Human Machine Interface
Looking good
When considering the use of display panels design engineers have to take into account not only their technical specifications but their visual appearance. As Mark Stephenson suggests, TFT LCD display modules provide an attractive solution
T
he perception of a good product is often derived directly from the appearance of the front panel or the graphical user interface: is it appealing to look at? Is it functional? Can it be easily used? For electronics systems and equipment requiring a graphical user interface, TFT LCD display modules are being used widely and extensively. For the design engineer, the technical specification is very important, but invariably display panels are chosen because of their visual appearance. The aesthetic characteristics of the display image and the resulting user experience it provides should therefore be given serious consideration.
In-plane-switching (IPS) TFT display technology addresses the two main limitations of a standard twisted nematic (TN) TFT display; that is limited viewing angles and poor colour reproduction.
Display readability
The ability of a display image to be easily viewed is referred to as the display readability. This is influenced by a number of factors which affect the optical performance of the display. The optical performance of a display is measured by the brightness and contrast of the display.
Brightness is defined by how much radiated light is emitted from the display.
20 November 2011
Brightness is measured in candela per square metre (cd/m”) or nits (where 1cd/m” = 1 nit).
Contrast is defined as the ratio of the luminance of the brightest colour (white) to that of the darkest colour (black) that the display is capable of producing. Typically, the higher or greater the contrast ratio, the easier it is to see the display image.
Twisted Nematic (TN) The TN mode TFT is the ‘workhorse’ technology for TFT displays. In a TN TFT display the liquid crystal molecules are aligned with the upper and lower glass substrates. The LC molecules align parallel to the rubbing direction of the alignment layer. The rubbing direction of the alignment layers of the two substrates is perpendicular to each other, which creates a 90 degree twist in the liquid crystal.
When a voltage is applied across the LC cell, the liquid crystal molecules untwist and align along the vertical electric field. A limitation with this basic TN technology is that the alignment of the liquid crystal molecules changes the further away they are from the optical electrode. This can have an impact on the passage of light through the cell causing reduced contrast, brightness and colour definition at wider viewing angles.
Components in Electronics
Vertical Alignment (VA) Vertical alignment was originally developed to address the limitations of TN mode technology. In VA displays the liquid crystal material exists naturally in a vertical state. With no voltage applied, the liquid crystal molecules remain perpendicular to the substrate. When a voltage is applied, the liquid crystal cells shift to a horizontal position, parallel to the substrate, allowing light to pass through. VA has achieved wide viewing angles and high contrast but with a reduction in brightness and average colour reproduction. Current VA panels can offer wide viewing angles - second only to IPS technology - good black levels and good colour reproduction. When VA panels are viewed off-perpendicular, a colour shift is present, but much less than that of TN panels. Multiple-domain (MVA) and super
patterned (SPVA) vertical alignment panels have addressed some of the limitations of VA technology and both technologies provide improved contrast particularly at wider viewing angles.
In Plane Switching (IPS) Developed and first introduced by Hitachi Displays back in 1996, IPS was the first LCD technology to give an outstanding level of picture quality particularly when
viewed at any angle. IPS technology provides impressive colour saturation, colour stability, contrast and black levels with a 176° wide vertical and horizontal viewing angle.
In an IPS TFT display, the liquid crystals
are aligned horizontally to the glass substrate. Both electrodes are horizontally aligned on the lower substrate. When a voltage is applied between the electrodes the liquid crystals rotate horizontally and remain in parallel with the electrodes and the glass substrate of the screen. The liquid crystal molecules are able to move freely into the required alignment. This approach keeps the liquid crystals parallel to the front of the panel, providing a viewing cone that is wide and symmetric.
A brief history of IPS The core IPS technology has since evolved into Super-IPS (S-IPS), Advanced-Super IPS (AS-IPS) and latterly IPS-Pro in 2008. Super-IPS was introduced in 1998 to combat the colour shift that was still apparent in wide viewing angles of the original IPS screens. More recently with IPS-Pro, a significant breakthrough was the refinement to the optical electrode pattern and structure, and a change from opaque to transparent electrodes. Medical, broadcast and professional monitoring applications demand
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