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| Backlighting |
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An
LCD is basically a reflective part. It needs
ambient light to reflect back to the eye. In
uses where ambient light is low or non-existent,
a light source must be placed behind the LCD.
This is known as backlighting. There are several
technologies used: |
| Electroluminescent (EL): EL backlights are very thin, light weight and
provide a very 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 80 to 100 VAC. This is supplied by an
inverter that converts a 5, 12 or 24 VDC input
to the AC output. Els have a limited life of
2,000 to 3,000 hours to half
brightness.¡¡ |
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Light Emitting Diode (LED): LED
backlights are used primarily for character
modules. They offer a longer operating life -
50,000 hours minimum - and are brighter than
Els. They do consume more power than Els. Being
a solid state device, they operate directly off
+5 VDC, so they do not require an inverter.
However, a current limiting resistor is
recommended for protection of the LEDs. LEDs are
mounted in an array directly behind the display,
which increases the thickness 4 to 5 mm. LEDs
come in a variety of colors, with yellow-green
being the most common.¡¡ |
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Cold Cathode Fluorescent Lamp
(CCFL): CCFL backlight offers low power
consumption and a very bright white light . Two
technologies are used: direct an edge lighting.
In both types a cold cathode fluorescent tube is
the light source. A diffuser distributes the
light evenly across the viewing area. Edge
lighting offers a thinner package and less
power. CCFLs require an inverter to supply the
270 to 300 VAC used by the CCFL tube. They are
used primarily in graphic LCDs and have a longer
life - 10,000 to 15,000 hours - than Els
do. |
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