Display size and shape http://www.radio-electronics.com/industry-currents/posts/semicustom-tft-displays-size-and-shape-everything is dictated by the system design – but the next consideration in display selection is the how easy is it for the intended end user to read the information on the screen? There are a number of considerations, but primary focus should be on the resolution and viewing angle.
Resolution: a processor AND a display issue
It is important to appreciate that the resolution is a function not only of the specification of the display itself, but also of the processor driving it. Frustratingly, system designers typically choose the display late in the design process. Their focus is often on the choice of processor, operating system and the writing of the software, so often, we see designs which demand a screen resolution not supported by the chosen processor. There are also instances where the processor will provide a higher resolution than is available from the chosen screen size. Thinking about how the system is going to be presented to its user at the start allows much more flexibility and ultimately a less challenging design journey in the long-run.
Assessing the viewing angle
Although display data sheets quote figures for viewing angle based on objective measurements, assessing it can be quite subjective. Whether or not a display can be read from a specific angle can depend on the light levels, the size of the text or graphics being viewed and the operator’s eyesight. With a handheld unit, generally operators will move the instrument so that they can clearly read the display. With a bench unit, they may be forced to view the display from an awkward angle, looking at the instrument from the side or from above. Working with customers to fully understand how the equipment will be used and where the operator will be sitting ensures we provide a product fit for purpose.
What limits the viewing angle on a display?
The standard viewing angles on a TN type portrait TFT are 60-70° to the left or right. An important point is that 2.8” TN displays are normally designed to be used in portrait mode – and the viewing angle is less from the bottom. You can see if that you turn the display through 90° then the viewing angle asymmetry appears to the left or right, depending on which way you turn the display
What technology offers the best viewing angle?
The best way to achieve a wide viewing angle is to use an IPS type TFT. These displays offer a 70-85° viewing angle from all four directions. This means they can be used in either orientation without penalty. IPS technology offers a lot of other advantages too – better contrast, a deeper black, and a very sharp image. It really is a great looking display technology - though inevitably it comes at a higher price.
How can I improve the viewing angle on an existing display?
If you’re committed to a standard TN type TFT there are still a number of things that you can do. A recently introduced technology is polarising o-film that redirects light improving viewing angles. Applying o-film gives you typically an additional 10 degrees of viewability in any direction, at the price of a slight loss of sharpness in the image.
An alternative is to add a ‘moth-eye’ film to the front of the display to reduce reflections which can affect viewability especially from oblique angles. For best results, this needs to be optically bonded to the display. The optically clear glue used in Optical Bonding prevents cracks in the cover glass and creates one solid part helping the display survive drop tests.
Optically bonding the cover glass to the front of the display doesn’t strictly speaking increase the viewing angle but it does improve appearance and readability in general. It eliminates reflection between the glass layers in the display by eliminating internal reflection between the layers. This increases display contrast so that it can be read more easily even in bright conditions. Even when switched off, an optically bonded display will appear nearly black rather than a murky grey.
Try before you buy
It’s extremely hard to visualise screen appearance in the abstract – the best way is to obtain samples of the target displays and load up the interface. Some suppliers have created platforms with pre-integrated motherboards and displays which are shipped working with popular operating systems and processors. Loading your application onto these platforms is quick and easy and you can easily make changes and even try different screen sizes to make an informed decision.
In his next post, Mike looks at the board driving the user interface