L.C.D.'s

by Kevin Ray

verywhere you look, you see Liquid Crystal Displays (LCDs) in one form or another. It may come as a surprise, but LCD technology started with the study of plants in 1888 when Austrian botanist, Friedrich Reinitzer, observed that, when he melted a curious cholesterol-like substance (cholesteryl benzoate), it turned into a cloudy liquid which cleared as its temperature rose. Once cooling began, the liquid turned blue before finally crystallizing. Eighty years later, in 1968, RCA made the first experimental LCD. First seen in watches and calculators, the technology has steadily improved to the complex levels that are now enjoyed in our computers and televisions.

We learned in school that matter has three states: solids, liquids, and gases. In solids, molecules basically stay in one place with respect to one another. In liquids and gases the molecules move more freely.

Liquid crystals are neither solid nor liquid, which makes the name a misnomer. Liquid crystals exist in an odd state that is sort of like a solid and sort of like a liquid. When in a “solid-like” state, their molecules tend to maintain their orientation and behave like the molecules in a solid, but their molecules can also move to different positions, like the molecules in a liquid.

So, do liquid crystals act more like solids or liquids? It turns out that liquid crystals behave more like liquids than solids. It takes a fair amount of heat to change a suitable substance from a solid into a liquid crystal, but it only takes a little more heat to turn that same liquid crystal into a real liquid. This is why liquid crystals are very sensitive to temperature, and why they are used to make thermometers and mood rings.

In LCD displays, electrical current is used to control the state and position of the liquid crystals. The liquid crystals are arranged in positively charged vertical columns (+), and negatively charged horizontal rows (-) that are sandwiched between two polarized glass panels coated with special polymers. These polymers contain microscopic grooves that are positioned at right angles to each other and behave like window blinds to assist in controlling the passage of light. When an electrical charge is introduced to liquid crystal molecules, they untwist! When they untwist, they straighten out, and the angle of the light passing through them no longer matches the angle of the grooves on the polarized glass. This blocks light in that area of the LCD and makes that area darker than the surrounding areas.

In all televisions, the screens consist of tiny squares called pixels. Color LCDs are no exception. They also have pixels which are divided into three subpixels. Each subpixel represents one of the primary colors, red, green, or blue. By carefully controlling and changing the voltage applied, the intensity of each subpixel can produce over 256 shades. When the subpixels are combined to form a pixel, they can produce as many as 16.8 million colors (256 shades of red x 256 shades of green x 256 shades of blue). Each subpixel must have its own transistor. A transistor is a semiconductor that regulates electrical current by acting as an on/off switch or an amplifier that increases electrical current. Consequently, the number of transistors required by LCDs is enormous. For example, a normal notebook computer supports resolutions up to 1,024 by 768. If we multiply 1,024 columns x 768 rows x 3 subpixels, we discover that there are 2,359,296 transistors in this normal computer! All it takes is one bad transistor to create a “bad pixel” on an LCD display. This is why most active matrix displays commonly have a few bad pixels.

This is also why LCDs are generally smaller than Plasma displays. Simply put, bigger equals more pixels and more transistors which equals a higher failure rate. Manufacturers of existing large LCDs often reject about 40 percent of the panels that come off the assembly line. This directly affects LCD prices since the sales of the good LCDs must cover the cost of manufacturing both the good and the bad. As advances in manufacturing occur, screen sizes will increase, and prices will decrease. Until then, if you want a flat panel display larger than 45”, Plasma is your only option.

This article provided by Kevin Ray of Custom Audio Video, LLC. Member CEDIA (Custom Electronic Design & Installation Association)