WEBVTT - How are LCD screens backlit?

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<v Speaker 1>Welcome to Brainstuff from how stuff works dot com where

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<v Speaker 1>smart Happens. Hi Am Marshall brand with today's question, how

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<v Speaker 1>do they backlight l c D screens? Obviously, these screens

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<v Speaker 1>are brightly backlit. A lot of people use their cell

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<v Speaker 1>phones or their laptops as flashlights, so they're using the

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<v Speaker 1>backlights to produce usable light. How do they make these

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<v Speaker 1>backlights so bright and so consistent? Currently? There are two

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<v Speaker 1>technologies for backlighting the screens. Today, most computer liquid crystal

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<v Speaker 1>display or l c D panels are lit with built

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<v Speaker 1>in fluorescent tubes that are placed either above, beside, or

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<v Speaker 1>sometime directly behind the l c D panel. A white

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<v Speaker 1>diffusion panel behind the l c D redirects and scatters

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<v Speaker 1>the light evenly, and that ensures a uniform display. This

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<v Speaker 1>is known as the backlight. A fluorescent light is most

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<v Speaker 1>often a long, straight glass tube that produces white light.

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<v Speaker 1>Inside the glass tube, there's a low pressure mercury vapor.

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<v Speaker 1>When ionized, mercury vapor emits ultra violet light. Human eyes

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<v Speaker 1>aren't sensitive to ultra violet light, although the human skin is.

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<v Speaker 1>That's how we get sunburns. The inside of the fluorescent

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<v Speaker 1>tube is coated with a phosphor that's going to turn

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<v Speaker 1>that ultra violet light into something human beings can see.

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<v Speaker 1>Phosphor is a substance that can accept energy in one

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<v Speaker 1>form and emit the energy in the form of visible light.

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<v Speaker 1>For example, energy from high speed electrons in a TV

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<v Speaker 1>tube are orbed by the phosphors on a normal CRT

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<v Speaker 1>screen and then turned into light. The light that we

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<v Speaker 1>see from a fluorescent tube is the light given off

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<v Speaker 1>by the phosphor coating inside the tube. The phosphor fluoresces

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<v Speaker 1>when energized, hence the name. A typical laptop display uses

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<v Speaker 1>a tiny cold cathode fluorescent lamp also known as a

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<v Speaker 1>CCFL for the back light. One of these small tubes

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<v Speaker 1>is able to provide an incredibly bright white light source

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<v Speaker 1>that can be diffused by the panel behind the l

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<v Speaker 1>c D In addition to providing ample light, CCFLs do

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<v Speaker 1>not rise far above the ambient temperature. This makes them

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<v Speaker 1>ideal for l c D panels since the light source

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<v Speaker 1>is close to the other components that could be ruined

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<v Speaker 1>by excessive heat. One amazing thing about these lamps is

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<v Speaker 1>their incredibly small size. They are extremely thin, about the

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<v Speaker 1>size of the graphite in a wooden pencil, and the

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<v Speaker 1>board that drives the lamp is very small as well. However,

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<v Speaker 1>it's not that hard to break these lamps, which is

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<v Speaker 1>why you're display may go dark if you drop your laptop.

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<v Speaker 1>The technology that's coming on strong today and is likely

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<v Speaker 1>to completely replace these fluorescent lamps in the future is

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<v Speaker 1>l e ED technology. L e d s are more

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<v Speaker 1>rugged than fluorescent lamps, they use less power than fluorescent lamps,

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<v Speaker 1>and they're more adjustable than fluorescent lamps. But they also

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<v Speaker 1>have this one other really interesting feature that's now used

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<v Speaker 1>on some of the high end l e ED backlit

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<v Speaker 1>h D t V screens, and that is you can

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<v Speaker 1>turn the l e d s in the backlighting on

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<v Speaker 1>and off separately, so you can have an array of

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<v Speaker 1>white l e d s that are behind the l

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<v Speaker 1>c D panel and in the areas that are supposed

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<v Speaker 1>to be dark, you can turn down or turn off

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<v Speaker 1>those l c D s. So when you look at

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<v Speaker 1>big l c D h D t V s today

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<v Speaker 1>and you see them with super high contrast ratios like

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<v Speaker 1>three million to one or ten million to one contrast ratios,

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<v Speaker 1>that's being done by having a panel of l e

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<v Speaker 1>d s that are behind the l c D panel,

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<v Speaker 1>and those l e d s are being controlled by

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<v Speaker 1>the TV signal to turn parts of the screen completely dark.

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