Flat-screen monitors (also called FPDs for Flat panel displays) are becoming more and more widespread, as they take up less space and are less heavy than traditional CRT monitors.
What's more, the technology used by flat-screen monitors uses less energy (lower than 10W, as opposed to 100W for CRT monitors) and emits less electromagnetic radiation.
Liquid crystal displays
LCD (Liquid Crystal Display) is based on a screen made up of two grooved transparent parallel plates, oriented at 90° to one another; the space between them holds a thin layer of liquid containing certain molecules (liquid crystals) which change direction when they are exposed to electrical current.
Combined with a source of light, the first plate acts as a polarizing filter, letting through only those light components whose oscillation is parallel to the grooves.
In the absence of electrical current, the light is blocked by the second plate, which acts as a perpendicular polarising filter.
When powered, the crystals align one by one in the direction of the electric field, and can cross the second plate.
By locally controlling the orientation of the crystals, it is possible to make pixels. There are normally said to be two types of flat screens, depending on which control system is used to polarise the crystals:
- "Passive matrix" displays, whose pixels are controlled by row and column. Pixels are given a row/column address using transparent conductors located in the monitor's frame. The pixel lights up when it is addressed, and turns off in when refreshed.
Passive matrix monitors usually use TN technology (Twisted Nematics). Passive matrix monitors often suffer from a lack of brightness and contrast.
- "Active matrix" displays, in which each pixel is controlled individually.
The most common technology for this kind of display is TFT (Thin Film Transistor), which can control every pixel using three transistors (which correspond to the 3 RGB colors). Under this system, the transistor coupled with each pixel can memorise its state, and keep it lit between refreshes. Active matrix monitors are brighter and display a sharper image. Whether the monitors are active or passive, they need a light source to function. The following terms define how the screen is lit:
- Reflection screens are light from the front, using artificial light or simply ambient light (as with most digital watches).
- Transmission screens use rear lighting to display information. This type of screen is especially well-suited for indoor use, or in dim light conditions, and normally provides a high-contrast, bright image. On the other hand, they become hard to read when used outdoors (in full sunlight)
- Transflective screens use rear lighting as well as a polariser made of a translucent material, which can transmit background light while reflecting some ambient light. This type of screen is especially suitable for devices that are meant to be used both indoors and outdoors (such as digital cameras and PDAs).
Plasma technology (PDP, Plasma Display Panel) is based on emitting light by exciting gases. The gas used in plasma screens is a mixture of argon (90%) and xenon (10%). Gas is contained within cells, each one corresponding to a pixel that corresponds to a row electrode and column electrode, which excite the gas within the cell. By modulating the voltage applied by the electrodes and the frequency of excitation, up 256 luminous values can be defined. The gas excited this way produces ultraviolet radiation (which is invisible to the human eye). With blue, green, and red phosphors distributed among the cells, the ultraviolet radiation is converted into visible light, so that pixels (made up of 3 cells) can be displayed in up to 16 million colors (256 x 256 x 256).
Plasma technology can be used to create large-scale high-contrast screens, but plasma screens are still expensive. What's more, power consumption is more than 30 times higher than for an LCD screen
The most common specifications for monitors are:
- The definition: The number of pixels that the screen can display. This number is generally between 640x480 (640 pixels long, 480 pixels wide) and 1600x1200, but higher resolutions are technically possible.
- The size: This is calculated by measuring the diagonal of the screen, and is expressed in inches (an inch is about 2.54 cm). Be careful not to confuse a screen's definition with its size. After all, a screen of a given size can display different definitions, although in general screens which are larger in size have a higher definition.
- The resolution: This determines the number of pixels per surface unit (given in linear inches). This is abbreviated DPI, for Dots Per Inch. A resolution of 300 dpi means 300 columns and 300 rows of pixels per square inch, which means that there are 90,000 pixels per square inch. By comparison, a resolution of 72 dpi means that one pixel is 1"/72 (one inch divided by 72) or 0.353 mm, which corresponds to one pica (a typographical unit).
- Response time: Defined by international standard ISO 13406-2, this corresponds to the amount of time needed to switch a pixel from white to black and back again. Response time (expressed in milliseconds) should be as low as possible (pragmatically, lower than 25 ms).
- Luminance: Expressed in candelas per square metre (Cd/m2), this is used to define the screen's "brightness" The order of magnitude for luminance is about 250 cd/m2.
- The horizontal and vertical viewing angle: Expressed in degrees, this is used to define the angle from which viewing the screen becomes difficult when the user is not looking at it straight-on.