Displays
Displays in the sun: useful pointers for designers
In this article, Mike Logan, display and input technology manager, andersDX, will consider some specific challenges of display design for a sunny, outdoor environment: viewing angle, sunlight readability, thermal management and component specification
S
unlight is a very long way from being the display designer’s friend, as anyone who has ever strained to read
an ATM screen on a sunny day will know. Sunlight on a display creates reflections, which can make a display much harder to read. It can also heat up the internals of the system, to levels beyond the specified operating temperature of the components. This is especially a problem for systems like marine units that need to be sealed against the elements.
Viewing angle
The best LCD display technology from a viewing angle perspective is IPS (In Plane Switching). These displays offer 85-88° viewing angles from all four directions – allowing them to be read almost side on. By comparison, a good standard TN technology offers around 60-70° from three directions and 50-60° from the fourth. IPS is clearly much better, and can be used in either orientation without penalty. It 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.
IPS does come at a price though. If
you’re committed to a standard TN 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.
Improving sunlight readability Although the display viewing angle is measured objectively, assessing it can be quite subjective – and as mentioned above sunlight can make it much harder to read a display even from a relatively favourable angle. A great way to improve the readability of a display in sunlight (or any other environment) is to use Optical Bonding between the cover glass and the display. This improves the appearance and readability of the screen in general by eliminating internal reflections that occur between the layer of protective glass and the display allowing the information on the display to be read more easily, even in bright conditions. Even when switched off, the screen of an optically bonded display will appear black rather than a murky grey.
The layer of optically clear glue used in Optical Bonding offers further resistance to external mechanical impact making it possible for the display to survive a force of impact to the front of the screen that would otherwise cause the glass to break. An alternative to optical bonding is to add anti-reflection films, otherwise known as ‘moth-eye’ films, in the gap between the cover glass and display to reduce internal reflections. There are other things you can do, for example increase the luminance of the backlight unit, but this comes with a penalty. The backlight will draw more power which will reduce battery life and increase the heat generated in the display.
Thermal management Thermal management needs to be carefully considered with a display that is likely to catch the sun. Fitting an IR filter overlay and using UV glues are always good measures to take, to reduce heating from the sun and prevent discolouration, but the electronics itself will always generate some heat which needs to be removed.
A marine or other outdoor system needs to be sealed to IP67, which precludes the use of forced cooling like fans, as well as the provision of apertures. The only solution really is to fit a heat sink, which will help some of the heat to dissipate out of the unit.
Component specification Unless the heat sink covers a sufficient amount of surface area, some degree of
Figure 1: Marine environments are extremely demanding for readability. Light levels can vary from full sun to complete darkness and crews are often working quickly under pressure
heating is inevitable. The components need to be specified to handle the anticipated maximum temperature. Two parts of a display that are particularly affected are the polarisers and the liquid crystal (LC) fluid itself. Standard polarisers have a temperature range of -20 - +70°C but extended temperature range solutions with -30 - +85°C are available. With the LC fluid, the specification to look out for is the ‘clearing point’. When the fluid gets too hot, it loses the ability to manipulate the polarised light, which causes the screen to go black. You’ll most likely have seen this phenomenon on your smartphone. Normal display operation is restored when you return it to a cooler environment. Liquid crystal ‘clearing point’ temperature can vary from 70°C to 85°C, but for displays where heat is a major issue, LC fluid with higher clearing points are advised.
Conclusion
It can be annoying to have to screen an ATM with your hand to read the display contents, but in many applications, this is impossible. Often the user will have his or her hands full controlling a machine, and their safety (marine navigation equipment) or their mission (fish finders) will depend on correct reading of the instrument. The techniques in this article are a very useful starting point, but consulting a display specialist such as myself or my colleagues at Anders is essential in order to achieve
www.andersdx.com
Figure 2a and 2b: 10” TFT Display using optical bonding to enhance sunlight readability 20 February 2017 Components in Electronics
www.cieonline.co.uk
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