Notes for Procurement - Sunglasses with outdoor facing digital displays

Ensuring the details that matter are considered for in-store digital projects

I was prompted to write this blog when walking through my local station on the way to catch a train. It was a beautiful sunny day and the station concourse, like many of the Victorian era, had a glass ceiling, meaning that direct sunlight was washing over the space. I was wearing sunglasses and, as I walked past Costa, noticed that I couldn’t see their digital window display’s content. Thinking that the display had broken, and being perennially interested in this sort of thing, I raised my glasses to check. I could now see that things were working fine. 

Before vs after view of digital signage through sunglasses.

It got me thinking. I don’t think I’ve ever seen a tender document specify that a screen should be visible when customers are wearing sunglasses. Of course, all the usual and obvious display specifications are present: brightness, resolution, size etc. are all very understandable details anyone can write into a tender, but my invisible-through-sunglasses situation presents a perfect example of when requirements on a page don’t tell the full story, and how that overlooked detail can negatively impact the solution’s efficacy. 

The specification in question comes down to the polarisation in the LCD panel. LCD screens work by polarising light created by a white LED backlight, usually passing it through a vertical filter to allow a certain amount of red, blue and green light through each pixel. If your sunglasses block all horizontally polarised light (which is standard for reducing glare) and the screen emits light in that same direction, the filters clash. This creates the perception of a black or heavily distorted image on screen, especially noticeable when the LCD is installed in a portrait configuration, as was the case with my coffee shop experience. 

So, the question is: how do we overcome this issue and what’s the specification that someone procuring a digital window display should think about including? If you want to express this technically, ask for the display to contain a circular polariser, but there’s really no need to get into this detail – simply requesting that the display content is visible through sunglasses in either portrait or landscape will do the trick. 

The downside to this requirement is that the displays able to meet it will be more expensive than those that won’t, since they’ll be employing higher-end LCD panels, which form most of the in-cost of manufacturing an LCD-based display. The additional benefit, however, which again is rarely mentioned in specification documents, is that higher-end panels specifically designed for window-based situations also employ anti-blackening technology in their panels,  allowing them to work in hotter conditions typical of window facing installations. Without this, when the sun’s really shining, less capable displays will often develop temporary black spots, because the liquid in the LCD panel stops being able to allow light to pass through when it reaches higher temperatures. 

If you’re wondering whether a direct view LED display suffers from these same potential issues, don’t worry – the clue is in the name. Direct view means that there is no LCD panel in front of the light emitting elements, so employing polarisation to create an image is not part of their fundamental design.