In 1888 German scientist Friedrich Reinitzer extract a compound known as cholesterol from carrots. He found that when he heated the liquid it changed from an orange colour to clear due to the crystal structure within the liquid.
In 1962 electronics engineer Richard Williams found that he could make the crystals move when he applied an electric current to them. A few years later the first liquid crystals displays (LCD) were built. These simple back and white (silvery-grey) displays first appeared in watches and calculators. Today LCD are also found in televisions, mobile phones and laptop computers.
LCDs consist of several layers sandwiched together. The middle layer contains the liquid crystals and on either side of this are two polarising filters – one rotated 90 degrees to the other. The liquid crystals in the middle layer align themselves to the same orientation as the polarising filter they are in contact with. When viewed side-on, the crystals in the middle layer appear to twist 90 degrees.
Most calculators and digital watches use a mirror at the back of the display, which reflects light that passes through the display. Laptops, mobile phones and televisions use a thin fluorescent light at the back of the screen.
Light waves passing through the display must travel each layer. When they pass through the back polarising filter they are polarised in that plane, for example, up and down. They then pass through the nemantic liquid crystal layer where their polarisation is rotated 90 degrees – now left and right. – which allows them to pass through the front polarising filter.
If an electrical current is applied to a small portion of the liquid crystal layer, the crystals straighten and are no longer aligned to the front polarising layer. This causes the light to become blocked by the front polarising layer and that part of the screen now appears black.
Colour LCD
If you look closely at a colour LCD with a magnifying glass you will see tiny dots of red, green and blue – the primary colours of light.
When no electrical current is applied to a section of the colour LCD, light passes from the fluorescent light, through the polarising filters and crystal layer, and then through the colour filter on the front of the display. When the current is applied, the light is blocked by the crystal layer and that part of the screen appears black.
Colour LCD can also adjust the intensity of the colour by adjusting the electrical current and allowing a percentage of light to pass through the colour filter.
Early colour LCD televisions were slow at changing the intensity of light that allowed through, causing ghosting images to appear, especially when watching fast action such as action movies and sport.
Today, some colour LCD televisions can change at frequencies of close to 200 Hz (times per second), much quicker than the human eye can detect.
This first appeared in the December 2009 issue of The Helix.