The exploding thin-film-transistor, liquid-crystal display industry has leaped beyond the traditional information technology applications of notebook PCs and desktop monitors, propelled by a wide array of technological advancements and attractive prices.

Today’s designers have an unprecedented variety of LCDs to choose from, along with a broad range of improved characteristics: exceptional brightness, enhanced contrast, advanced resolution, fast transmission speeds and much wider viewing angles. These parameters have improved markedly to optimize screens for everything from high definition televisions and digital information displays to monitors and handsets, providing a compelling array of options through which equipment makers can truly differentiate their product lines.

Over the past few years, growing production capacity, as well as a fiercely competitive landscape in the mainstream market segments, has led makers of LCDs to turn toward specialty applications to enhance their mass production investments. As a result, manufacturers of appliances, vending machines, voting machines, retail kiosks, medical electronics, scanners and sensors, test and measurement gear, portable consumer electronics, and other display-laden equipment are in various stages of incorporating LCDs in their products.

Improved view

The display is one of the most critical components in the human interface for most appliances, and a dim, low-contrast display can give consumers a dim view of the product as a whole. Two key LCD performance metrics are brightness and contrast ratio: the former specified in candelas per square meter (or nits), the latter described as a ratio, X: Y, between the brightest (X) and darkest (Y) points in a display.

Mainstream LCDs today have a typical brightness of 250 nits to 300 nits, up from about 100 nits in the early days of LCDs. This will trend upwards in the future beyond 500 nits. The dramatic improvement in brightness can be attributed to several factors, including brighter backlights, innovative light-dispersion designs and more efficient polarizers and color filters, as well as advanced structures that increase a display’s aperture ratio. (The aperture ratio is the percentage of a pixel area that actively manipulates light.) For specialty applications, LCDs with brightness ratings as high as 1,000 nits are available for use under very high ambient light conditions, including outdoor environments.

Contrast ratios also are rising, fueled in part by the tremendous improvements in brightness. From early levels that had less than a 100:1 contrast ratio, today’s mainstream LCDs have increased to the 500:1 range, with a recent additional boost to the 700:1 level. This year, the introduction of a new generation of color filter resin technology, uniaxial compensation films, and a vertically aligned cell structure will take this value to 1,000:1. Specialized displays targeting medical and industrial applications will deliver upwards of a 1,500:1 contrast ratio. These ultra-high contrast levels are particularly useful for clinical patient monitoring and electronic imaging applications, such as radiology, where displays are increasingly being used as a replacement for standard X-Ray films.

Fast and pure

Samsung's i730 smartphone has an exceptionally bright 2.2” LCD screen with a touchscreen display and slide-out mini-keyboard for busy managers and consumers in the fast lane.

LCD technology also has matured in the area of response time, the time it takes to change the state of a liquid crystal cell from “on” to “off” or vice versa, and all gray-to-gray transitions in-between. Early LCDs, with 100 ms to 200 ms response time, were only suitable for static images. Advanced thin-film transistor LCDs improved the picture substantially, but stalled at a plateau 25 ms, still less than ideal for high-quality, rapid video signaling.

A few years ago, some of the leading displays reduced on/off response time to 12 ms (20 to 30 ms for changes in gray level). This improved to 8 ms last year, and Samsung will advance this further in 2006, reducing on/off response to approximately 5 ms through a technique called Dynamic Capacitance Compensation. DCC-1 provides an over-shoot voltage to the LC cell, while DCC-2 utilizes advanced look-ahead algorithms in the frame prior to DCC-1, to partially drive pivotal pixels toward their next gray scale state. The DCC-2 effect is similar to an athlete getting a head start on the 50-m dash. Fast response is critical for applications displaying TV-style video, as well as in medical diagnostic and industrial gear focused on motion imaging.

Color purity, or color gamut, is another critical display trait that deserves close scrutiny in medical, chemical analysis and control panel instrumentation, where different hues symbolize various states of activity. This is also important in touch-screen, kiosk-type applications, since one of the major causes of merchandise returns at retail kiosks and in virtual Internet kiosks is that the purchaser’s perception of electronically displayed images did not adequately represent what was expected.

Color gamut is defined as a percentage of the color capabilities of the National Television System Committee’s NTSC television standard. The 72 percent of NTSC achieved by LCDs around 2004 represented a major advance in chromaticity, with purer colors providing a truer representation of real-world images. Most cathode ray picture tubes have 72 percent NTSC, a level with which consumers have been satisfied for years. However, new backlighting methods with wide color-gamut, cold-cathode, fluorescent tubes and LEDs have extended this even further to 92 percent and 105 percent, respectively.

Another improvement in LCDs can be found in their resolution, the number of pixels per inch of display area. Where 75 ppi was the norm in early LCDs, resolution is now typically in the 83 ppi to 122 ppi range. For even greater resolution accuracy in industrial and professional applications, LCD suppliers have developed pixel resolution as high as 302 ppi, thanks to special photolithography techniques used for semiconductor processing. This level of resolution has eluded most emissive displays, including plasma, EL and OLED.

A wider view

Samsung’s d307 wireless handset features a vibrant LCD screen that swivels from “portrait” viewing for standard communications to “landscape” viewing for mobile digital multimedia broadcasts.

One LCD performance metric that merits special attention in non-PC applications is viewing angle. Traditionally, conventional LCDs have had a relatively narrow viewing angle. This presents little concern where the end user looks at the screen head-on. However, as screen sizes increase in most applications, the tendency for multiple viewers to watch the same screen also increases, which makes a wider viewing angle desirable. Technologies such as Super Patterned Vertical Alignment (S-PVA) have been developed, in part, to expand viewing angle and enhance contrast and off-axis color reproduction.

Traditional LCDs have a helix liquid-crystal pattern, and here viewing angle is typically 40 Deg in the vertical axis and 50 Deg from right or left. The Samsung BTN family extended those limits to the 60 Deg to 75 Deg realm a couple of years ago, with >80 Deg slated for production in 2006 thanks to a wide-angle S-PVA (and previously PVA) panel structure. This panel structure utilizes a precise vertical alignment and multiple molecular domains within each liquid crystal cell. PVA, with its four domains and S-PVA with eight domains, initially provided a symmetrical 80 Deg viewing angle from all directions. The angle was extended to 88 Deg in 2004, 89 Deg in 2005 and 90 Deg in 2006.

Wide viewing angle can be a specification of the highest priority for those applications where difficulty in off-angle viewing or off-angle color shifts can have significant negative ramifications. In certain maintenance scenarios for example, sometimes the end user has to adopt an undesirable posture while viewing a portable instrument at an unusual angle. In medical instrumentation, too, the display must provide a clear image from any angle of view.

As a result, Samsung adapted S-PVA (used in televisions) for small- and medium-sized LCDs using a proprietary mobile Super Wide View+ (mSWV+) technology, providing a wider viewing angle for the mobile display industry.

Wide viewing-angle LCDs are inherently slower than traditional LCDs in response time, but gains have been made in technology such as S-PVA by employing an advanced form of DCC to overcome this limitation. The 25 ms on/off response of PVA displays was shortened to 16 ms in 2004, and the 30 ms to 40 ms required for a gray/gray transition was reduced to just 8 ms. With S-PVA LCD TV panels, that has been improved to 5ms.

The view ahead

The boom in LCD capacity has prompted LCD manufacturers to recognize the exacting needs of specialty markets and to extend the technological capabilities of their product lines to meet those needs. As a result, designers in these segments can now select LCDs with the specific technical characteristics they need to optimize their product designs. And as LCD technology continues to improve, designers will have even greater freedom to specify the technical attributes they need.