Gunter Wagschal, Project Manager Business Unit Industrial as Sharp Microlelectronics Europe considers High Brightness LCDs and how they are interfacing as the backbone of modern industrial HMIs
Transflective displays represent state-of-the art technology that ensure readability under changing and very bright ambient light conditions. In effect, incident light is reflected at microstructures within the LCDs supporting the backlight. This method covers a wide range of ambient light levels with CCFL backlit displays, even though the luminosity of CCFLs can be controlled only in very narrow boundaries.
But the reflective mode only works well under direct sunlight and there is a certain shift in colours compared to the transmissive mode. At bright but diffused ambient light conditions transflective displays quickly lose their contrast compromising the readability of the LCD.
Contrarily, high brightness displays with LED backlight constantly render a good image across the whole spectra of ambient light levels. Also touch-screen HMIs need LCDs of higher luminosity since the touch sensitive surface reduces the transmissivity of the complete module by up to 10 percent.
For the great variety of HMI solutions Sharp offers several very bright displays with diagonals from 5.7” to 23.1” with a luminance up to 1,200 cd/m_ (image 1). They provide sufficient brightness to ensure good readability even in sunlit environments.
In dark ambience, thanks to the LED backlights, the displays can be dimmed correspondingly without compromising on the lifetime as it would be the case with CCFL backlights. To support design engineers in using of the broad dimming capabilities, particularly the newer LCD models with LED backlight are already fitted with driver units for pulse width modulation (PWM) dimming.
Energy efficient and quick starts
An effect of the good dimming capability is the improvement of the energy efficiency. Reducing the display brightness in relation to ambient light levels substantially decreases the power consumption – in dark environments to just about a fifth compared to full load needed at sunshine. Solution providers committed to eco friendly designs therefore cannot omit displays with LED backlight particularly since they don’t contain mercury as well.
The fast reaction time of the LEDs positively affects starting characteristics of display as well. Already at normal ambient temperature CCFLs need several minutes to reach their full brightness – cold environments expand these starting phases considerably. LEDs on the other hand provide full luminance instantly independent from ambient temperature as soon as tension is applied.
Consequently, TFT LCDs with LED backlight can be controlled easily depending on the ambient light level and actual usage. Many applications only need the screen to be fully lit when a user is approaching the application.
Particularly for ATMs and vending machines this is of interest: for long periods during the day they remain idle, and depending on the day time ambient light levels are quite different.
Screen brightness and henced power consumption of such systems bear significant potentials for optimisation, which can easily be realised by integration of ambient light and distance measurement sensors as they are also offered by the company (image 2).
Additionally, LED backlights are more robust compared to CCFL: while fluorescent tubes begin losing their brightness from the first operating hour, this effect starts significantly later with LEDs. Cold environments even accelerate the degradation of CCFLs while they even extend the lifetime of LEDs.
Operated at the low end of the temperature scale specified LED backlight may reach a lifetime of 200,000 hours and more. At the high end of the operating temperature range most decisive is, how efficiently the dissipative heat can be released into the environment.
For industrial LED backlit LCDs the company has developed a special design of the chassis where the back side of the display module functions as a heatsink. As a result, most of the company’s high brightness displays are specified for operation temperatures of up to 60°C, some of them even endure up to 75°C, at lifetimes of 50,000 hours.
The integration of LED backlit LCDs facilitates the design-in of HMIs, since the low voltage direct current power supply bears several advantages. First of all, with LEDs using direct current substantially improves the EMI characteristics: CCFLs, in contrary, are operated with alternating currents of 50 – 60kHz making it necessary to shield the tubes and the inverter.
Sometimes this remains insufficient causing a slight flicker of the display image. Secondly, the high voltage power supply of the CCFLs with up to 1.5kV requires special precautions which can be omitted with LED backlights, since they run on only 32 V and less.
Typically high brightness LCDs with LED backlight are used in industrial applications such as control units of manufacturing equipment operating under bright working light conditions or outdoors. However the technology also finds use in portable test and measurement equipment, heavy machinery for building, forestry and farming, which all require powerful backlights.
Sharp Microlelectronics Europe
