The displays for old-fashioned TVs are based on a device called a cathode-ray tube. These consist of a glass tube from which the air has been removed, with a fluorescent screen at one end and an electron gun at the other.

The original televisions
The electron gun uses a heater, much like the filament from a lightbulb, to heat a cathode (a terminal through which positive electric current leaves its source, or negatively charged electrons enter). The heated cathode releases a cloud of electrons, which are turned into a beam by two anodes (the terminal where current flows into a device, or electrons leave).

Credit: SXC/hberends
Credit: SXC/hberends

One anode attracts the electrons and directs them towards the screen while the other focuses them into a tight beam, which is steered by electromagnetic coils that surround the tube. When the beam of electrons hits the screen, it excites a fluorescent coating – usually phosphor – which then glows.

The beam is scanned across the screen in a pattern of horizontal lines called a “raster”.  Controlling the intensity of the beam at a particular point allows a picture to form. Colour televisions use three electon guns and three types of screen coating that glow red, blue and green, combining these in various proportions to produce any colour.

Flat screens
As cathode-ray tubes are large, heavy and fragile, they have been largely replaced by flat-screen displays. These were once too expensive to be commercially viable but have become more common as costs have fallen.

Flat screens are usually LCDs, or liquid crystal displays. Liquid crystals are made up of molecules that are arranged in a regular way, like crystals, but are free to move around, like liquids. Applying an electric field disrupts the alignment of liquid crystals’ molecules and changes the amount of light they let through.

Screens consist of many pixels made up of a layer of liquid crystal between transparent electrodes – in colour LCDs the pixels are divided into three cells coloured red, green and blue using filters. Changing the intensity and colour of each pixel forms a picture.

Adding depth
Television programmes are usually broadcast as a flat two-dimensional image, but 3-D TV is becoming more popular. There are several ways by which the impression of depth can be given to a TV image, the most common of which is stereoscopy.

This involves filming with two cameras separated by the same distance as a person’s eyes. The two images are sent to different eyes using 3-D glasses. 

Credit: SXC/fodor - 3-D glasses
Credit: SXC/fodor - 3-D glasses
Credit: SXC/fodor - 3-D glasses

The first 3-D glasses were “passive” and simply used different coloured lenses. The TV shows two slightly offset images identical to each other except that one has a blueish tint and the other a reddish tint. The glasses’ red filter blocks out all of the red light so one eye can’t see the red-tinted image, and similarly for the blue picture. 

The brain is fooled into thinking it’s looking at the same object, and the difference in focus gives the illusion of depth.

New 3-D-ready TVs are different. They flick between the two different images more than 120 times every second, and require “active” 3-D glasses. Active glasses use liquid crystal lenses to rapidly switch between transparent and opaque in time with the TV image changing, often controlled by radio or infrared.

The change happens more quickly than can be detected by the brain, which simply combines the two images. Active 3-D doesn’t compromise the colour of the images as much, but they are very expensive.

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