The major challenge with industry displays is to combine the advantages of LED backlighting with the standards required of industrial applications in terms of temperature resistance and robustness. The wish list of requirements that design engineers place on backlighting for industrial displays is a long one. They require a high level of brightness with good uniformity, extensive dimmability, low weight, low power consumption, long life cycle over a broad temperature range and high mechanical resilience - and these are just a few parameters. Further, requirements include long-term availability and low costs. None of the backlight technologies that are currently on the market cover all these aspects equally well. CCFL (cold-cathode fluorescence lamps) have long proven themselves as luminants for industry-display backlighting, and with good reason: they supply great brightness and, above all, a high lighting efficiency with large-format displays. Another advantage of CCFLs is their long life cycle. They can last for approximately 50,000 hours—particularly at high operating temperatures. Another advantage that they offer is the high availability and reliability of the established technology.
Overcoming CCFL disadvantages
A few disadvantages of CCFLs cannot, however, be dismissed, starting with their high operating voltage, which can exceed 1kV. The required voltage inverters and voltage-protection arrangements make CCFL backlighting a comparatively complex system. Furthermore, the fluorescent tubes quickly reach their limits, particularly at low temperatures, when the backlight is dimmed—this also reduces the life cycle—and in situations where strong vibrations and other mechanical loads are present. 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 exception will only apply until any adequate alternative becomes available.
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 tubes. LEDs are mechanically substantially more robust compared to CCFLs, a particularly crucial factor for displays targeting the extensive bandwidth of industrial applications. They also do not contain any mercury and only require a direct-current, low supply voltage; thus, the high-voltage converter required for CCFL lamps is no longer necessary. As a consequence, in addition to reduced system complexity, LED-backlit LCDs can easily be used where high voltage is not desirable, for example in hazardous environments and areas where there is a risk of explosion. When used as backlighting, LEDs also have their strengths, particularly at lower ambient temperatures. These advantages include prompt-response characteristics.
CCFLs provide the full light output only when they have reached their normal operating temperature, which can take a few minutes at room temperature. LEDs, however, attain their full luminosity almost immediately, as soon as the operating voltage is connected. The LEDs' brightness can be regulated continuously over the entire range from 0V to the maximum operating voltage without any negative impact on the life cycle of the backlight. Moreover, cold environments do not affect their performance—in fact, the result is quite the contrary: ambient temperatures substantially below the standard temperature of +25°C extend the life cycle of LEDs to in excess of 50,000 hours.
Doing away with heat accumulation
Industry LCDs with LED backlighting require special heat management because of the environments in which they are expected to operate. The challenge in the design of LED-backlit industry displays still lies in the LEDs' greater dissipation compared to CCFLs. The electricity absorbed by the lighting module is partially converted into heat, whereby LED backlights produce substantially greater quantities of heat than comparably bright CCFL backlights. In this way, heat accumulation can occur inside the display and affect the LCDs and the backlight, as the heat is not transported away. Industry LCDs with LED backlighting therefore require a carefully developed concept for heat dissipation (Figure 1) to avoid this problem.
Many display manufacturers, however, simply replace the CCFL cartridge with an insert fitted with LEDs. As there is no direct link from the insert to the external panel housing, it is hard for the heat to escape. The result is often heat accumulation in the backlighting area, which can damage the liquid crystals, the colour filter and, not least, the LEDs themselves. Conventional LCDs with LED backlighting are therefore only designed for a comparatively low maximum temperature and have a shortened life cycle, compared to the CCFL variant. It is becoming the case, however, that some LED-backlit industry displays are more appropriate for industrial applications. For example, Sharp has developed displays with a housing that has been specially conceived for LED backlighting. The displays have a direct thermal coupling between the backlighting and the mechanical parts of the LCD module. The waste heat of the LEDs is thus transported to the rear outer side of the panel, where it can be easily dissipated into the environment, thus preserving the display's performance. These LED-backlighting variants have also been engineered to fulfil the full industry specification with a life cycle of 50,000 hours, so that their performance is comparable to that of CCFLs. Depending on the module, the operating temperature lies in the range of between -30 and up to +80°C.
As a result of the engineering process, the LED-backlit LCDs offer optical brilliance with viewing angles of up to 160° in all directions, 262,144 colours and a special anti-reflection coating (Figure 2). The new TFT LCDs are thus particularly suitable where tough application conditions meet rapidly changing light conditions, such as in outdoor areas for forest, agricultural and construction machinery, as well as in transport, where these conditions apply.
RGB LEDs for life-like colours
RGB LEDs are particularly superior to CCFLs as a backlighting option when it comes to true-colour image rendition. The latter is a necessity for displays used in the medical sector and an advantage for those deployed in the advertising and design industries. Compared to the NTSC standard, values of 120% and more can be achieved through LED backlighting, thus achieving a considerably more life-like image rendition than CCFL tubes, which are capable of creating a colour gamut of only approximately 75%. The extremely high NTSC values are achieved specifically with RGB LEDs, as it is only through separate control of the red, green and blue LEDs that the spectral composition of the backlighting can be optimised in correlation to the screen content. Possible applications include medical diagnosis equipment or in products earmarked for the graphics, advertising and multimedia markets, where life-like colour rendition is particularly important.
If a more compact design is required, an improvement of the phosphorous coating of the LED chips results in higher NTSC values (Figure 3). Conventional white-light LEDs consist of a blue diode that is coated with yellow phosphorous. The wave spectrum of these pseudo-white-light LEDs consists of a sharp blue light peak at 450 nm and a broad cone of between approximately 500 and 630nm. Through a mixture of red and green phosphorous, Sharp resolves the broad cone of the wave spectrum into two broad peaks, with intensity peaks at 540 and 640nm, respectively. The result of this process is significantly greater coverage of the natural colour spectrum with a NTSC value of up to 80%.
LED-backlit industry displays supplement the existing portfolio of industry LCDs with CCFL backlighting as new important model variants. Both technologies cover different application areas. LED backlights are more advantageous particularly where aspects such as great mechanical robustness, a broad brightness range, the avoidance of mercury and explosion protection are important. However, thanks to their proven features and cost-effectiveness, displays with CCFLs continue to be a good choice for a large number of applications in the industry sector. They are particularly suitable for use as luminants for backlighting in environments that have high ambient temperatures for a longer period of time.
LED backlights that are destined for use in industrial displays still have room for improvement over CCFLs. Above all, it is important to bear in mind that, until further notice, CCFL-backlit LCDs remain the more cost-effective alternative. Those designers however, who already opt for the new backlighting technology today for specific reasons—such as explosion protection—must ensure that the chassis of the LCD modules are specially designed for the LED backlighting (Figure 4) and that they have a direct thermal connection between the backlighting and the housing. As is the case with the examples quoted here from Sharp, the LED-backlit LCDs can only attain the long life cycle required by the industry if the the increased waste heat of the LEDs can be released to the environment via the housing.
Figure 1: Heat dissipation has to be developed specifically for industrial-use LCDs.
Figure 2: LED-backlit LCDs can offer optical brilliance and versatility. They often have viewing angles of up to 160° in all directions, 262,144 colours and a special anti-reflection coating for clarity of vision.
Figure 3: Phosphorous coating of the LED chips results in higher NTSC values.
Figure 4: For industrial applications, an LCD-module chassis has to be custom-designed to meet specific, harsh and demanding environmental conditions.
