Large-screen television technology

Large-screen television technology (colloquially big-screen TV) developed rapidly in the late 1990s and 2000s. Prior to the development of thin-screen technologies, rear-projection television was standard for larger displays, and jumbotron, a non-projection video display technology, was used at stadiums and concerts. Various thin-screen technologies are being developed, but only liquid crystal display (LCD), plasma display (PDP) and Digital Light Processing (DLP) have been publicly released. Recent technologies like organic light-emitting diode (OLED) as well as not-yet-released technologies like surface-conduction electron-emitter display (SED) or field-emission display (FED) are in development to supersede earlier flat-screen technologies in picture quality.

Large-screen technologies have almost completely displaced cathode-ray tubes (CRT) in television sales due to the necessary bulkiness of cathode ray tubes. The diagonal screen size of a CRT television is limited to about 100 cm (40 in) because of size requirements of the cathode-ray tube, which fires three beams of electrons onto the screen to create a viewable image. A large-screen TV requires a longer tube, making a large-screen CRT TV of about 130 to 200 cm (50 to 80 in) unrealistic. Newer large-screen televisions are comparably thinner.

Before deciding on a particular display technology size, it is very important to determine from what distances it is going to be viewed. As the display size increases so does the ideal viewing distance. Bernard J. Lechner, while working for RCA, studied the best viewing distances for various conditions and derived the so-called Lechner distance.

As a rule of thumb, the viewing distance should be roughly two to three times the screen size for standard definition (SD) displays.

The following are important factors for evaluating television displays:

A pixel on an LCD consists of multiple layers of components: two polarizing filters, two glass plates with electrodes, and liquid crystal molecules. The liquid crystals are sandwiched between the glass plates and are in direct contact with the electrodes. The two polarizing filters are the outer layers in this structure. The polarity of one of these filters is oriented horizontally, while the polarity of the other filter is oriented vertically. The electrodes are treated with a layer of polymer to control the alignment of liquid crystal molecules in a particular direction. These rod-like molecules are arranged to match the horizontal orientation on one side and the vertical orientation on the other, giving the molecules a twisted, helical structure. Twisted nematic liquid crystals are naturally twisted, and are commonly used for LCDs because they react predictably to temperature variation and electric current.

When the liquid crystal material is in its natural state, light passing through the first filter will be rotated (in terms of polarity) by the twisted molecule structure, which allows the light to pass through the second filter. When voltage is applied across the electrodes, the liquid crystal structure is untwisted to an extent determined by the amount of voltage. A sufficiently large voltage will cause the molecules to untwist completely, such that the polarity of any light passing through will not be rotated and will instead be perpendicular to the filter polarity. This filter will block the passage of light because of the difference in polarity orientation, and the resulting pixel will be black. The amount of light allowed to pass through at each pixel can be controlled by varying the corresponding voltage accordingly. In a color LCD each pixel consists of red, green, and blue subpixels, which require appropriate color filters in addition to the components mentioned previously. Each subpixel can be controlled individually to display a large range of possible colors for a particular pixel.

The electrodes on one side of the LCD are arranged in columns, while the electrodes on the other side are arranged in rows, forming a large matrix that controls every pixel. Each pixel is designated a unique row-column combination, and the pixel can be accessed by the control circuits using this combination. These circuits send charge down the appropriate row and column, effectively applying a voltage across the electrodes at a given pixel. Simple LCDs such as those on digital watches can operate on what is called a passive-matrix structure, in which each pixel is addressed one at a time. This results in extremely slow response times and poor voltage control. A voltage applied to one pixel can cause the liquid crystals at surrounding pixels to untwist undesirably, resulting in fuzziness and poor contrast in this area of the image. LCDs with high resolutions, such as large-screen LCD televisions, require an active-matrix structure. This structure is a matrix of thin-film transistors, each corresponding to one pixel on the display. The switching ability of the transistors allows each pixel to be accessed individually and precisely, without affecting nearby pixels. Each transistor also acts as a capacitor while leaking very little current, so it can effectively store the charge while the display is being refreshed.