Main Difference – CRT vs. LCD
CRT and LCD are two display technologies used by monitors. CRT is an older technology. For domestic applications, CRT screens have largely been replaced by LCD and plasma screens. However, CRTs continue to be used in science and medicine, where they are used in as cathode ray oscilloscopes (CROs). The main difference between CRT and LCD is that the CRT screens use electron guns to shoot beams of electrons in order to display images whereas LCD screens use the “twist” in liquid crystals to display the images.
What is CRT
CRT stands for cathode ray tube. In CRTs, there are heated metal filaments called cathodes. These filaments emit electrons which are then accelerated by anodes, forming beams of electrons. An anode-cathode pair producing an electron beam is called an electron gun. The intensity of the electron beam can be controlled by changing the voltage applied to the cathode.
These accelerated electrons travel through a vacuum and strike the television screen. The screen of a CRT is coated with a phosphor, so that when electrons strike the screen, a glow is produced. The brightness of the glow depends on the intensity of the electron beam. The screen is made of many pixels, each pixel consisting of regions coated with different phosphors that would give off a red, green or blue light when electrons strike it. There are three electron beams produced by three electron guns, each beam made to strike a particular phosphor and produce a specific colour. Since the blue, green and red lights are produced in a small region we do not see the individual red, green and blue lights. Instead, depending on how much red, green and blue is present, we can see various colours.
To make sure that electrons from each beam ends up on the intended pixel and not on a neighbouring pixel, a shadow mask is used. This consists of a metal sheet with holes, and it sits behind the screen (some CRTs make use of a filter called aperture grill instead of a shadow mask). To produce an image, electron guns need to illuminate one pixel at a time. They do this at a very fast speed, however, so that we do not notice each pixel lighting up one by one.
The following video gives a good explanation of how a CRT works, along with animations:
What is LCD
LCD stands for liquid crystal display. An LCD has two polarising filters placed behind the screen, with their angles of polarisation perpendicular to each other. Normally, if two polarising filters are placed in this way, light cannot reach the screen. However, LCDs have a material called “twisted nematic liquid crystals” sandwiched between these two polarising filters. Liquid crystals are a special type of molecules which are arranged like molecules in a solid, although they have the ability to move about. In particular, twisted nematic liquid crystals can twist. Because they twist, they rotate the plane of polarisation of light passing through them.
In LCD screens, the liquid crystals are placed in such a way that their twist allows the light coming through one polarising filter to pass through the other filter. The “amount of twist” in the molecules, and thereby how much of the light passes through the filters, can be altered by means of a potential difference applied across the liquid crystal layer. The diagram below shows the different layers present in an LCD screen:
Just like a CRT, the LCD screen is also made of many pixels, each pixel consisting of three subpixels to produce red, green and blue light. Each subpixel is given an electrode so that by altering the voltage of this electrode, it is possible to alter the brightness of each coloured subpixel. The following video describes how an LCD screen works, with animations:
Difference Between CRT and LCD
Mechanism of Operation
CRT screens use electron guns to shoot a beam of electrons onto the screen. The screen is coated with a phosphor, which glows when electrons strike it.
LCD screens use an electric field to untwist molecules of liquid crystals sandwiched between two polarising filters so that the electric field can control the .
Contrary to popular belief, CRT screens are technically capable of producing better-quality images with a high contrast, since they do not need to be backlit, like LCD screens.
CRT screens cost more to produce, and they consume much more power compared to LCD screens.
CRT screens are bulkier and heavier compared to LCD screens.
“Old TV and 15 inch monitor” by Alpha (Own work) [CC BY-SA 2.0], via flickr
“Reflective twisted nematic w:liquid crystal display.” by ed g2s (Own work) [CC BY-SA 3.0], viaWikimedia Commons