Feature Displays
A clear view on display selection S
electing a display solution for a new product design should not be considered a trivial task. A colour TFT display is often the most
expensive single component on the bill of materials, and there are many brands, formats, technologies and performance qualities to choose from, with price varying from budget to premium. If it resides on the front panel of a product, the display can have a signifi- cant impact on the user’s perception of the product, therefore a number of questions have to be asked. Is the display bright and colourful? Is the information presented on the display easy to read? Can the display be viewed in all application environments? Along with these optical performance and aesthetic properties, it is also essential to consider the cost implications of the display solution. The type and quality
panels side by side will allow a direct visual comparison of optical performance to be easily made. This can often be confirmed by measuring the brightness of the display panel with a light meter.
The front surface of the display will have a polariser film attached. Anti-glare polarisers help to minimise surface glare by reducing specular – mirror like – reflection from the front surface of the panel, reducing image ‘washout’ under bright ambient light environments. Additionally, finger prints on an anti-glare polariser surface will not be so visibly apparent. Glare polarisers, although allowing a higher level of light transmission, and hence higher brightness and contrast, are more susceptible to surface reflections, finger prints and scratches.
The brightness and contrast
‘Optical performance is a measure of the brightness and contrast of the display, and contributes to the ease of viewing and readability of a display image. This is likely to be the most important factor when making a display selection’
of materials and components used will influence the production costs, and hence the supply cost of a display; while the attributes of the design imple- mentation can also impact long term performance and reliability. Investing in a high quality, premium TFT display solution can help to ensure reliability and contribute to the sales success of a product. As it is essential that the best display is chosen for a particular application or environ- ment, optical performance, technical specification, mechanical design, and environmental and commercial needs should all be considered.
Optical performance
Graphical user interfaces need to be capable of displaying bright, vivid display images and information. Optical performance is a measure of the brightness and contrast of the display, and contributes to the ease of viewing and readability of a display image. This is likely to be the most important factor when making a display selection. Placing two competing display
Instrumentation FEBRUARY 2011
performance can also be impacted by limitations in display performance such as colour shift and crosstalk. Colour shift impacts the colour tone of the display image which changes when the display is viewed off-perpendicular, and can result in image inversion when the display is viewed from an acute viewing angle. Crosstalk can create shadowing and ghosting effects on the display image.
Many user interfaces now require highly consistent optical performance and image reproduction; and contrast and colour characteristics must be maintained over a wide range of viewing angles. Wide viewing angle technologies such as in-plane switch- ing (IPS) and vertical alignment (VA) are now widely used in smartphones, tablet PCs, medical monitors and LCD televisions.
In-plane switching and vertical alignment TFT display technology has overcome the limitations of con- ventional TFT display technology by providing a stable colour image (from almost any viewing angle up
The benefits provided by the features and design implementation of a premium TFT display module should not be underestimated, says David Potts
to 176° in both horizontal and vertical planes), excellent colour saturation and high contrast ratios.
Backlights
White LED backlights have effectively replaced cold cathode fluorescent lamp (CCFL) backlights in many small to medium size (3.5” to 12.1”) industrial specification TFT LCD display mod- ules. Solid state LED backlights are more inherently robust and reliable than the CCFL backlights they are replacing, with the additional advan- tage of not requiring a high voltage DC/AC inverter to drive the backlight. One key issue to consider is the backlight operating lifetime in extreme environments, especially in cold conditions. An LED backlight is less affected by operating temperature which in a harsh environment is essential for ensuring a reliable, maintenance free display installation. By contrast, the operating life of a CCFL is limited in low temperature conditions: operating CCFLs between 10°C and 50°C has no effect on life- time whereas starting and operating a CCFL below 10°C greatly shortens its life. Typically at 0°C, a CCFL will last approximately 1,000 hours.
Backlight lifetimes are specified in terms of the time to reach half- brightness. LED lifetime characteristics are important with lifetime estimations of between 70K to 100K hours being quoted by display manufacturers. Close scrutiny of manufacturers’ data sheets is often necessary to determine exactly how the LED lifetime is being speci- fied: many display manufacturers use the same white LEDs in backlight units and the difference between a specified
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The display can have a significant impact on the user’s perception of a product, therefore before selecting a display solution for a new product design it is essential that attention is paid to such features as optical performance, technical specification and mechanical design. David Potts, divisional marketing manager at Anglia Displays, comments
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