LCD, TFT and CCD Technology

The liquid-crystal display (LCD) is a flat-panel technology that forms images by controlling how much light passes through tiny cells rather than emitting light itself. Each cell contains liquid crystal, a material whose rod-shaped molecules can be aligned by an applied electric field. Sandwiched between two crossed polarising filters, the liquid crystal acts as a controllable light valve, twisting the polarisation of passing light by an amount that depends on the voltage applied across the cell.

The classic arrangement is the twisted nematic effect. With no voltage the molecules form a quarter-turn helix that rotates the light's polarisation so it passes the second polariser and the cell looks bright; applying a voltage straightens the molecules so the light is blocked and the cell looks dark. Because liquid crystals only modulate light, an LCD needs a separate light source, typically a backlight, and colour is produced by red, green and blue filters over groups of subpixels. In a thin-film-transistor (TFT) active-matrix display each subpixel has its own transistor that holds its state, giving the fast, high-contrast images used in monitors and televisions.

A related optoelectronic device is the charge-coupled device (CCD), an image sensor rather than a display. It is an array of light-sensitive elements that convert incoming photons into electric charge, accumulating charge in proportion to the light falling on each pixel. The stored charges are then shifted across the chip and read out to build up an image. CCDs were long the dominant sensor in digital cameras, scanners and astronomical instruments before complementary CMOS sensors became common.

Frequently asked questions

Does an LCD produce its own light?

No. Liquid crystals only modulate light, so an LCD needs a separate light source such as a backlight. The crystals act as valves that let more or less light through.

What is the twisted nematic effect?

It is an LCD mode where liquid-crystal molecules form a twisted helix that rotates light's polarisation. Applying a voltage untwists them, switching the cell between bright and dark.

What does a CCD do?

A charge-coupled device is an image sensor that converts light into electric charge at each pixel, then shifts the charges out to be read, forming a digital image.