Wednesday, August 4, 2010

I left my H-T exposed to sunlight for several hours. When I finally retrieved it, I was horrified to see that the LCD was completely black...

Q I left my H-T exposed to sunlight for several hours. When I finally retrieved it, I was horrified to see that the LCD (liquid-crystal display) was completely black. However, after it cooled for a few minutes, the display returned to normal. What happened to the display, and how did it recover?

A Liquid crystals used in most LCDs are long, straight mol ecules that tend to line up with each other, and anything else that’s nearby. If you sandwich a film of liquid crystals between glass plates that are ridged like a miniature corrugated roof, the molecules will line up with the ridges. If you rotate one plate by 90° the molecules near that plate will orient themselves at right angles to the molecules near the other plate. Between these plates, the rest of the crystal lattice forms a smooth one-quarter twist. This twist rotates the polarization of light by 90° as it passes through the liquid crystal. 

The liquid crystals used in displays are electrically unbalanced: one end of the molecule is slightly negatively charged, the other end slightly positively charged. So applying a small voltage across the glass plates causes all the molecules to “stand on end,” and the liquid crystal loses its ability to twist the polarization of light. Switch off the voltage, and the lattice returns to its previous state.

To create a display, the glass plates are replaced with polarizing filters, also out of alignment by 90°, and a reflecting surface is put behind them. Incoming light is polarized by the first filter, twisted 90° by the liquid crystals, passes through the second filter, is reflected and reverses its journey.

Apply a voltage, however, and the incoming light passes unchanged through the liquid crystal and so can’t pass through the second polarizing filter. Consequently, the display goes black. By using segmented electrodes, letters, numerals and other shapes can be displayed.

The liquid crystal state is a phase between solid and liquid cool it and it solidifies; heat it and it melts. Melted liquid crystals lose their ability to change the polarization of light, becoming ordinary liquids. That’s when you see the display going completely black or blue. When cooled, it returns to the liquid crystal phase and reflects light again.

Don’t make it a habit to leave your rig in the sunlight for prolonged periods of time. Repeated abuse can, over time, permanently alter the chemical properties of the display and render it inoperable.

From QST December 2000