You might not notice it when viewing a daylight scene through a picture window, but no glass surface is absolutely transparent. Indeed, a typical air-to-glass interface reflects a significant percentage of the visible spectrum of light.
Each air-to-glass interface reflects about 4% of incident light. Because a single pane of glass has two air-to-glass interfaces – a front and a back – as much as 8% of light viewed through the glass is lost to reflection and 8% of the background light is reflected back to the viewer. The combination results in dimming of the desired light, as well as distracting glare.
The lenses of cameras, telescopes and binoculars are treated with anti-reflection coatings to maximize the amount of light transmitted to the camera sensor or the observer’s eyes. Similarly, when camera or telescope lenses consist of multiple, closely spaced glass elements (and they usually do) the elements are often bonded together using a transparent adhesive that has a refractive index that closely matches that of the glass it’s joining, replacing the interior air-to-glass interfaces with less-reflective adhesive-to-glass interfaces. When gathering the scant photons that make it to earth from distant galaxies, you don’t want to lose any to reflections produced by refraction interfaces. Similarly, when viewing images produced by computer displays, you don’t want those views to be overwhelmed by reflections of background light.
The reflective properties of air-to-glass surfaces can make it very difficult to view computer displays in bright daylight. Touchscreen displays are constructed of multiple layers, including the TFT-LCD component and an overlay of tempered glass, usually separated by a thin layer of air, creating three or more reflective air-to-glass interfaces. Bonding the layers with an optical adhesive eliminates reflective interior interfaces and, when combined with an anti-reflective first-surface coating, can dramatically reduce glare-inducing reflections.
Optical bonding of small, round lens elements is one thing; bonding large, rectangular display layers is quite another. It’s expensive and difficult, but Advantech accomplishes it brilliantly, and affordably, as well. The optical-bonding and anti-reflection systems I’ve described are precisely the technologies deployed in Advantech’s IDK-1107W 7-inch, wide-screen display to reduce total reflectivity of the cumulative screen layers from 13.5% without these systems to less than 0.2% with them, making the IDK-1107W perfect for use in full daylight.
Sunlight has a brightness of about 100,000 candlepower (a/k/a “candela” or “cd”) per square meter of illuminated surface. (Please note that I’m equating candela, lux and lumen for sake of simplicity.) 13.5% reflection of 100,000 cd/m2 of sunlight equals 13,500 cd/m2 versus the 200 cd/m2 represented by the ’s negligible reflectivity of 0.2%. Given that the typical computer display has a maximum brightness of only 250 to 350 cd/m2, it’s easy to see why the images produced by highly-reflective standard displays disappear when overwhelmed by the full 13,500-cd/m2 of reflected sunlight.
Advantech’s IDK-1107W has a display brightness of from 400 to 500 cd/m2, much greater than that of its 0.2% or 200-cd/m2 reflected background sunlight. The result is easy readability, even in broad daylight. As an added bonus, this system also yields a display with enhanced mechanical strength and scratch resistance. These optical bonding and anti-reflection treatments will soon also be available on Advantech’s IDK1121W industrial-grade, 21.5-inch wide-screen monitor, as well. More on Advantech’s optical-bonding system is available here.