How do they backlight LCD screens?

In this episode, Marshall explains the two technologies that are used to backlight LCD panels.

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Episode Transcript

Speaker 1 (00:00):
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

(00:23):
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

(00:47):
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.

(01:11):
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

(01:33):
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

(01:55):
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

(02:19):
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

(02:42):
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

(03:02):
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

(03:23):
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,

(03:44):
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.
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(04:04):
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