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SEEING THE BIG PICTURE: SUNLIGHT READABLE DISPLAYS
Steve Varley from the Ginsbury Displays Division of Solid State Supplies investigates some of the technologies involved in making an LCD screen suitable for use in bright lighting conditions
I
ncreasingly, LCD displays are replacing static advertising posters
and billboards in outdoor locations as well as being used for road signage and other information-giving applications. However, many of these outdoor installations are subject to bright lighting conditions, making conventional displays difficult to read. There are two main aspects to
producing a viable sunlight-readable LCD display. Firstly, increasing the display brightness to levels that will allow it to be read in bright light and, secondly, adding an anti-glare layer to the front of the display to mitigate the effects of direct light. ‘Solar loading’ can be a challenge with
LCD displays that are exposed to direct sunlight. This is a mechanism whereby the infrared component of the light heats up the Liquid Crystal Structure of the display so, although the display surface may not feel hot, the internal temperature can become elevated. If an LCD gets to a temperature above about 70-90°C (known as the ‘clearing point’), then it loses its isotropic properties, often causing at least part of the display to go black. Although this effect reverses as the display cools, if it happens too frequently, the effect can become permanent. Fortunately, some LCD displays are now available with higher clearing temperatures of 100-110°C to allow displays to be used in direct sunlight without this issue occurring. A typical LCD display for indoor use
will have a backlight brightness of around 250-300 candela-per-metre- squared (cd/m2) (In some circles, the term ‘nits’ is used in place of cd/m2 – the two are identical and can be used interchangeably). For use in sunlight or other bright conditions, the backlight brightness has to be increased to around 1000-1500 cd/m2 – an increase of 5-6 times. This is normally achieved through fitting a replacement backlight with an increased number of LEDs. Most commercial LCDs for indoor use
have an edge lit backlight LED strip with some form of optics, including a
14 NOVEMBER 2017 | ELECTRONICS
the display difficult to read. Effective optical bonding is
light guide, to uniformly distribute the light across the rear surface of the LCD. High brightness LCDs for sunlight readable use tend to have an array of LEDs across the back surface thereby providing direct lighting. The required power for sunlight readable displays does increase as there are more LEDs to drive, but the thermal impact is not significant due to the inherent efficiency of LEDs. The second part of the process for
converting an LCD display to be daylight readable is to install an optical filter to the front of the display. The two types commonly used are anti-glare, which disperses incident light or anti-reflective, which absorbs light, thereby cutting down on reflection. When bonding the filter to the front of
the LCD, it is critical that no air gaps are present, otherwise changes in the refractive index between the filter, the unwanted air and the display will increase the number of reflective surfaces (the front and back of the filter as well as the front of the LCD will all reflect), making
Figure 1: Filters must be bonded to the LCD without air gaps to ensure optimum readability
accomplished by filling the space between the LCD screen and the filter with a transparent liquid adhesive that sets to be a transparent solid with a similar refractive index to the filter. Performed correctly, this process improves the contrast of the display substantially and significantly improves readability. In fact, with the way the human eye works, contrast is at least as important as brightness in making a display readable. As well as improving contrast and
readability, the filter also acts as a pseudo-heatsink for the LCD allowing excess heat to escape into the ambient air. This is particularly useful in fully sealed outdoor LCD applications where there is no other air circulating. Nevertheless, it is still considered
good practice to think carefully about how the assembly is placed – techniques such as angling the display so that it does not point directly at the sun will significantly reduce the heat problem by reducing the effects of direct infra-red radiation. Ginsbury has been in the display
business for almost forty years. The company provides a full range of solutions from individual components such as an LCD panel, backlight driver, controller card or touch screen right the way through to fully integrated bespoke solutions. Over the years Ginsbury has supplied
tailor made and bespoke display solutions to a variety of markets including fighter jets, bomb disposal vehicle systems, helicopters, industrial control systems and retail applications. The company is able to advise on
application requirements and then design a complete turnkey solution including fully ergonomic packaging that is suitable for the application and the environment.
Ginsbury Displays
www.ginsbury.co.uk T: 01634 298900
/ ELECTRONICS
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