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3. brief history of tv screens          

Television monitors have undergone several generations of modification and technological changes. All the older once used a cathode ray tube (CRT) to project images onto a phosphor coated screen. The German Physicist Dr Karl Ferdinand Braun invented the CRT in 1897. The key components of the tube are shown in the diagram below. A heated filament called cathode (not unlike an ordinary electric bulb) generates a cloud of electrons that are attracted to positively charged rings called anodes. Coils surround this arrangement to surround it with a magnetic field, which accelerates the electrons in the direction of the screen. Inside of the tube is vacuum it facilitate rapid movements of electrons. A stream of electrons is focused by a focusing anode in to a tight beam and then accelerated by an accelerating anode. To excite the atoms on the phosphor the electrons must have considerable energy. That is way the beam has to travel a considerable distance before they are allowed to impact the screen. As the beam of electrons impinge up the chemical coat on the inner surface of the screen, light is emitted. The beam is controlled by electronics, to scan the surface a line at a time in a Zigzag fashion.

 In diagram on the right, the blue lines show the scanning path of the electronic beam as it impinges on the screen, while the red lines are the reverse path when the beam is switched off. The green line is the retrace path to the top, again while the beam is switched off. The green line is the retrace path to the top, again while the beam is switched off.

As the beam paints each line from left to right, the intensity of the beam is changed to create different shades of black, grey and white across the screen. Because the lines are spaced very closely together, your brain integrates then into a single image. A TV screen normally has 525 lines visible from top to bottom. One such sweep of line from the top to the bottom is done in 1/30th of a second. So in one second 30 frames (of 525 lines) are created. This is what gives you the sense of continuity of motion of a picture.

Commercial broadcast began in the 1920s. Colour television was the next development in the 1950s. The trick was to cost the screen with dots (or stripes) of three types of phosphors, rather like the way a coloured paged in printed in a paper. Three beams simultaneously, scan the screen, and each beam activates the phosphor for a different colour. On the inside of the tube, very close to the phosphor coating, there is a thin metal screen called a shadow mask. This mask is perforated with very small holes that are aligned with the phosphor dots on the screen. The holes of the mask ensure that the beam of a specific colour hits the dots of its own colour and no other.

When a colour TV needs to create a red dot, it fires the red beam at the red phosphor. Similarly for green and blue dots. To create a white dot, red, green and blue beams are fired simultaneously- the three colours mix together to create white. To create a black dot, all three beams are turned off as they scan past the dot. All other colours on a TV screen are combinations of red, green and blue.

In the intervening before the advent of flat panel TVs, the shape of the screen changed. First the rounded corners were made sharp, giving the viewer the impression of watching a square screen. Next, the surface of the screen became flatter, until today all TVs have a flat screen CRT. So you have what are called `flat-screen flat-square' screen. The technology for this was mainly in the electronics, to ensure that there was no distortion of images along the edges and the corners in particular.

Research work on the plasma effect discovered in 1966 was conducted at the University of Illinois in 1968. Fujitsu began its research and development work on plasma display panels in the late 1960s, and in 1990 it produce the world's first commercially available 21 inch colour display. The colours in the earlier models appeared washed out; this has been remedied with better dyes. The first generation flat monitors were small sized. Large screen plasma TVs were made later. In 1997, Fujitsu introduced a 42-inch TV. In the same year several others introduced bigger and more impressive flat TVs. Technical advances lead to improving the contrast ratio and ultra high resolution and in the following year flat TVs started becoming a commercial success. Philips is the first to introduce the flat TV in India.