Transflective Screens

Transflective Screens- the solution come rain or shine

Martin Whitehead is Systems Manager of Display Solutions; he talks about why the hybrid product of transmissive and reflective displays has produced such a winner.

 Knowing what you want to achieve after a technical brief is always a useful starting point. With the goal of providing split screen information points across the city of London, informing visitors of the latest delights on offer in the capital, the first consideration was to ensure that the screens would be legible and therefore of use. 

Anyone who has ever worked in front of a computer screen near a window, tried to use an ATM in daylight hours or looked at their mobile phone's screen while outside will be only too familiar with the problems daylight can cause in terms of readability. It isn't the brightness that causes the displays to become unreadable, but the lack of contrast, it is a question of how the human eye perceives a screen, a design fault if you will of homo sapiens. The information screens for London held precisely the same challenge for the team at Display Solutions who were supplying the main contractor.

 With most of the sites for the 20” screens specified facing South, it became evident that keeping the visitor information readable in varying degrees of bright-or not so bright- sunlight would be the practical goal. Delivering this without having all the units under constant maintenance through overheating would be the technological challenge. In addition to providing sunlight-readable displays, the chassis into which they were to be mounted had to be capable of withstanding the challenge posed by the British weather not to mention the British hooligan, and assorted tiny British wildlife, namely thunderflies. All units were mounted on posts and consisted of the display, chassis and a computer on the back with a simple function of storing data and providing communication.

"Obviously we try to find the most simple and straightforward solution to the problem, but the big consideration when making a screen readable in sunlight is always going to be heat dissipation. Taking a 20" screen as an example, it will push out 30 watts of heat just for starters.  In order to make something readable in sunlight you can choose two routes. The first is the transmissive display solution, the bull in the china shop approach if you will, which involves simply cranking up the brightness. This dictates using more power to drive the light source-it's a simple solution but has one major flaw and that is the increase in heat that is generated. It's a linear relationship, on a one-to-one basis, i.e. the more watts you are using, the more heat you will be producing. So if you are using three times the power you'll be producing three times the amount of heat.

" More power also means greater energy consumption and so a higher running cost, it can also involve shorter component life and so higher downtime as well as a bulkier casing needed to house the greater number of backlights. The second route is the transflective choice, a relatively new option, and a hybrid of the transmissive and reflective modes, that has opened up an almost optimum path for industrial applications as a large percentage of the heat is reflected rather than stored within the unit, making it ideal for use in high ambient light situations especially within sealed units. To recap on the reflective mode, it uses the light source i.e. sunlight or natural daylight which shines onto the monitor and is then reflected back from a reflector positioned behind the LCD; although this method has the advantage of using little power, it is inefficient in some situations, particularly in low light and of course is completely unreadable at night..

"With a transflective display, you have the best of both worlds; low power consumption and therefore low heat generation together with excellent performance and contrast even in varying light. This is due to its ability to deliver the optimum contrast ratio (CR) even in varying degrees of sunlight, for example when a shadow is cast over a small percentage of the screen, instead of upping the brightness across the total area rendering some parts unreadable, it is able to balance the contrast in just the area necessary. By incorporating both reflected light and a backlight it can operate through the range of ambient light, utilising natural light to its maximum benefit, maintaining total readability even in the brightest sunlight while also delivering low heat output and low energy consumption, an important consideration for many companies who are conscious of their environmental footprint. The transflective method also means a slimmer product can be produced as there are fewer components to be incorporated within the chassis. It has to be said that this marriage of techniques has meant a really workable solution to the problem of industrial displays providing readable information in all conditions, and now the transflective film is being produced in large physical sizes up to 60"`at a cost-effective price, I expect it be specified a lot more.

"Since there were certain immovables in the application i.e. the chassis had to be a closed system; sited outside they obviously had to be sealed for the reasons mentioned previously.. Enclosed systems therefore retain more heat, high temperatures cause malfunctioning of equipment so it's essential to address the issue and maintain as low a constant operating temperature as possible.This can be done via three routes; firstly stop heat getting into the enclosure. Ideally this will be done by installing an infrared heat shield to absorb the heat-carrying long wavelengths of light, common to all LCDs. It is common practice to install an ultraviolet filter, but this is designed to prevent component degradation by filtering out short wavelength light, and does nothing to stop absorption of heat. Secondly you look at ways to stop generating heat, and that's where the transflective method comes into its own and thirdly you must look at ways to get rid of the heat you cannot avoid generating and this can be via either conduction, directly or indirectly to the chassis, or convection where the air inside the chassis is moved from where it is hotter to where it is cooler by means of a fan.

"Although this is always the most vulnerable part of the whole operation, the fans we would specify have a minimum life span of  50,000 hours or five years of continuous useage. The use of transflective displays is the key element to success in providing London's visitors with up-to-date information where they need it, but while Display Solutions can provide the technology, I’m sorry, but there are no guarantees about the weather!  "