Modern society has depended on LCD screens since the digital watch was invented in the 1970s. Since then, they have gradually permeated our lives in the form of mobile phones, laptops, TVs, microwaves and even ovens. So how do they work, exactly?
There are millions of colour pixels in every kind of screen, whether it is CRT, LED or plasma. In every screen, these pixels are rapidly turned on and off in order to create a moving image. Each type of screen does this differently. Liquid crystal displays (LCDs) turn pixels on or off by rotating polarised light. This may sound simple enough, but the chemistry behind it is pretty complex. For more extensive chemistry knowledge, head to Chemicals.co.uk.
What are Liquid Crystals?
Until 1879, there were 3 known states of matter: solid, liquid and gaseous. Then plasma was discovered and a fourth was added to the list. Scientists thought that all states of matter had been identified – until 1888, when Friedrich Reinitzer discovered something very peculiar.
Reinitzer had discovered a substance that was neither solid nor liquid, but something in the middle. These substances became known as liquid crystals (LCs), and these mesophases are used in many things besides your TV screen. They are also commonly found in:
- Some clays
- Mood rings
- Cell membranes
- Soaps and detergents
Like the molecules in a solid, liquid crystals maintain their orientation and display similar symmetry properties. Unlike solids, however, liquid crystals can move around and flow like a liquid. Therefore, liquid crystals can be defined as having the order of a solid and flexibility of a liquid.
Liquid crystals can exist in several states. These can be categorised into 3 main phases: thermotropic, lyotropic and metallotropic. Most liquid crystals are either thermotropic or lyotropic, but it is thermotropic LCs that make LCD screens possible.
Thermotropic Liquid Crystals
Thermotropic LCs are sensitive to temperature changes. At high temperatures, the components of this liquid crystal will gain energy and undergo a phase transition into an isotropic liquid. At low temperatures, the thermotropic phase cannot be supported and the LC will enter a glass phase.
There are many sub-states to thermotropic liquid crystals, but the one that LCD screens are interested in is the nematic phase. When in this state, LC molecules can flow like a liquid whilst maintaining an order.
Their natural twisted structure can also be straightened out by an electric current. This behaviour is important because it is exactly how pixels in an LCD screen are turned on or off. But before the liquid crystals can do this, they first need polarised light.
What is Polarised Light?
Normal sunlight is classified as unpolarised because its light waves vibrate in every plane, i.e. horizontally, vertically, diagonally and every other direction. Polarised light, then, is simply when light waves have been manipulated so that they only vibrate in a single direction, known as a plane.
When light is polarised, it appears darker because only certain light waves are able to pass through. This is how polarising sunglasses work. They contain a filter that only allows light waves vibrating on a single plane, e.g. vertically, to pass through. Light waves that vibrate horizontally, for example, will not be able to pass through the filter, making the light appear darker.
In fact, if you put 2 pairs of polarising sunglasses on top of each other and turn one of them to a 90° angle, no light will be able get through and the glasses will turn black. This exact same principle is used in LCD screens to turn pixels on and off.
How do LCD Screens Work?
In order for an LCD screen to operate, a few materials are required. First, a backlight is positioned at the back of the screen. This is needed because LCD screens cannot produce light by themselves and require a source of illumination in order to create an image.
Then two pieces of glass are coated on one side with a polarising film. The film on the first piece of glass faces the backlight and will only let vertical light waves, for example, pass through. The second polarising filter is then placed at a 90° angle to the first piece so that only horizontal light waves, for example, will be able to pass through.
The back of the first piece of polarising glass is also coated with a special polymer that creates microscopic grooves in the direction of the filter. A layer of nematic light crystals is then added on top so that their orientation is aligned to the direction of the grooves and filter.
Between the two pieces of pieces of polarising glass is also a negative and positive electrode. These are important because they cause the light crystals to either twist or straighten. Here’s how it works:
- The light from the backlight travels forwards. When it reaches the first polarising filter, only light waves vibrating on a single plane (e.g. vertically) can pass through.
- The light waves then pass through the layer of liquid crystals. If there is no electric current, the LCs have a twisted structure and are able to rotate the polarised light as it passes through.
- The polarised light waves are now vibrating horizontally and are able to pass through the second polarising filter which is at a 90° angle to the first one.
- The light can then reach the relative pixels and switch them on. There are millions of red, blue and green pixels in a screen and when they light up, they create an image.
- However, if the electric current is switched on, the liquid crystals straighten up and do not rotate the polarised light as it passes through.
- Since the light is still vibrating on the vertical plane, it cannot pass through the second, horizontal filter and the relevant pixels remain dark and switched off.
LCD screens can employ this process multiple times every second, resulting in the rapid activation and deactivation of pixels. It happens so rapidly that it creates a moving image.
A lot of chemistry takes place inside your TV, laptop or phone screen, showing how integral science really is in our everyday lives.