| LCDs create their display with the manipulation if visible ambient
light. In the absence of this light, we must add backlighting to
make these LCD displays visible. There are many choices to
consider when backlighting an LCD. Once again the choice comes down
to appearance vs. cost vs. features. Each approach has its
advantages and disadvantages, and no one method is right for all
applications. The data below will only give the highlights of each
technology with general comments. For our LCD modules we integrate
most of these types of backlights into our displays. Here are
the 3 most common methods of backlight and information on how
to drive LED backlighting.
LED Backlighting
Light Emitting Diode, or LED, backlight is the most popular
backlighting for small and medium LCDs. The advantages of LED
backlighting are its low cost, long life, immunity to vibration,
low operational voltage, and precise control over its intensity.
The main drawback is it does require more power that most of the
other methods, and this is a major drawback if the LCD size is
large enough. LED backlights come in a variety of colors,
with yellow-green being the most common, and now white is becoming
cost effective and very popular. LED backlights offer a longer
operating life - 50,000 hours minimum - and are brighter than ELPs.
Being a solid state device, they are configured to operate with
typically a +5VDC power (and optionally 12VDC power), so they do
not require an inverter. The LED backlight has two basic
configurations; Array and edge lit. In both types the LEDs
are the light source that are focused into a diffuser that
distributes the light evenly behind the viewing area. In Array lit
configuration there are many LEDs mounted uniformly behind the
display, it offers more uniform and brighter lighting and consumes
more power. In Edge lit configuration, the LEDs are mounted
to on side (typically the top) focused edge on into the diffuser,
it offers a thinner package and consumes less power. Electroluminescence Panel (ELP) Backlighting Electroluminescence Panel, or ELP, is a solid state phenomenon
which uses colored phosphors, not heat, to generate light. EL
backlights are very thin, lightweight and provide an even light.
They are available in a variety of colors, with white being the
most popular for use with LCDs. While their power consumption is
fairly low, they require voltages of 100 VAC @ 400Hz. This is
supplied by an inverter that converts a 5, 12 or 24 VDC input to
the AC output. ELPs also have a limited life of 3,000 to 5,000
hours to half brightness. The biggest drawbacks to an EL panel is
that it requires an inverter to generate the 100VAC, consistent
brightness, and limited life.
Cold Cathode Fluorescent Lamp (CCFL) Backlighting
Cold Cathode Fluorescent Lamp, or CCFL, backlights offer low power
consumption and a very bright white light. The primary CCFL
configuration used in LCD backlighting is edge lighting. A cold
cathode fluorescent lamp is the light source with a diffuser
distributing the light evenly across the viewing area. CCFLs
require an inverter to supply the 270 to 300 VAC @ 35KHz used by
the CCFL tube. They are used primarily in graphic LCDs and
have a longer life - 10,000 to 20,000 hours - than ELPs do.
Their biggest drawbacks are: cold weather will reduce the light
output by as much as 60% (see graph below), they require an
inverter to generate the 350VAC (please note that the inverters do
not function well at low temperatures), the light intensity cannot
be varied (it is either on or off), and vibration can reduce the
life expectancy of up to 50%.
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