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Backlights
An LCD subpixel acts like a light valve, allowing light to pass or not. Transmissive and
transflective color LCDs have backlights to provide the white light to shine through
the liquid crystal material. It is important that the light be spread evenly across the LCD.
The very purpose of the polarizers and color filters is to always block light of unwanted
colors and orientations, so the backlight must be very intense to insure that the
quantity of unblocked light is sufficient. Thin, cool and low power are important attributes
of backlights. See the Pen Computing article
Backlights, Sidelights, Frontlights for a better explanation of this terminology.
Fluorescent Tube Backlights
Cold fluorescent light is the most popular backlight technology. Common abbreviations are
CFL, CCFL or CCFT. Gas in a sealed fluorescent tube is excited by a high voltage
alternating electric field. Gas molecules emit their characteristic radiation which strikes
the fluorescent material coated on the inner surface of the tube causing it to fluoresce
white light. Kyocera LCDs use a straight CFL tube on one side or opposite sides of the LCD
or a U-shaped CFL tube on three sides. Other LCD vendors who lack transflective technology
have been known to use arrays of 8 or more CFLs in efforts to achieve sunlight readability
by overpowering the sun.
When a CFL is mounted on one edge of the LCD, there will be a reflector around three quarters
of the tube, reflecting the light into the edge of a white or clear diffuser sheet or
light guide. The light guide takes light from an edge and spreads it. Different light
guides are designed for two CFL tubes on opposite edges. Scattering films are used to make the
backlight more uniform.
CFLs in Kyocera LCDs require controlled milliAmps of alternating current at hundreds of volts
and kHz frequencies, as given in section 13 of each LCD specification. Starting voltages are higher.
Inverters are the devices which provide this kind of power for CFLs. The specified CFL life depends
on using an inverter that limits the current. Refer to the web page on inverters.
CFL lifetimes of 54,000 hours are typical of newer, standard temperature LCDs. Kyocera promises long
product cycles to match the long life cycles of our customers' industrial products. Some of our
legacy KCS- and KHS-series models date back to a time when CFL lives were 10,000 hours or less, so
they have field replaceable CFLs. These CFLs are mounted in plastic modules, affixed to the
LCD by one screw or by a latch. On two-tube LCDs, the top tube module is not interchangeable with
the bottom tube module. It is debatable whether the CFLs in the 10.4 inch LCDs are field replaceable,
but they can be replaced by a technician using only pliers. These CFLs are not in a module, so identical
CFLs are used in the top and bottom positions.
LED Backlights
High-intensity LEDs can be used as backlight sources. A diffuser is still required.
The advantage is that no inverter is required, producing a net power saving
and avoiding a kHz frequency within the customer's product.
Kyocera is offering standard low-power LED backlit LCDs on V-series 4.7 inch LCDs,
which include a constant-current backligh power circuit, driven by 3.3 or 5.0 Vdc power.
Kyocera is offering high-power LED backlight on several models of 5.7 inch V-series LCDs.
These 5.7 inch backlights require the customer to provide a constant-current supply at
around 27Vdc. They are almost as bright as their high-brightness CFL backlight counterparts.
In 2006, Kyocera plans to release a constant-current LED power board
which accepts commonly available 5.0 or 12Vdc input voltage.
Electroluminescent Backlights
EL light panels produce light over their entire surface. No diffuser is needed, so the
LCD can be thinner and lighter compared to CFL backlight. Kyocera can produce EL backlit
LCDs on a custom basis. EL is much less bright than CFL, so it has not been chosen for Kyocera
standard LCDs.
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