Television and display devices
A display device turns electronic signals into a visible image, and several distinct technologies have done this over the history of television and computing. Each works by controlling many small picture elements, or pixels, across a screen and refreshing them fast enough that the eye perceives smooth, continuous motion. The way each technology generates light and colour gives it a characteristic set of strengths in brightness, thickness, viewing angle and power use.
The cathode-ray tube (CRT), used in older televisions and monitors, sweeps one or more electron beams across a phosphor-coated screen; where a beam strikes, the phosphor glows, and colour is produced by separate red, green and blue phosphors. The liquid-crystal display (LCD) instead uses a layer of liquid-crystal material whose molecules twist under an applied voltage to control how much light from a backlight passes through coloured filters; it is thin, light and energy-efficient. A plasma display uses a grid of tiny gas-filled cells that are excited into a glowing plasma, which in turn lights coloured phosphors, allowing large flat panels with strong contrast.
Closely related is the charge-coupled device (CCD), an image sensor rather than a display. A CCD is an array of light-sensitive elements that convert incoming light into electric charge; the charge packets are then shifted out in sequence to be read and digitised. CCDs were widely used in video and still cameras and in scientific imaging, capturing the pictures that display devices reproduce.
Frequently asked questions
- How does an LCD produce colour?
- Liquid crystals twist under voltage to vary how much backlight passes through each subpixel, and red, green and blue colour filters combine to make the visible colour.
- What makes a plasma display different from an LCD?
- A plasma panel lights tiny cells of excited gas that glow directly, while an LCD blocks or passes light from a separate backlight; plasma offered strong contrast, while LCD is thinner and more efficient.
- What is a CCD used for?
- A CCD is an image sensor that converts light into charge and shifts it out to be read, capturing images in cameras and scientific instruments rather than displaying them.
