E6D05: What is a liquid-crystal display (LCD)?
A. A modern replacement for a quartz crystal oscillator which displays its fundamental frequency
B. A display using a crystalline liquid which, in conjunction with polarizing filters, becomes opaque when voltage is applied
C. A frequency-determining unit for a transmitter or receiver
D. A display that uses a glowing liquid to remain brightly lit in dim light
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 30 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. We will distill the LCD innards down to the fundamental principle of operation, but be aware that an LCD is somewhat more complex in its solid state architecture and especially in its crystal molecular dynamics.
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.
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.
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.
The answer to Extra Class question E6D05, “What is a liquid-crystal display (LCD)?” is “B. A display using a crystalline liquid which, in conjunction with polarizing filters, becomes opaque when voltage is applied.”
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