Flat panel displays (FPDs) are rapidly replacing CRTs as the primary human-machine interface in the industrial world because of their thin form factor, light weight, low power consumption and high-quality, distortion-free images.

In its recent Liquid Crystal Display (LCD) Monitor Strategy Report, DisplaySearch (Austin, Texas) predicted that between 1999 and 2005 the LCD monitor market will grow 3.5 times faster than the CRT market on a revenue basis and 2.8 times faster on a unit basis. But CRTs have one major advantage over FPDs-low cost. You can get a 17-inch CRT for as little as $200, whereas an LCD of equivalent size will cost five or six times as much. In addition, CRTs have wide viewing angles, high image quality and excellent response times.

Then why go to FPDs? Because they offer many advantages that make them worth the initial cost and some of those advantages save money in the long run. For example, a CRT weighs three or four times more than an LCD of comparable screen size. Weight is a major disadvantage in the industrial environment, where the heavier the monitor, the more money you need to spend on designing and building shock and vibration protection. As a result of their lower weight, LCDs are easier to install, don't require as much strength in their enclosures and are ideal for swing-arm applications.

In addition, a CRT typically consumes twice as much power as an LCD of comparable size and so it costs more to operate. Over time, the higher operating cost can be substantial for a factory with many monitors. More power means more heat and that means more money must be spent to cool the system as well.

To operate in high ambient light conditions, industrial monitors have to be pretty bright, requiring 250 to 350 nits. (A nit is the metric unit of emitted or total surface luminance, expressed in candelas/meter2.) Displays to be used in direct sunlight can require as much as 1,200 nits. CRTs are simply not capable of that much brightness. And because the backlight on an LCD can be replaced, a brighter version can be installed if needed. That's not possible with a CRT.

A CRT eventually burns out, and when it does, you have to replace the whole CRT at a cost of at least $200. The newer LCDs have long-life backlights with approximately the same longevity as a CRT. And when the backlight burns out you can replace it at a cost of roughly $25.

Space is another important consideration. An LCD is typically one-fourth as thick as a CRT. In some embedded applications, it is impossible to use a CRT because the space is simply not available. In addition, designing or buying a larger enclosure for a CRT adds cost.

Finally, LCDs are not affected by magnetic fields, so degaussing is not necessary. CRTs require bulky and expensive purging systems not needed for LCDs.

Those LCDs designed for the PC and laptop environments are not rugged enough to withstand the harsh conditions on the factory floor, where heat, moisture, dust, vibration, shock and high electromagnetic interference (EMI) are common. In addition, conventional PC-type LCDs usually do not have wide enough viewing angles for images to be clear from wherever an operator may be standing. A conventional LCD typically has viewing angles of 100 degrees to 110 degrees from side to side, whereas the industrial LCD has to provide a viewing angle of as much as 170 degrees .

Nor are conventional LCDs bright enough for high ambient light conditions. The LCDs in the PC marketplace are typically 150 nits. As already stated, as much as 350 nits may be needed in an LCD for the industrial market.

Yet today there are a variety of high-quality color FPDs designed specifically for industrial environments. These displays are rugged enough to withstand severe environmental conditions and have the enhanced viewing angles and high brightness that industrial applications require. The industrial displays currently on the market range from five-inch LCDs to full-size displays of 20 inches or more.

The specific type of FPD you choose for your industrial automation and/or control system depends on a variety of factors, and care must be taken to make choices that fit your particular application. The one- or two-line flat panels formerly used to display simple information are no longer adequate for today's complex embedded devices, which have acquired the intelligence, bandwidth and processing power of a computer. Consequently, the designer of industrial embedded systems must now make choices not only as to whether or not to incorporate a large-screen display, but which technology to employ and how to do so.

Whether you are designing an embedded or a standard system, the following choices must be considered: types of LCDs; analog vs. digital LCDs; reflective vs. transmissive backlighting; and whether or not to use touch screens.

The two primary types of LCDs are active-matrix and passive-matrix. Although passive-matrix displays are much cheaper than active-matrix (thin-film transistor) displays and operate in a similar manner, they are generally not a good choice for industrial environments. The images are not as sharp, the viewing angles are not as wide, and the response time and contrast ratio are lower than those of an active-matrix LCD. Those are all major issues in an industrial environment.

Various choices

For active-matrix LCDs, a variety of viewing modes and backlighting technologies are available. Choices are usually determined by ambient viewing conditions and factors like viewing angle, brightness and contrast ratio.

For industrial applications the decision between an analog or a digital display primarily depends on the type of signal the display will receive from the host computer.

When driving a digital LCD from a PC that outputs analog video signals, an interface card converts the signal and image quality is degraded in the process. When driving a digital LCD from a device that outputs digital signals, however, analog-to-digital (A/D) conversion is not necessary and signal integrity is preserved.

If your host computer outputs an analog signal, you may want to consider an analog LCD. NEC makes fully analog LCDs that receive analog signals without internal A/D conversion. The displays provide easy-to-interface, virtually plug-and-play replacements for CRTs and have the added advantages of maximum color depth and lower system cost. The disadvantage of analog LCDs is that the world is going digital and eventually all PCs and host devices will be capable of outputting digital PC and video signals.

Reflective LCDs have a mirror or other reflective material attached to the backside of the panel so that when light is reflected off the surface of the material, the selected pixels are visible. That type of backlighting is best suited for small panels that will be viewed indoors or outdoors in high-ambient-light conditions.

Transmissive LCDs usually have cold cathode fluorescent lamps (CCFLs) as backlights. The lamps provide polarized lighting and need to be complemented with light guides such as polarizers, defusers and enhancement film to spread the light evenly. They are suitable for viewing either in well-lit indoor conditions or dark environments. CCFL backlights offer high brightness, long life and low cost. They provide an efficient source of illumination in varying strengths.

Touch screens are increasingly popular for machine and process control, but have their own design issues. When a touch screen is added to an LCD, light from the backlight must pass through an additional layer of material, which can reduce the light intensity.

Touchy transmissions

As a result, the designer needs to pay attention to the touch screen's transmissivity rating. The higher the transmissivity percentage rating, the more light is transmitted through the panel. Resistive and capacitive touch screens have transmissivity values ranging from 60 to 80 percent, while surface acoustic wave and infrared scanning technologies can have transmissivity ratings as high as 90 to 100 percent. Transmissivity degradation in touch screens can be compensated for by selecting flat panels with higher backlight brightness ratings and with higher transmissivity through the LCD panel itself.

When extra protection is needed for extremely harsh industrial environments, enclosures must be designed that provide the needed protection without affecting electronic performance. Care must be taken when buying enclosures, however. The degree of protection against liquids, dust and ice varies, and some enclosures offer little protection against dust and moisture.

The National Electrical Manufacturers Association (Nema) and the International Electrotechnical Commission (IEC) publish standards for the design and testing of enclosures for protection of electronic equipment in harsh environments. However, Nema does not provide any verification testing, nor does it certify conformance. Standards for commercial and industrial displays are Nema 12 and Nema 4/4X (roughly equal to IEC levels IP55 and IP66, respectively).

Protect yourself

The best flat-panel displays and enclosures also protect against heat, vibration and EMI. Those conditions are not covered in the Nema standards, so you will need to ask the manufacturer to provide specifications of heat, vibration and EMI thresholds to verify that the display you choose has the protection you need.