How are LCD screens backlit?

In this episode, Marshall explains the two technologies that are used to backlight LCD panels. Tune in to learn more about the science behind LCD panels.

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Episode Transcript

Speaker 1 (00:00):
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Welcome to Brainstuff from how stuff works dot com where

(00:22):
smart Happens. Hi Am Marshall brand 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

(00:45):
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

(01:05):
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 backlight. A fluorescent light is most
often a long, straight glass tube that produces white light.

(01:26):
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.

(01:48):
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 orbed 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

(02:09):
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

(02:31):
that can be diffused by the panel behind the l
c D In addition to providing ample light, CCFLs 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

(02:52):
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 may go dark if you drop your laptop.
The technology that's coming on strong today and is likely

(03:14):
to completely replace these fluorescent lamps in the future is
l e ED technology. 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

(03:34):
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 V s today

(03:55):
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.

(04:18):
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