Liquid Crystal Displays
If you have never seen a liquid-crystal display (LCD), you have either been living on a deserted island or under a nice slab of granite for the last 40 years. They are quite literally everywhere these days. But, in spite of their ubiquity, very few of us have a solid clue about how these electronic visual displays work. Since LCDs are just as prominent for ham radio information display as for every other type of electronic appliance, it’s worth taking a closer look at what’s going on inside those flat little amalgamations of pixels. Let's distill the LCD innards down to the fundamental principle of operation.
An LCD is a sandwich of liquid crystalline material in between two polarizing filters, and typically with a source of light behind the sandwich. The rear polarizing filter
(closest to the light source) and the front polarizing filter (closest to the viewer) have opposite polarizing effects – imagine the rear filter to be horizontally polarizing and the front filter vertically polarizing. As randomly polarized light from the backlighting source passes through the rear polarizing filter, only horizontally polarized light rays pass through. If no change occurs, these horizontally polarized rays will not be allowed to pass through the vertically polarized front filter. The display area (or pixel) appears dark.
Twisted Crystals: However, if the liquid crystal material between the filters twists the polarization of the light as it passes through the crystal, and if that newly twisted polarization matches that of the front filter (vertical), the light will pass through the front filter and the display area appears bright. This is the key to the workings of the LCD. The molecules of the crystal are long, twisted structures that change the polarization angle of light as it passes through the material.
Left in its normal state, these twisted crystals will rotate the polarization of transiting light from horizontal to vertical, allowing it to pass through the front filter. But, when a voltage is applied to the crystal its molecules straighten out somewhat, or untwist. This spoils their ability to rotate the light’s polarization, and the light rays will no longer match the polarization of the front filter. No longer shall they pass out of the display, thereby making a dark area or dark pixel.
The liquid crystalline material can be laid down within the sandwich in patterns, such as in segment displays or pixel displays. By selectively activating segments or pixels of LCD material with applied voltage, a variety of displays or images can be produced. Digital logic or a microprocessor may be used to selectively control applied voltages to produce desired characters or images. Variable voltage application to a pixel results in variable amounts of crystal twisting, thereby controlling the amount of polarized light that passes -- this affects brightness control of the pixel.
Color LCDs: "But..." you might ask, "...how is color incorporated into LCD displays?" Great question! It's not that much more complicated.
In a color LCD display, each "pixel" is actually a set of pixels positioned very close together. For instance, a color pixel cluster may include a tiny red, green, and blue pixel, each of which can be independently controlled in light throughput by its twisted crystals. A white light source may be used behind the entire array, and each pixel has a tiny color filter that allows only the red, green, or blue portion of the white light to pass through its area. By varying the throughput of light of each color in the pixel cluster, a blended pixel color is created. Generally, the sub-pixels of each color are so small that their light is perceived as a blended pixel. But, if you get up really close to the display and look close, you may be able to discern the individual color pixels in the LCD array of your monitor.
See how easy that was? LCDs can be used for everything from television and computer monitors to your transceiver's clever waterfall display. Now you have a basic idea of how they work.
-- Stu WØSTU