Sunlight readability has been a hot topic for our customers lately—perhaps because desert maneuvers have become a mainstay of modern warfare, or perhaps just because it is high summer in the Northern Hemisphere—so we have been fielding a number of questions about our display designs, like how do we make our rugged displays sunlight-readable? Is it just about dialing up the brightness? What does “next gen” readability look like? In response, Tad Ihns, our owner and president, penned the explanation below.
Avalex custom designs LCD display assemblies to guarantee sunlight readability. Display sunlight readability is, at its foundation, a measure of contrast. However, making a sunlight readable display is somewhat more complex, requiring a combination of high bright backlighting, reflection minimizing techniques, and sufficient native contrast in the LCD panel.
The luminance of an LCD is limited by several factors. Typical industrial LCDs use lower efficiency LEDs for backlighting, which electrically limits the backlight output. Custom LED backlights with high efficiency LEDs and light guides can raise the maximum output threshold significantly. The output of each display can then be tuned to meet the desired maximum luminance.
Heat is the most significant constraint on LED output—every LCD panel has a maximum operating temperature where the liquid crystals stop responding. Backlight tuning needs to be refined to maximize daylight readability without threatening the performance of the LCD at its specified environmental operating temperature…the thermal path is key. When we design our displays, we take great care to account for the thermal path, not only of LED backlight assemblies, but also for the display electronics. For displays intended for hotter operating environments, we have found that heat fins added to the periphery of the bezel provide the necessary additional surface area to passively cool high-brightness displays.
The target luminance for all current display designs is 1,000 nits. To ensure we hit our mark on readability, we design backlights with additional luminance capacity, and then tune back to meet customer requirements. For instance, thanks to a backlight innovation for a recent customer project, we achieved a luminance of 2,500 nits while maintaining a +71 °C operating temperature, but we derated the unit to 1,100 nits to better fit the customer application. Achieving very high, very bright backlighting like this thrills optical engineers, but in most cases is unnecessary for sunlight readability.
LCD assembly reflections are controlled by using optically bonded glass layers. We use a proprietary silicone bonding process to create a seamless transition between the cover glass (rugged for durability) and the LCD module, eliminating any internal reflections from multi-layer stacks. We use HEA-treated optical grade cover glass, which is certified to have a 99.8% reflectivity efficiency rating in the visible spectrum, ensures low reflectivity, and anchors our formula for sunlight readability.
Anatomy of an Optical Stack
The most critical factor in sunlight readability, though, is the normal contrast of an LCD panel. Contrast ratios for LCDs have increased markedly in the last 5 years, and, when we source industrial LCD panels, high contrast is at the top of the list of considerations. For instance, in DVRs, we use a commercial green OLED with a native contrast of 10,000:1, which is amplified by a specialty filter glass. In direct sunlight, the DVR display provides flawless readability, even with the brightness turned down to less than 200 nits.
No discussion of viewability is complete without consideration of video quality and image processing. Sunlight readability cannot make a fuzzy image sharp, so we make a point of designing our own display boards to provide the most pristine sensor imagery possible. These efforts pay off in crisp imagery with sharp edges and perfect scaling—maximizing the detail.