by Luo Luo, Global Product Manager - TFT, Densitron Already widely-used in industries

Optical Bonding - Exploring its Potential in the Broadcast Sector 9th November 2017

by Luo Luo, Global Product Manager - TFT, Densitron Already widely-used in industries like healthcare and automotive, optical bonding has great potential to increase the performance and durability of displays used across the broadcast sector. In this article, we look at the technology; its key benefits and how it can best be applied in the broadcast environment. Defining Terms Optical bonding is the process of laminating a touchscreen or cover lens to a display with an optically-clear layer. The materials and processes used vary by application, but all fulfil the same fundamental function - filling the air gap between the touchscreen or cover lens and the display. This reduces the number of internal surfaces off which light is reflected, thereby decreasing internal reflected light loss and increasing contrast ratio. The improved contrast ratio delivered by optically-bonded touchscreens and displays means more defined blacks, brighter whites and exceptional colour brilliance - all of which are growing in importance as high-dynamic-range (HDR) imaging becomes an increasingly integral part of both production and broadcast. The most obvious benefit of the increased display contrast is the reduction of wash-out effects in bright conditions. When an air gap is present, this can create a light reflection of as much as 8%. It is this fact that has given rise to its popularity in the medical and automotive sectors, both of which rely heavily on display accuracy. This also makes it particularly beneficial for displays and touchscreens used in studio environments with large volumes of ambient light, as well as during live outside broadcasts of sporting and music events. Increasing readability By eliminating the air gap between the cover glass and display, optical bonding also reduces the potential for visibility issues that occur when light reflected in such a gap creates the illusion that pixels move depending on the viewer's line of sight. As a result, optical bonding increases contrast and improves readability, thereby increasing the range of angles from which the display can be viewed without distortion, and often perceived as widening the overall viewing angle by the end user: a key benefit in busy broadcast control rooms and on-air video walls. Optical bonding isn't only used between touchscreens and displays though. Indeed, manufacturers are increasingly asking us to optically bond cover lenses to displays to provide added protection. In equipment used in busy studio environments, optical bonding can prevent dust and grime from collecting between the cover lens and display. Similarly, optically bonding the cover lens to the display in equipment that may be exposed to the elements can prevent damage caused by ingress of moisture or airborne dirt. In tests carried out internally, optically-bonded displays showed no operational change after undergoing stringent heat and humidity cycles. An additional benefit of optical bonding is that extremely strong bonds are created when air is removed from the gap between the cover glass and display. This can increase the overall strength of a touchscreen or display. In turn, this increases impact and shock resistance, improving the product's durability and reducing the likelihood of faults or warranty claims. Some of our customers have undertaken their own tests on the durability of various displays and optically bonded displays passed according to EN 6050-1 standards. Selecting the right optical bonding solution The materials typically used in optical bonding include optically clear adhesives (OCAs), like those used in smartphones and tablet computers for the consumer technology market, optically-clear resins (OCRs), like those used in point-of-sale terminals, ATMs, touchscreen monitors and satellite navigation systems and optically-clear gels such as those used with the small TFT and PMOLED displays incorporated in wearable devices and broadcast monitoring equipment. Which material is selected and which process used to create the bond will vary according to the substrate used for the overlay, the nature of the display, whether the solution needs to be reworkable and what the primary reasons are for needing optical bonding in the first place. One recent example of where material choice was key was with a Densitron customer based in the US. With an end product that needed to operate constantly and reliably in both high heat and humidity, the customer was looking for an integrated display that could cope with these harsh environments. Initially, we advised on a flush mount cover lens and IP rated gasket, together with an optically bonded sunlight readable display. In initial tests everything was performing well, however, over time the customer noticed that in using this combination, delamination was occurring between the cover lens and the optical bonding and deduced it was a problem because of their end product. There had been no allowance made for thermal expansion of components under such harsh environmental conditions. With our advice, they then looked at using different materials to allow for the extended rating they obviously needed, while we optimised our bonding process to incorporate these findings and meet the new requirements. This example shows why, although the concept of optical bonding is relatively simple, it requires expertise to ensure the right interactions occur between all the factors. However, the improvements it brings to the performance and lifespan of touchscreens and displays, regardless of whether it's used as part of a solution for video monitoring or audio mixing, in instrumentation for live production switching or a post-production colour-grading, suggests this careful approach will undoubtedly be worthwhile over time.

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