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Speaker 1: Welcome to Brainstuff from how stuff works dot com where
Speaker 1: smart Happens. Hi Am Marshall brand with today's question, how
Speaker 1: do they backlight l c D screens? Obviously, these screens
Speaker 1: are brightly backlit. A lot of people use their cell
Speaker 1: phones or their laptops as flashlights, so they're using the
Speaker 1: backlights to produce usable light. How do they make these
Speaker 1: backlights so bright and so consistent? Currently? There are two
Speaker 1: technologies for backlighting the screens. Today, most computer liquid crystal
Speaker 1: display or l c D panels are lit with built
Speaker 1: in fluorescent tubes that are placed either above, beside, or
Speaker 1: sometime directly behind the l c D panel. A white
Speaker 1: diffusion panel behind the l c D redirects and scatters
Speaker 1: the light evenly, and that ensures a uniform display. This
Speaker 1: is known as the backlight. A fluorescent light is most
Speaker 1: often a long, straight glass tube that produces white light.
Speaker 1: Inside the glass tube, there's a low pressure mercury vapor.
Speaker 1: When ionized, mercury vapor emits ultra violet light. Human eyes
Speaker 1: aren't sensitive to ultra violet light, although the human skin is.
Speaker 1: That's how we get sunburns. The inside of the fluorescent
Speaker 1: tube is coated with a phosphor that's going to turn
Speaker 1: that ultra violet light into something human beings can see.
Speaker 1: Phosphor is a substance that can accept energy in one
Speaker 1: form and emit the energy in the form of visible light.
Speaker 1: For example, energy from high speed electrons in a TV
Speaker 1: tube are orbed by the phosphors on a normal CRT
Speaker 1: screen and then turned into light. The light that we
Speaker 1: see from a fluorescent tube is the light given off
Speaker 1: by the phosphor coating inside the tube. The phosphor fluoresces
Speaker 1: when energized, hence the name. A typical laptop display uses
Speaker 1: a tiny cold cathode fluorescent lamp also known as a
Speaker 1: CCFL for the back light. One of these small tubes
Speaker 1: is able to provide an incredibly bright white light source
Speaker 1: that can be diffused by the panel behind the l
Speaker 1: c D In addition to providing ample light, CCFLs do
Speaker 1: not rise far above the ambient temperature. This makes them
Speaker 1: ideal for l c D panels since the light source
Speaker 1: is close to the other components that could be ruined
Speaker 1: by excessive heat. One amazing thing about these lamps is
Speaker 1: their incredibly small size. They are extremely thin, about the
Speaker 1: size of the graphite in a wooden pencil, and the
Speaker 1: board that drives the lamp is very small as well. However,
Speaker 1: it's not that hard to break these lamps, which is
Speaker 1: why you're display may go dark if you drop your laptop.
Speaker 1: The technology that's coming on strong today and is likely
Speaker 1: to completely replace these fluorescent lamps in the future is
Speaker 1: l e ED technology. L e d s are more
Speaker 1: rugged than fluorescent lamps, they use less power than fluorescent lamps,
Speaker 1: and they're more adjustable than fluorescent lamps. But they also
Speaker 1: have this one other really interesting feature that's now used
Speaker 1: on some of the high end l e ED backlit
Speaker 1: h D t V screens, and that is you can
Speaker 1: turn the l e d s in the backlighting on
Speaker 1: and off separately, so you can have an array of
Speaker 1: white l e d s that are behind the l
Speaker 1: c D panel and in the areas that are supposed
Speaker 1: to be dark, you can turn down or turn off
Speaker 1: those l c D s. So when you look at
Speaker 1: big l c D h D t V s today
Speaker 1: and you see them with super high contrast ratios like
Speaker 1: three million to one or ten million to one contrast ratios,
Speaker 1: that's being done by having a panel of l e
Speaker 1: d s that are behind the l c D panel,
Speaker 1: and those l e d s are being controlled by
Speaker 1: the TV signal to turn parts of the screen completely dark.
Speaker 1: For more on this and thousands of other topics, does
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