Wikijunior:How Things Work/Television

Most televisions and other displays around today use cathode ray tubes (CRT) or light emitting diodes (LED). The CRT television has three important parts. First is the cathode, which sends out a spray of electrons. Second are the focusing and deflecting coils, which shape that spray into a stream and then aim it at the third part, the screen, which has a phosphorus coating. The phosphorus lights up when it gets hit by electrons, and by lighting up different parts of the screen and leaving others dark, the CRT makes an image.

Who invented it?
Several people contributed to the invention of television. Each person did a little at a time. These three people seem to have been the major inventors:

Vladimir Kosma Zworykin developed a primitive television camera.

Philo Taylor Farnsworth was the first to successfully demonstrate the transmission of television signals.

John L Baird achieved the first transmissions of images of face shapes by means of television.

How dangerous is it?
A very high voltage is needed to produce the energy needed to start and run a CRT and there is a risk of this high voltage causing severe electric shocks.

Televisions have many dangerous parts secured in a cabinet to prevent people from hurting themselves.

What does it do?
A television is essentially a radio receiver with pictures. Signals similar to a radio's are received by the television antenna and sent to the receiver where they are sorted out.

Part of the signal is used to provide the sound you hear. Another part is used to create an image.

To get the image to appear like it is moving lots of still pictures are sent out. In the system used in the UK it is 25 each second. These are displayed one right after another to give the impression that the image is actually moving.

The first black and white televisions used a series of lines which were thick in some places and thin in others. If you looked very closely at a black and white television screen which was showing the letter "A". It would appear like the image below.

-==-    ---==--==---     --========--     -==--==-     ====

Today, all color televisions display a series of dots which change color depending on the image being displayed. If you look very closely at this screen with a magnifying glass you will be able to see this.

How does it vary?
There are many types of television sets. Some of them are explained below.

Cathode Ray Tube (CRT)
Cathode Ray Tube televisions have been around since the mid-1930s. The size of the screen is between 13 and 40 in.

Rear Projection
Rear projection screens are usually quite large in comparison to other television sets. The video image is projected from inside the unit onto a mirror that reflects the image back to the screen. The size of the screen ranges from 50 to 73 in. Rear projection television sets are also divided into many types of rear projection screens including CRT (three separate CRT projectors), DLP (Digital Light Processing), 3LCD (three LCD projectors), LCoS (Liquid Crystal over Silicon) and Laser televisions.

Front Projection
In Front Projection television sets, the image is projected from an over-head projector. Due to this light needs to be minimized. The size of the screen ranges from 40 to 100 in-plus.

Plasma
Plasma televisions are generally mounted on a wall since they are only a few inches thick. Plasma televisions offer excellent color and black level performance like the CRT. However the Plasma TVs tend to heat up, are heavier and consume more power than LCD televisions of the same size. The size of the screen ranges from 42 to 56 in.

Liquid Crystal Display (LCD)
LCD televisions do not have the same color and black level performance of plasma and CRT sets. They however have a low power consumption, cheap and glare-free visibility. The size of the screen ranges for 15 to 100 in or more.

How has it changed the world?
Communication has become so easy, masses are now accessible to the entertainment which was a privilege of the classes earlier. with the content only improving with time, we now get things unimaginable. News, movies, music, and educational channels have all improved our standard of living and have made us informed. At the same time one must be cautious of the time to be invested in such things as this box is also blamed for wasting people's time and making life miserable. TV sets are therefore also called "Idiot Box", so be careful spend time wisely.