Light emitting diodes are used in an increasing number of medium- and large-format displays as backlight. As could be seen recently at the ifa, the Consumer Electronics sector is assuming a pioneering role here " all liquid crystal display manufacturers presented ultra-flat LCD TV appliances with LED backlighting. But is the new backlight technology also suitable for industrial applications?
Challenge for industrial LCDs
The challenge above all with industry displays is to combine the advantages of LED backlighting with the standards required of industrial applications with regard to temperature resistance and robustness. Sharp is presenting solutions here and expanding its product portfolio of LCDs for industrial applications to include LED-backlit versions.
The wish list of requirements that design engineers place on backlighting for industrial displays is a long one: high level of brightness with good uniformity, extensive dimmability, low weight, low electricity consumption, long life cycle over a broad temperature range and high mechanical resilience are just a few aspects. Further requirements include long-term availability and low costs. None of the backlight technologies that are currently on the market cover all the aspects equally well. Cold cathode fluorescence lamps (CCFL) have long proven themselves as luminants for backlighting industry displays, and with good reason: They supply great brightness and, above all, a high lighting efficiency with large-format displays. Another plus point for the CCFLs is their long life cycle of approx. 50,000 hours (particularly at high operating temperatures) and the high availability and reliability of the established technology.
Industrial LCDs with LED backlight
A few disadvantages of the CCFLs cannot, however, be dismissed, starting with the high operating voltage of up to more than 1kV. The required voltage inverters and voltage protection arrangements make CCFL backlightings comparatively complex systems. Furthermore, the fluorescent tubes quickly reach their limits, particularly at low temperatures, when the backlight is dimmed (as this also reduces the life cycle) and in cases of strong vibrations and other mechanical loads. This is exacerbated by the gas filling of the tubes, which contains mercury. Although there is an exception to the RoHS regulation for CCFLs, this applies only for as long as there is no other adequate alternative available.
LEDs also for large-format displays?
As a basic principle, LEDs represent an alternative to the backlighting of medium and large-format displays. First of all, they compensate for the aforementioned disadvantages of the tubes: a particularly crucial factor for displays for the extensive bandwidth of industrial applications " LEDs are mechanically substantially more robust compared to CCFLs. They also do not contain any mercury and only require direct current low-volt supply voltage: This means that the high-voltage converter required for CCFL lamps is no longer necessary. The consequence: in addition to reduced system complexity, LED-backlit LCDs can easily be used where high voltage is not desired, e.g. in areas where there is a risk of explosion. As backlighting, LEDs also have their strengths, particularly at lower ambient temperatures; these include their prompt response characteristics. CCFLs provide the full light output only when they have reached their normal operating temperature. This can take a few minutes at room temperature. LEDs, however, attain their full luminosity almost immediately, as soon as the operating voltage is connected, whereby the brightness can be regulated continuously over the entire range from zero volts to the maximum operating voltage without this having a negative impact on the life cycle of the backlight. Cold environments are unable to affect it " quite the contrary: Ambient temperatures substantially below the standard temperature of 25°C extend the life cycle of LEDs to way over 50,000 hours.