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