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How LCDs Work

by Rick Prescott

Liquid Crystal Displays, or LCDs, are quickly becoming the display technology of choice. This simple but extremely useful invention is being used in almost everything we can think of. From the displays of digital watches, to calculators and even computer monitors, LCD's provide a way to display information in a more effective way than we ever could before.

Inside a LCD, there are two thin plates of glass with a special liquid crystal solution between them. This liquid crystal solution has an interesting property. If there is no current going through the liquid crystal solution, the solution is clear. But when there is an electrical current going through the solution, the solution crystallizes and goes from clear to dark. The solution acts like a shutter when current is passed through it, changing from clear to dark. So for instance, in the display of a calculator, by changing certain sections from clear to dark, the calculator can form different numbers. (more)

HOW THEY WORK

The term LCD stands for Liquid Crystal Display. The liquid crystal fluid is the active medium that is used to create an image. It consists of a large number of elongated crystals suspended in a fluid. This reservoir is sandwiched between two thin sheets of glass. Each piece of glass has a transparent conductive pattern bonded to it (see Figure 1). The crystals are aligned in a spiral pattern until an electric field is impressed on the conductors.

Figure 1

A sheet of polarizing material is bonded to the outside surfaces of both the front and rear glass covers. As incident light of random polarization enters the top polarizer, it is stopped except for that which is polarizerd in the proper direction. With no electric field applied, the light is twisted or its polarization is changed by the spiral pattern of the crystals. The bottom polarizer is aligned opposite of the top one but the "twisted" light is now aligned with the bottom polarizer and passes through. The display is now transparent and appears light (see Figure 2).

Figure 2

When an electric field is applied to the conductors, the crystals align themselves with that field. The ambient light now passes through the crystal without being twisted (see Figure 3). This makes it out of phase with the bottom polarizer where it is absorbed. The display now looks dark or opaque. By selectively applying the field voltage, the desired pattern can be created.

Figure 3

Advances in LCD technology have produced the Super Twisted Nematic (STN) display with a twist angle of >200ý versus 90ý for TN type. This results in greater contrast and a wider viewing angle. (more)

BACKLIGHTING (from Seiko)

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 nonexistent, a light source must be placed behind the LCD. This is known as backlighting (see Figure 4a). There are several technologies used:

• Electroluminescent (EL)
EL backlights are thin, lightweight, and provide an even light. They are available in a variety of colors, with white being the most popular for use with LCDs. Although 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 3000 to 5000 h to half brightness.

• Light Emitting Diode (LED)
LED backlights offer a longer operating life—50,000 h minimum—and are brighter than ELs. But, 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. LEDs come in a variety of colors, with yellow-green being the most common.

• Cold Cathode Fluorescent Lamp (CFL)
CFL backlight offers low power consumption and a bright white light. Two technologies are used: direct and 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. CFLs require an inverter to supply the 270 to 300 VAC used by the CFL tube. They are used primarily in graphic LCDs and have a longer life—10,000 to 15,000 h—than ELs do.

VIEWING MODES (from Seiko)

LCDs are offered in three basic light transmission modes:

• reflective
• transflective
• transmissive

Reflective LCD
In the reflective mode, ambient light is used to illuminate the display. This is achieved by combining a reflector with the rear polarizer. It works best in an outdoor or well-lighted office environment.

Reflector bonded to the rear polarizer reflects the incoming ambient light. Low power consumption.

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