Many
of you will have a television at home and you will certainly have seen one working. To
understand something of how they work it is useful to compare them with the
CR0.
Like the CR0, the important part of the television is its tube. This is
evacuated and contains an electron gun that fires a beam of electrons at a fluorescent
screen. This glows when electrons hit it. The more electrons the brighter it glows.
Television screens are usually bigger than CR0 screens, their size is measured in
centimeters (or inches) from corner to corner.
Unlike the CR0 the beam is moved by a set of electromagnetic coils at the base of the tube. The beam 'scans' the screen in a series of lines, starting at the top left and finishing at the bottom right. In modern televisions there are 625 lines, the odd ones are scanned first then the even ones. This stops the top left part of the picture fading out before the bottom right. The beam only takes 1/25 s to scan the whole screen once. You can see a simplified version of this in the diagram.
The
signal to the electron gun comes from the aerial and this controls the brightness of the
beam at any point on the screen at any instant. This can be seen in the very simple
example screen. The beam is on in the white area but off in the black area. In an actual
picture there are shades of grey as well and the picture is changing all the time.
If
you look carefully at a television screen you will see that it collects a lot of dust and if you
put your finger near it when the set has been on for a while you may feel a tingling in your
finger or even get a small spark. This is because of the build-up of static charge on the
screen which attracts dust and is earthed through your finger.
Colour television sets are a little more
complicated. As you know you need three colours to make any other colour - red, blue and
green.
In a colour television set the screen is covered with dots of fluorescent
paint arranged in threes. One glows red when an electron beam hits it, one blue and the
third one green. There are one million of these dots - 333 333 of each type.
There are
also three electron guns, each one lined up so that electrons from it only hit dots of one
type. This is done by a 'shadow mask'; a metal plate with a million holes between the
electron guns and the screen. Changing the brightness of each gun will change the colour
of the screen.
Some colour televisions work slightly differently, they have only one
gun and a series of vertical lines instead of dots. This reduces the power used by the set
and gives a very good picture.
There have been
some exciting developments in televisions in the past few years and soon flat screen
televisions, of both the LCD and plasma type will become much more common. In fact by 2013 few homes in Britain still use the traditional cathode ray tube televisions.
The LCD sets are much lighter and thinner, consume less energy than electron beam TVs and some are designed to hang on the walls of rooms like pictures.
The screen of an LCD TV is made of millions of liquid crystals. Each crystal is like the shutter of a camera either blocking the light or allowing it to pass through. You can control the amount of light that passes through by applying a voltage to a crystal or pixel. It does this by rotating the plane of polarisation of the light.