A bright future for display technologies

The value of worldwide shipments of electronic displays was estimated at roughly $64 billion in 2001 and is expected to grow to $114.8 billion by 2006, according to US analysts. Yet the price of the displays is continuing to come down and the market for displays will continue to see high growth for many years to come.

So with new markets and ideas popping up all the time, this is an area which the Australian electronics industry can harness. Emerging markets are where Australia needs to be and what follows are some of the technologies available to enter these new and now viable markets.

What follows is a selection from some of the highlights that have occurred over the last few years.

In 2005 NEC produced trans-flective technology based on NLT (natural light technology). It was first introduced in 2001 in 2" (120 x 160 pixels) for mobile phones and PDAs and has now been introduced into the larger displays from 6.5" and above for the industrial market with increased viewing angles.

There have been improvements in contrast ratio with Samsung earlier this year releasing TFT displays for LCD TV with up to 1200:1 contrast ratio.

Meanwhile luminance has improved with standard and industrial panels. Now with the improvements in backlight films it is possible to achieve 600 cd/m2 while maintaining a wide viewing angle and long life.

Samsung has released the 82" TFT for LCD TV technology that will ach-ieve extra-wide viewing angles.

NEC released LED backlighting in LCD TFTs up to 21.3" to improve the life of a display, while the combination of a new LED backlight system and TFT technology produces a display of vivid color that has not been possible with conventional displays such as LCD modules with cold cathode fluorescent lamp (CCFL) backlight units.

Pixel pitch has also been improving. Mobile phone technology enhancements around 2" size require high pixel pitch using the latest in sunlight readable technology. In this case the human eye can see 220 pixels/inch. A display with this resolution allows for a small size panel for video, mapping and photo applications where resolution is vital.

Viewing angles of 85° from all angles in flat displays are now with us. Gone are the days when you had to have someone looking over your shoulder to see what you were writing on your laptop, or PDA.

Overall, I believe we are not moving as quickly as Moore's Law has predicted for technological advancements, but close in terms of pricing. This is dominated due to competition in the market and the uptake of the latest technology with the 'must have' colour display.

There are still many hurdles to jump in the coming years to improve the display that will replace a mainstream CRT for colour, brightness, contrast and its ability to be viewed in full daylight.

These are some of the issues flat panel displays face and it is always a trade-off for sunlight readability vs cost.

I remember a comment made to me back in 1995 about the cost of a plasma display in terms of US$/inch. The projection was for US$100/inch by 2000. This would put the current 42" plasma displays somewhere in the region of US$4200 in 2000. In 2000 this would have been quite an accurate figure. I have not heard such comments or predictions again from the display manufacturers. Perhaps this is due to the competition in the market. But the cost of TFT displays will always have an aggressive target to meet from consumers.

Technical hurdles

• Flexible displays: Both Philips and E-Ink are working on flexible displays that could have a vast economical and environmental impact. They not only could replace bulky CRT units and rigid flat screens, but even further away they could supplant paper in books, newspapers and other media.

• Display life: There are two problems associated with panel life to be overcome.

  One concerns the panel in terms of MTBF and the half life of the backlights (when using CCFL tubes) although LED backlighting will greatly improve this. The other concerns the life of a panel's production. What choices will be available to the user who has a long-term project with a long lifecycle? This can be referred to as managing obsolescence.

• Disposal What are we to do with the displays when they have expired? The materials used in CCFL tubes are carcinogenic so how do we dispose of them safely and to what agency do we turn for support?

Although these are hurdles for the future, they are matters we should look at today but let's see what panels are being offered to us currently.

Consumer displays

This market requires high volume, low cost and the devices are products such as PDAs, mobile phones, portable DVD players (including aftermarket automotive), tablet PCs and PC monitors.

A typical issue when selecting a TFT display from this area is how long the display will be available for your needs. The price for the display will be very good, but may only be available for 12 months. If this is not the lifecycle of your product then it could mean that selecting a display from this market could lead to disaster.

However, there are other types of displays available. STN is normally black and white with a high contrast ratio and is typically reflective. CSTN (colour super twist neumatic) has a lower contrast ratio, requires some unusual voltages to run but is still a reflective screen. Many people would be familiar with the first edition of the Gameboy Advance where the display was not very good for low light conditions.

Consumer TFTs are typically wide screen or 16:9 aspect ratio in the smaller sizes and used for portable DVD players.

The PC market is a high volume segment, but low contrast/brightness is not good for sunlight readable or industrial applications.

Industrial displays

Industrial TFTs are the display segment for longer life where a supplier will commit to a display being available for a certain time or that a replacement will be available with the same mechanical dimensions and electrical connection.

These panels are typically more expensive than the consumer displays but offer additional features such as higher brightness, contrast and some transflective qualities. Applications include: medical, marine, POS, military, transportation (automotive) and instrumentation.

Enhanced displays

Enhanced TFTs refer to (in most cases) the enhancement of an industrial panel to meet certain needs such as improving sunlight readability.

What does Transmissive, Reflective and Transflective mean?

So far I have mentioned terms you may, or may not be familiar with that include transflective, reflective and transmissive. Perhaps I should now explain what they mean.

According to NEC, a standard TFT is transmissive which means that ALL light to the panel is provided by the backlight of the display or transmitted light - in this case a light pipe and CCFL tubes. So as the ambient light increases you need to increase the backlight brightness to overcome more ambient light.

A typical example of this is placing your laptop outside and trying to read the display - not easy to do.

The other option available is reflective where all light is provided by a light source from the front of the display. You may have seen this on PDAs or mobile phones where you can see the LEDs on the side illuminate the front of the display.

As you can imagine, as the ambient light increases, the display contrast also appears to increase.

Transflective - I believe the word does not even exist in the dictionary. It is a combination of a transmissive panel with some reflective qualities. Typically a display type such as this returns between 1 and 5% of ambient light.

It gets brighter in direct sunlight, yet works well in low light due to the backlight and is sometimes called NLT (natural light technology). Just having this quality does not guarantee that the display will behave well in direct sunlight but it is one step closer to achieving this.

Enhanced displays are required for a number of reasons, but primarily to improve sunlight readability of off-the-shelf displays.

These solutions enable users to set themselves apart from their competitors and so offer a superior product, yet the back-end electronics could be the same.

Sunlight readable displays

To achieve sunlight readability there are three options available or a combination of all of them.

• Backlight enhancement. Both active and passive enhancement;
• Direct bonding of a glass substrate to the front of the display panel;
• Films and filters to index match the display to the environment the display will be working in.

Backlight enhancement is, as the term suggests, "An enhancement of the backlight to make a display brighter." There are two ways of doing this:

Passive backlight enhancement is where additional filters are added to magnify the available backlight to achieve a higher brightness. This can mean a loss of viewing angle as more light is collected from the peripheral areas and transmitted through a narrower viewing angle of the screen.

Typically on a good screen you can take a 450 cd/m2 display and through passive enhancement make this display achieve 500-600 cd/m2.

Active backlight enhancement is where additional CCFLs tubes are added to a standard industrial panel to increase the Candela rating of the panel. In most cases the tubes are added in a side-lit format and try to keep to the standard mechanical footprint for the panel. An example of what can be achieved is to take a 450cd/m2 panel and raise it to 1000 cd/m2.

Higher ratings can be achieved, but at the risk of washing out the contrast of the screen where black becomes grey due to the intensity of the backlights showing through. One of the main issues with active enhancement is the increase in power and heat generated. However, the improvements can be drastic on a good panel to start with.

Another way of enhancing an industrial panel is to directly bond a glass substrate to the front of a panel. Thickness depends on the application up to a depth of 5 mm and improves contrast in high ambient light levels.

Other improvements include mechanical rigidity, resistance to wet environments and chemicals and this system can also mean less luminance required from the backlights for sunlight readability.

Direct bonding works like this. If you take a standard panel with a standard glass or polycarbonate filter in front of a good TFT display, the total reflections are 13.5% and you would need a higher candela rated panel to overcome these reflections for a sunlight readable display.

Anti reflection (AR) coat the panel and you have a reduction of 4% out of the total reflections, which now are 9.5%. As part of the transflective technology most of these panels come as AR coated so the enhancement of a good transflective panel is lost by not looking after the filter.

With a poor filter we can AR coat both sides to correct the problem. Both sides must be coated to reduce the total reflections to 1.5% but the downside to this is that the filter is expensive with both sides AR coated and we still have an air gap. Not a cheap solution.

Now direct bond the glass filter to the display and index match the adhesive to the glass of the panel and the filter. There is only one AR coating and the total reflections are now reduced to 0.2%. A cheaper (non-transflective) display can be used in this situation and looks very good.

So, a little thought on the use of the display up front can save money in design and enhancements to achieve the desired effect.

The best panel is a combination of all the enhancements available - direct bonding, AR films and active enhancement - but do you really need all these enhancements?

An interesting question with never the same answer as it is really dependent on your application and cost. The NEEDs are what defines the panel and the top of the range panel is great, but does it meet your needs/costs?

There are other reasons why you would choose a direct bonded enhanced panel over a standard panel. These include: increased readability; ruggedness; no condensation; water resistant; improved durability; lower power requirements; reduced total costs.

Some applications for enhanced displays would include: avionics, fuel pumps, kiosk, military, handheld, tablet, instrumentation, transportation and medical.

The wrong panel choice could mean 'poor sales' for your end product. Regardless of how good the product works, in the end the user will choose the product that looks to be a better display if the price is the same.