pixel. In front of the LCD shutters are small colored filters, one for each RGB sub-pixel. Since the colored filters cut out all but a narrow band of the white light, the amount of light that reaches the viewer is always less than 1/3 of what left the polarizer. Since the color gamut is produced by selectively reducing the output for certain colors, in practice much less light makes it through to the view, about 8 to 10% on average. In spite of using highly efficient light sources, an LCD display uses about the same power as a CRT of the same size.

LCD shutters consist of an encapsulated liquid that changes its polarization in response to an applied electrical field. This response is fairly linear, so even a small amount of leaked power that reaches surrounding shutters causes the image to become blurry. To counteract this effect, and improve switching speed, LCD displays use an Active matrix addressing of transparent thin-film transistors to directly switch each shutter. This adds complexity to the LCD screen and makes them more difficult to manufacture. The shutters are not perfect and allow light to leak through, which means that the contrast ratio of an LCD is less than that of a CRT, and this causes the color gamut to be reduced as well. Additionally the use of a polarizer to create the shutter limits the viewing angles where this contrast can be maintained. Most importantly, the switching process takes some time, on the order of milliseconds, which leads to blurring on fast moving scenes. Massive investment in the LCD manufacturing process has addressed all of these concerns to some degree.

The SED produces light directly on its front surface. Scenes are lit only on those pixels that require it, and only to the amount of brightness they require. In spite of the light generating process being less efficient than CCFLs or LEDs, the overall power efficiency of an SED is about ten times better than a LCD of the same size. SEDs are also much less complex in overall terms – they lack the active matrix layer, backlighting section, color filters and the driver electronics that adjusts for various disadvantages in the LCD shuttering process. Despite of having two glass layers instead of one in a typical LCD, this reduction in overall complexity makes SEDs similar in weight and size as LCDs.

Canon's 55" prototype SED offered bright images of 450 cd/m2, 50,000:1 contrast ratios, and a response time of less than 1 ms. Canon has stated that production versions would improve the response time to 0.2 ms and 100,000:1 contrast ratios. SEDs can be viewed from extremely wide angles without any effect on the quality of the image. In comparison, a modern LCD televisions like the Sony KDL-52W4100 claims to offer 30,000:1 contrast ratios, but this uses the "dynamic contrast" measurement, and the "on-screen contrast ratio" is a more realistic 3,000:1. Contrast ratios of LCD televisions are widely inflated in this manner. The same set claims to offer viewing angles of 178 degrees, but the useful viewing angles are much narrower, and beyond that the color gamut changes. Sony does not quote their response times, but 4 ms is common for larger sets, although this is also a dynamic measurement that only works for certain transitions. SEDs are very closely related to the field emission display (FED), differing only in the details of the emitter. FEDs use small spots containing hundreds of carbon nanotubes whose sharp tips give off electrons when placed in a strong electrical field. FEDs suffer from erosion of the emitters, and require extremely high vacuum in order to operate. For this reason, industry observers generally state that the SED is a more practical design. FEDs have one advantage the SED does not offer; since each sub-pixel has hundreds of emitters, "dead" emitters can be corrected by applying slightly more power to the working ones. In theory, this could increase yields because the chance of a pixel being completely dead is very low, and the chance that a screen has many dead pixels is greatly reduced. Sony has demonstrated a 26" FED drawing only 12 W showing a bright scene, SEDs should be even lower powered. Throughout the flat-screen introduction, several other technologies had been vying with LCDs and PDPs for market acceptance. Among these were the SED, the FED, and the organic light-emitting diode system that uses printable LEDs. All of these shared the advantages of low power use, excellent contrast ratio and color gamut, fast response times and wide viewable angles. All of them also shared the problem of scaling up manufacturing to produce large screens. Example systems of limited size, generally 13", have been shown for several years and are available for limited sales, but wide-scale production has not started on any of these alternatives.