Seeing is believing

Whatever you’re designing, chances are you’re using displays to either design it or integrate into it.

There’s never been a wider choice of display technology. But, there’s always a trade off between price and performance, mobility and capability.

For desktop displays, you can’t go far wrong with the CRT (cathode ray tube). This mature technology has established manufacturing processes, and massive demand makes it the cheapest option with the widest number of suppliers. If image quality and flexibility are key, then it’s still the best choice and, in the desktop display market, CRT dominates sales by 8 to 1.

In terms of resolution, today’s 17in. CRTs can handle displays of up to 1,600 by 1,200 pixels, and the larger 23in. tubes can display 2,048 by 1,536 – now supported by the majority of current graphics chipsets. Nor are they tied to a single resolution like LCDs and other matrix-based displays, making them highly flexible in a desktop display environment.

Less apparent is the CRT’s colour gamut, which is still superior to that of competing technologies.

To combat the size penalty of the CRT, short-neck tubes reduce the length of the yoke and bell by around 30%. However, this also reduces the angle of incidence where the electron beam strikes the phosphors at the corners of the screen.

Quadrupole focussing arrays and wider grille pitch towards the sides of the screen help reduce these effects, but short-neck CRTs are prone to colour variation and misconvergence in these areas due to electron overspill onto surrounding phosphors.

Flat-screen tubes have the same problem because the phosphor layer is on a considerably flatter internal surface. While it reduces fishbowl distortion and glare, the glass of the front face is thicker, partly to create the flat external surface, but also to maintain the vacuum envelope’s integrity against external pressure. This can create distortion as the refractive properties of the glass bend the light (creating the appearance of an inverted curve) and adds to the weight of the unit.

The CRT hasn’t stopped developing though, with Sony announcing its Gravuretron CRT with a 15mm grille pitch screen (equivalent to 500dpi).

LCDs, on the other hand, eliminate these problems but have their own limitations. Comprised of a sandwich of glass substrates, polarising and transistor layers with a mercury backlight and diffuser, LCDs are lighter, flatter and much thinner than CRTs, and benefit from reduced EMI and power consumption. But their manufacture is complex and low yield and, despite high demand, they still cost much more than CRTs. The process of injecting and anchoring liquid crystal into the screen assembly can often leave gaps or errors that don’t appear until after the display is complete.

Interestingly, IBM claims that it’s developed a new ion gun technique to replace the velvet roller for aligning the liquid crystal, but this has yet to hit the mainstream.Other issues, like poor screen uniformity, can be caused by inefficient backlighting, especially as the screens are made larger (Samsung now has third-generation glass in production, as demonstrated by its latest 24in. Syncmaster 240T LCD), and viewing angle is typically restricted in the vertical axis due to the arrangement of the transistor layer. Although the screen doesn’t refresh in the same manner as a CRT, response time between off and on states is slower, especially in the cold, and contrast ratio is poorer.

For large displays and installations where image projection isn’t possible, a practical option is the plasma display panel (PDP). Current passing through cells of plasma gas creates ultraviolet emissions that cause the phosphors on the inside of the screen to glow. However, the response time is much slower (as the plasma still emits UV light as it cools) which can cause latency and blurring on moving objects, and the screens can get quite warm without the use of fans.

As well as being its main advantage, size is also the PDP’s biggest disadvantage, as it can’t compete with the resolution and compactness of LCDs. PDP remains firmly in the niche market of specialist information displays, and, because demand is lower in this area, prices are high (over £5,000 for a 42in. display).

Perceived market value varies across Europe, but it’s apparent that LCD share is growing and the drive for thin, full-colour displays is high, especially in the expanding PDA and mobile device sector. It’s here that the LCD is the best choice for integration by OEMs. As these products become more capable, operation and user interfaces become more complex, and simple monochrome alphanumeric displays are no longer up to the task. The use of graphic displays also makes the implementation of multiple character sets a more viable option.

With a dedicated device like a PDA, the display doesn’t need to support multiple resolutions and can be driven by a simple graphics chip with a small amount of buffer memory, and the lower colour gamut isn’t likely to present a problem until we start running CAD applications on our palmtops. New developments in reflective backplates mean that the displays are still usable in strong daylight, with LED sidelights replacing the power-hungry mercury backlights of larger products.

Monochrome LCDs are still cheaper, but for how much longer will consumers be satisfied with 16 shades of grey?

So it’s unsurprising that several companies are racing to provide alternative solutions for future, full-colour displays, and there are three big candidates for mobile devices.

Eastman Kodak’s Organic Electro-Luminescent displays (OELDs) are the result of ten years’ worth of R&D, and appear to be the closest to the tape. Sandwiched between two glass plates, the carbon elements emit light when subjected to current from the active matrix transistor layer, and produce enough light to remove the need for mercury backlights while being brighter and requiring less power than LCDs.

They provide a viewing angle greater than 160° in both axes, have a quoted contrast ratio of 100:1 and don’t suffer from the same depreciation in response time when used in cold environments. But they do apparently degrade dramatically when exposed to oxygen, so a sealed unit is still required. However, according to Carl F. Kohrt, Executive Vice President of Eastman Kodak, ‘Vivid, full-colour active matrix is the next step for OEL to reach its full potential as the display technology of the 21st Century’.

Similar in principle to the OELD, Cambridge Display Technologies’ Light Emitting Polymers (LEPs) use red, green and blue polymers in place of the carbon elements.

An LEP display is a couple of years from mass production, but Philips Electronics is currently manufacturing prototypes in Eindhoven, and is confident that it will be able to apply this technology to irregular or even flexible display surfaces in the future. CDT’s Technical Director, Jeremy Burroughes, says that the most recent LEP display has ‘the same colour range as the screen in a Sony VAIO’.

Alternatively, Candescent’s radical ThinCRT uses a design that’s more akin to typical CRTs, only in a component that’s less than a centimetre thick. A cold-cathode process reduces power consumption and heat, while millions of tiny spindt cathodes replace the bulky electron gun and yoke, bringing the advantages of a large CRT to a light, pocket-sized display.

However, these cathodes still require a vacuum, and because the envelope is much flatter and thinner, ceramic spacers are required to keep the sides apart and maintain structural integrity. So a ThinCRT solution is likely to be less robust than OEL and LEP alternatives.

Until these products become widely available, LCD will maintain its role as the user interface choice, and will probably continue to make inroads into the established CRT desktop display market as companies like Samsung, NEC, Mitsubishi and LG Electronics build bigger, cheaper products. But there are alternatives in the pipeline, so if your product design is still some way from the fabrication plant, you might want to consider your options now.