LCD Optical Stack Films for Improved Readability in Direct Sunlight

As an increasing number of LCD flat panel displays (both commercial grade as well as more ‘ruggedized’ units) find their way into outdoor applications, the demand becomes significantly larger for these products to not only be able to withstand wider temperature extremes (cold and hot), but the viewable images from these displays must also be readable when exposed to extreme optical conditions such as direct sunlight and/or high ambient lighting condition, especially where glare and reflection are most common and subsequently create undesirable imaging results.

The majority of commercial displays are not properly designed to address these demanding requirements. A very common approach by many LCD product manufacturers is to simply integrate higher wattage CCFL or LED backlights to increase brightness of the display image. This approach and way of thinking has a multitude of flawed aspects…one being ‘heat’. The more wattage one integrates to merely boost luminance, the greater the need for complex thermal dissipation. There are even instances where these manufacturers incorporate no additional thermal management solutions to dissipate the heat generated by the enhanced backlights. Heat, regardless of backlight type (CCFL or LED), will significantly shorten the life expectancy of not only the backlights themselves, but as well many other internal electronics and optical materials. Consider further reading our whitepaper article LCD Operating Temperatures where we discuss “Going Isotropic”.

On the opposite spectrum of things, some LCD product manufacturers are integrating “transmissive” enhancements only and, although many panels today are offered with higher “white” luminance of more than 400 cd/m2 (nits), these products do not adequately provide enough brightness and/or contrast improvements to be considered practical in high ambient lighting conditions and certainly not in direct sunlight.

The Myth

A common myth is that simply increasing a display’s backlight brightness alone will make it readable in direct sunlight or other high brightness environments. The reality is that merely boosting levels of brightness is only a benefit if it does not adversely affect the contrast of the display image at the same time.

A distinct characteristic of human vision is contrast or the difference in visual properties that makes an object (or its representation in an image) distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view. Because the human visual system is more sensitive to contrast than absolute luminance, we can perceive the world similarly regardless of the huge changes in illumination over the day or from place to place. Subsequently, it is for this very reason that contrast plays the biggest role in viewing images in direct sunlight or high ambient light conditions. Luminance is only a small factor. Try this exercise….as the contrast level decreases images become less visible even though the background brightness remains the same. The human eye significantly notices contrast changes, so simply making an LCD display brighter does not necessarily make it visually appear better nor easier to view in direct sunlight. One can increase the luminance of a display to the point of greatly reduced contrast and thus progressively diminishing image visibility.

For example, let’s consider that an LCD product manufacturer enhances the backlights to increase the luminance of a particular display that has a white luminance of 200 cd/m2 (nits) and a contrast ratio of 300:1 (black luminance of 0.66 nits). This hypothetical manufacturer boosts the backlights by ten-fold to a white luminance of 2,000 nits. While the display luminance output is without argument considerably brighter, the increased brightness will be of little or no actual value in direct sunlight if the “black luminance” is correspondingly increased from 0.66 nits to 660 nits (contrast ratio of 3.03). These factors, as well as other now inherent integration issues such as increased power consumption and the need now for complex thermal ‘heat’ dissipation, must now be considered as a collective when designing a display for true sunlight readability. Not to mention, that if optical bonding of the LCDs frontal protective window or touch screen isn’t added, reflection still exists. More on this can be learned from reading our article titled Optical Bonding.

For many of its products, VarTech Systems integrates a number of active and passive enhancements to its ‘sunlight readable’ solutions. The first is the integration of traditional value-add backlights to increase the luminance of the display well above normal levels expected for indoor use. The display contrast must, however, as well be increased. Because of these measures, precautions must be taken to avoid direct specular reflections from the display surface.

The second is to incorporate VBOND, a unique optical bonding technology which combines an innovative bonding process with an industry-leading proprietary adhesive to optically bond an anti-reflective glass, plastic or touch sensor directly to the front of an LCD display. VarTech’s bonding technology enhances display performance by improving sunlight readability up to 400% and impact and scratch resistance up to 300%. It is ideal for use in consumer and industrial applications requiring outdoor viewability and the durability to withstand impact, vibration, extreme temperatures, altitudes and dust.

But the third modification is to integrate an innovative optical stack enhancement (OSE), a unique optical design which boosts efficiency of the backlight’s light utilization and minimizes the surface reflection of ambient light. The result is a transmissive enhancement that features higher contrast and a wider range of colors, even in bright outdoor light, than traditional reflective LCDs. This is accomplished by:

  • Recycling what would be wasted off axis light and then redirecting light towards the viewer;
  • Reflecting and recycling polarized light which under normal circumstances 50% of the backlight output would have been absorbed into the LCD and subsequently wasted;
  • Finally, the use of a multilayer, ultra-high reflectivity, mirror-like optical enhancement film that is used as a very efficient light guide and cavity reflector with a reflectivity of 98.5% across the visible spectrum which provides a brighter more efficient display.

And while VarTech does add increased backlighting to its sunlight readable products, the integration of theses innovative films to the LCDs optical stack allow for much lower wattage value-added backlights to be needed. So, in contrast to those companies who are simply adding significantly higher wattage value-add backlights to its products, displays enhanced with VarTech’s unique technology do not require huge amounts of additional power. This benefits the end-user with cooler running units.

Since the display’s lamps are not being overdriven, the VarTech enhancements do not adversely affect the brightness half-life of the bulbs. A direct correlation exists between the brightness half-life of the bulbs and the suitability and cost of ownership of the integrated solution. For example, most bulbs are rated for 50,000 hours when operated at their intended drive current. As a rule of thumb, the bulbs will lose 10% of their initial brightness during their first 500-2,000 hours and 1-5% for every 1,000 hours of use thereafter. Longer brightness half-life also reduces the cost of ownership by reducing the frequency in which the backlights must be replaced.

Those models which VarTech incorporates its unique Optical Stack Enhancement (OSE) technology are typically combined with its optical bonding process as well. This process (VBOND, as mentioned above) produces an optical bond between any display cover glass or touch panel, and any size LCD.

Utilizing advanced proprietary optical bonding technology; VarTech Systems overcomes optical challenges for display product manufacturers at an affordable price. In a wide range of applications, standard liquid crystal displays appear to “washed out” in high ambient lighting conditions. This wash out is due to excessive reflections and glare caused by bright light. Commercially available LCDs, especially when protected by a separate cover glass or plastic shield, can not deliver enough brightness to make the display functional in outdoors or in other high ambient light applications. The exclusive VBOND process provides a significant reduction of ambient light reflections at an affordable price, compared to other display enhancement technologies.

VBOND seals either a top cover glass or touch screen directly to the face of the display bezel. Our bonding process eliminates the air-gap between the display and the cover glass, vastly reducing reflective light, which causes visual washout of the display image. VBOND also enhances structural integrity by supporting the LCD assembly with the cover glass. The bond maintains perfect display uniformity while providing shock protection, unlimited humidity protection, and elimination of fogging caused by trapped moisture accumulating in typical air-gap assemblies.