Design News for Mechanical and Design Engineers

Technologies from coating to in-plane switching are used to widen viewing angles of LCDs.

There are various display technologies to consider, depending on the requirements of the application. Electro-luminescent (EL) displays, vacuum fluorescent displays (VFDs), and light emitting diodes (LEDs) all have excellent temperature-range and luminance characteristics, but limited color representation (see table). EL displays have done well in instrumentation and military applications, VFDs likewise in automotive dashboards, and LEDs in public information displays and traffic lights. Organic LEDs (OLEDs) are just now coming to the market. Many industry pundits believe that the OLED is a viable technology to unseat the LCD over the next decade. All these displays fall into the category of flat-panel displays (FPDs).

But the current champion of flat-panel display technology is the active matrix liquid crystal display (AMLCD). According to Ross Young, president of the market research firm DisplaySearch, "AMLCDs accounted for 71.5% of the $23.97 billion FPD market in 2000." Active matrix is one of the three basic LCD subtypes, the others being character displays and passive matrix displays. Character displays, used in the common wristwatch, gas-pump, and some cellular phones, are often monochromatic. Passive matrix display applications range from cellular phones and personal digital assistants (PDAs) to some notebook computers. Passive matrix displays are monochromatic or of limited color (65k colors or less). Active matrix display uses span from high-end cellular phones and Internet appliances to notebook PCs and flat-panel monitors. Active matrix displays range from color to full color (16.7 million colors).

LCDs have proliferated, partially due to their scalability to large sizes—up to 30 inches so far—and their high resolution of more than 300 pixels per inch. More importantly, they can be used throughout a fairly wide temperature range, reproduce a wide spectrum of colors, have high contrast ratios for easier viewing, and recently gained the capability to support full motion video. AMLCD applications include ATM machines, oscilloscopes, slot machines, cash registers, in-flight and in-vehicle entertainment, and medical diagnostic and imaging devices.

Reverse scan allows designers to optimize the viewing direction of an LCD flat panel, accounting for specific viewer position -- such as this normally 6 o'clock scan panel reversed to allowing viewing by an observer at 12 o'clock (top row).

Most high-information-content AMLCDs use twisted nematic technology. This technology is very stable, high yielding in manufacturing, and offers high contrast ratios and fast response times. Often the display includes optical compensation film to improve the viewing angle. Also available is in-plane switching, which features an excellent viewing angle but lower contrast ratios and slower response times. Vertically aligned (VA) and multi-domain vertically aligned (MVA) display technologies feature wide viewing angles, fast response, and high contrast ratios, but are harder to build and tend to look better when viewed slightly off axis than when viewed straight-on.

Low-temperature poly-silicon (LTPS) is a type of AMLCD that utilizes a different approach to creating the active matrix elements, and results in brighter displays that consume less power and have smaller form-factors and improved reliability. But LTPS requires more manufacturing steps, making it more expensive, and its yields are lower. Many manufacturers have made large investments in LTPS, so the benefits may outweigh declining costs over the next couple of years.

Design tips and choices. When considering which type of display to select, a designer should consider which factors are most important to the customer. For example, if a product must provide information in an easy-to-read format, yet show some motion video from time-to-time, such as in a point-of-order kiosk, an engineer may want to consider a lower-resolution display, such as VGA (640 × 480 pixels) to provide large-text characters and the ideal resolution for NTSC video.

Important characteristics could include a high contrast ratio and high luminance for easy readability in high ambient light environments. Thus the designer might consider a wide-brightness viewing angle (most display specifications consider only the contrast ratio for viewing angle) so that the display will attract the attention of passers-by.

It is also important to consider if the user will typically be viewing the display from a position above (12 o'clock) or below (6 o'clock). Most displays are optimized for viewing from one direction or the other (see figure). Fortunately, displays designed for non-PC products have a "reverse scan" capability, allowing the designer to turn the display over so that it can be viewed from the opposite direction without having to reprogram the controller. Be aware, however, that most display panels are mechanically asymmetrical so you must know the optimum-viewing angle before beginning the mechanical design.

AMLCDs frequently use backlighting, which requires an external power supply inverter. To achieve the luminance necessary for high ambient light environments, as well as for fault-tolerance, AMLCDs have two or four backlights. This means the mechanical designer must accommodate not only the display but also multiple small inverters, or a single, larger, multi-channel inverter. Inverters should be physically close to the display, to avoid noise effects. So expect serious performance degradation if the wires from the display's backlight do not plug directly into the inverter.

The glass in AMLCDs is typically only 0.7-mm thick with a polarizing film laminated to the front. This combination is not good for withstanding point impacts. Applications that require a considerable amount of faceplate durability are best served by placing another sheet of glass in front of the display.

Most touch-screens for LCDs use a layer of thick glass to protect the display. It is important to have a small gap between the protective glass and the display glass. If the two pieces of glass should touch, small, concentric circles appear in the displayed image, disappearing after contact ends.

Overall, AMLCDs are actually quite rugged, and can often withstand more than 100g of shock and 1g of vibration, largely because the thin glass flexes slightly. That said, the displays tend not to withstand torque very well. Designers should try to isolate the display from the chassis as much as possible, and never allow the display to become a stressed member.

With many modern products using flat-panel displays to have more functionality, it is very important for designers to understand the limitations of the technology, and design products that can take advantage of the available benefits.