Frontline Technology

Challenges for Outdoor Digital Displays

Challenges for Outdoor Digital Displays

When designing an outdoor digital-signage system, location, hardware, sunlight readability, durability, and reliability are key components to consider.  To meet these challenges, Delphi Display Systems has developed fully integrated and sealed outdoor display systems that incorporate embedded media control and robust remote management.

by Benjamin Medvitz

DIGITAL SIGNAGE is appearing in more and more areas of our lives.  Significant technological advances are making it cost effective for almost any business to incorporate LCDs in sizes up to 60 and 72 in. with only a moderate investment.  System-on-chip (SoC) technologies are enabling not only smart TVs for consumers but integrated displays with media players and embedded computers for interactive signage applications available out-of-the-box.

With the unprecedented speed of innovation and mass-market ability to deliver LCD screens at rock-bottom prices, many customers want to utilize these technologies outdoors or provide combined indoor–outdoor systems and expect that similar technologies at similar price points will deliver an equally impressive experience.

However, the outdoor environment is harsh and unforgiving.  With temperature extremes from –40°F to +125°F and above, direct sunlight day in and day out, sand, rain, snow, sleet, fog, and high winds, the challenges of making electronic displays work reliably outdoors are significantly greater than in indoor controlled environments.  The mass-market technologies that have made ultra-inexpensive LCD TVs ubiquitous cannot be fully leveraged for outdoor use, and sales volumes for outdoor displays are low enough that economies of scale do not provide the same results as those seen in indoor displays.

There are many companies that provide outdoor display systems today, with widely varying features, reliability, performance levels, and price points.  Selecting the right outdoor display for digital signage can be complicated.  Each project’s critical parameters, including display placement, brightness, maintenance, required display lifetime, and others, have a significant impact on display selection and ultimate project success.

In this article, we will review many of the challenges unique to outdoor digital signage for smaller displays from 12 to 72 in. – those ideal for LCD technology.  Backlight luminance, heat management, sunlight management, polarization, serviceability, and lifetime are all factors.  We will also discuss options and solutions recommended by our company’s researchers.

Bright Enough?

Display luminance is measured in candelas per square meter, or “nits.”  Indoor TVs and monitors typically provide white luminance levels of 300–400 nits.  By comparison, a white outdoor surface in direct sunlight can reflect 10,000 nits!

We perceive a world of almost constant brightness throughout our day.  Thanks to the human eye, which is able to automatically adjust to widely varying luminance levels, we do not often have to consciously think about the brightness of a room or environment.  We are able to discern features and objects in our environment by the contrast between light and dark or between differing colors.  In very bright outdoor environments, we typically only see high-contrast features, while in darker environments we can see more subtle variations between light and dark.  While this adjustment is usually automatic, occasionally we notice it, for example,  when we enter a darker building on a sunny day and need time for our eyes to “adjust” before we can see any detail or when we leave a dark theater on a sunny day and everything appears too bright to be seen until our eyes adjust.

Information on digital displays is presented with a constant level of contrast between light and dark areas.  In a dark room, the luminance of an LCD backlight provides the maximum light level in the lightest areas of the display.  The display technology determines the minimum light level in dark areas of the display, commonly measured in reference to the maximum light level as a contrast ratio.  When displays are used outdoors, however, the entire environment is much brighter due to ambient light from the sun.  Our eyes adjust for this brightness, reducing our sensitivity and requiring greater contrast for us to be able to discern features or information on displays.  Displays that appear bright in darker rooms may be so dim outdoors as to be unreadable if the backlight luminance is not high enough to overcome the high levels of ambient light from the sun.  If the backlight is bright enough but the display’s contrast ratio is too low, or if too much of the outdoor environment’s ambient light reflects off the face of the display, it gets “washed out” as our eyes become unable to clearly discern between light and dark areas of the information on the display.  For outdoor displays to provide clear information and crisp images, they must be able to provide both enough luminance and enough contrast ratio to overcome the very bright ambient light from the sun.

Displays marketed as “high brightness” or “sunlight readable” start at a luminance of 700–800 nits, with many going beyond 2000–3000 nits.  An 800-nit display looks far better outdoors than a 300-nit display, but will still require shade structures, ambient-light management methods, or other means to avoid being washed out in direct sunlight.  Each layer of the touch screen, protective face glass, and other coatings on the front of the display will further reduce the available luminance by 10–20%, requiring additional backlight energy or even more careful management of environmental conditions to maintain readability.

As mentioned above, the contrast ratio between bright and dark areas of the display is also critical to outdoor readability, as human eyes become less sensitive to small luminance changes in environments with high ambient light.  Anti-reflective coatings on the front face of the display can reduce the reflected light outdoors, greatly improving the observed crispness and contrast of the display.  Displays that exceed 1500 nits and dark-room contrast ratios of 1000:1 or more will usually appear clear and crisp in direct sunlight, similar to those shown in Fig. 1.

Fig. 1:  These outdoor LCDs (with some key features listed above) are used for a restaurant drive-thru menu and order confirmation application.

Another thing to consider when selecting an outdoor display is the expected lifetime.  All backlight technologies reduce in luminous output (or efficiency) as they age.  This characteristic of a display is registered in a standard “mean time to half-brightness”  (MTTH)  measurement.  For an 800-nit display operated 24 hours 7 days a week, an MTTH of 50,000 hours means that after 5.7 years of operation, the peak white display luminance will now only be 400 nits.  The challenge for MTTH is that it is also highly dependent on temperature.  When a display is in direct sunlight most of the day, it gets hot.  Add this to the internal heat from the display electronics itself and it can get really hot inside the display enclosure!  A display with a 50,000-hour MTTH at a temperature of 25°C (77°F) may only have a 20,000-hour MTTH at a continuous temperature of 55°C (131°F).

Keeping Cool in the Sun

Think about how hot the inside of your car gets when it is left outside on a sunny day.  The heat is unbearable, even if the outdoor temperature is comfortable, due to the greenhouse effect of trapped infra-red energy from the sun inside the car.  The effects are the same for outdoor displays.  If an LCD panel gets too hot, the liquid crystals inside the panel may be permanently damaged.  The liquid-crystal fluid gets too hot and the entire system changes its phase state.  Once the liquid-crystal fluid overheats, it becomes an isotropic liquid and is no longer able to translate the polarization angle of the light based on electrical stimulation and natural birefringence alignment.  We call this effect “clearing” and the temperature at which the LCD exhibits this phenomenon is known as the “clearing temperature.”  In displays that have a normally black mode, this results in large areas of the LCD blackening and becomes unreadable, similar to the artifacts shown in Fig. 2.

Fig. 2:  The liquid-crystal mixture in the LCD at right has suffered from isotropic blackening effects, which can occur as a result of high temperatures.

Recent advancements in the technology of LCD materials and construction have yielded great improvement in the high-temperature operation of displays.  While some conventional TN-LCD panels have clearing temperatures as low as 50°C, new fluid formulations such as those in LG’s high-brightness in-plane-switching (IPS) displays allow for clearing temperatures up to 110°C.

There are two general ways of managing temperature inside outdoor displays.  Some utilize outside air to cool the components of the display, similar to rolling down the windows in your hot car.  It keeps the interior temperature from getting too hot, but allows dust, dirt, sand, and all kinds of contaminants into the display.  These displays often feature advanced inlet filters to keep these hazards out, but will require regular cleaning and replacement.  If you do not have a robust maintenance staff and budget, this maintenance will soon be overlooked and these displays are likely to fail after only a short life, either due to overheating from clogged filters or from contamination let in from degraded or failed filter elements.

Utilizing outside air is also a problem if the LCD panel is not optically bonded to cover glass.  If there is any space between the LCD and the front glass of the display, outside air provides an excellent opportunity for condensation and fog to form between the display and the glass, obscuring the display.

The other main option for temperature management is to seal the unit and utilize advanced thermal management within the display to keep the internal temperature from getting too high (Fig. 3).  Sealed units offer the advantages of being maintenance-free and more flexible on installation locations because they do not require a supply of outside air.  Sealed units often represent a higher up-front cost because the structure of the display needs to be more robust to provide the environmental seal, but this pays off over the life of the system by avoiding many of the challenges and maintenance requirements of utilizing outside air for cooling.

Direct sunlight, especially on the face of the display, also adds a significant amount of heat to the enclosure.  With sunlight power at around 93 W/ft.2, the solar heat gain in an enclosure through the display face can often exceed the heat generated by all of the internal display and power components combined (Fig. 3).

Fig. 3:  Solar loading is one of the many environmental stresses faced by outdoor displays.

A variety of advanced films and coatings are commonly used to reduce this solar heat gain before it enters the display.  Infrared light (IR) blocking and reflecting filters can dramatically reduce the solar heat gain of an outdoor display while maintaining excellent readability.  Critical parameters of filters include the percentage of IR rejection (good filters are 98–99% blocking), optical transmissivity or percentage of visible light passing through the filter (many exceed 80% today, some over 90%), and the ratio of IR absorbed to IR reflected.  Filters that reflect IR are preferred over absorptive filters as the absorptive filters result in thermal gain at the filter surface from absorption of the infrared light energy.

Polarization

Typically, all the light emitted from an LCD panel is linearly polarized.  Standard LCDs have this light vertically polarized along the short side of the display (vertical when the display is in landscape mode).  Some specialized LCDs are horizontally polarized for optimal viewing in portrait orientation or at an arbitrary angle.  Our eyes perceive all polarizations of light, so we do not notice the polarized nature of LCD panels.

When outdoors, however, many people wear polarized sunglasses to cut down on outdoor sun glare.  These sunglasses are vertically polarized, as most sun glare comes from horizontal surfaces (like water).  If a standard linearly polarized LCD panel is mounted outdoors in portrait orientation, the light will be horizontally polarized from the LCD and will be entirely blocked by the sunglasses.  The display will appear to be black to anyone wearing polarized sunglasses.  Using special vertically polarized LCDs can mitigate this problem, but the selection of display then becomes dependent on its intended orientation (landscape or portrait) and can increase project costs and complexity.

Even standard displays mounted in landscape orientation or specialized displays with arbitrary polarization angles may suffer readability and brightness problems if people with sunglasses view the display at an angle, as shown in Fig. 4.  A viewer who tips her head by 45° will see less than 70% of the brightness of the display.

Fig. 4:  Depending on the orientation of the display, polarized sunglasses affect an LCD’s visibility in different ways (top).  Displays outfitted with circular polarizer films (bottom) are still viewable by viewers with polarized sunglasses.

Some advanced displays available today incorporate ¼-wave circular polarizing wave-plate filters, also known as wavelength retarders.  These filters marginally reduce native brightness of the display, but they convert the linearly polarized light from the LCD to circularly polarized light so that it can be easily viewed through polarized sunglasses in any orientation.

What Happens When Something Breaks?

The market opportunity for outdoor LCD signage has been recognized by many of the major display brands.  Samsung recently released a line of fully sealed, 2000–2500-nit full-HD displays in sizes ranging from 47 in. up to 72 in.  NEC and LG Display provide high-brightness systems in “open frame” formats for system integrators.

Fully factory-sealed units can make on-site repair simple – just replace the entire module with a new one.  That simplicity can be blunted, however, by the cost and bulk of shipping full display units whenever anything goes wrong inside the unit.  Fully sealed displays without adequate media players incorporated into the units can also result in complex systems with multiple sealed modules (display, media player, power source) and the need to environmentally seal multiple video and network interconnections.

Modular display systems provide a compromise between the simplicity of full-unit replacement and the complexity of component-level repair and maintenance.  These systems provide an integrated display, media player, and power supplies designed to operate together.  They are often in one enclosure and provide for modular replacement of only the elements that may fail.  This saves considerable cost and time in keeping an outdoor digital display operating.  Smaller modules can be pre-stocked and replaced when needed, and the large and bulky display panel itself is only removed and replaced if it is the element that fails.

What About the Source Video?

Most outdoor displays available today provide for a variety of video signal inputs similar to indoor displays, including HDMI, DVI, and DisplayPort.  Thus, it can be tempting to utilize the same source player hardware as for indoor units.  Unfortunately, the temperature and environmental issues that displays face, as well as security issues in unprotected outdoor spaces, make this selection much more difficult.

Most player hardware is designed for indoor use and to utilize external air for cooling.  Many systems, including compact Intel Next Unit of Computing (NUC) player PCs and industrial fan-less indoor media players, are only rated for temperatures up to 50°C.  Even high-temperature players must be mechanically attached to the displays and power, video, and network connections and secured against tampering.

Display manufacturers such as Samsung and LG include integrated player software for their branded solutions and a small subset of third-party solutions.  Other companies, such as Delphi Display Systems, include full-media-player computers in the same sealed enclosures as the display.  These integrated players have the advantage of being secure and designed as a unit for full outdoor operation.

Updating Media

Whether your media player is integrated into the display or attached to it, media updates can also be challenging outdoors.  Choices are similar to indoor environments, but the complexity and cost structure can be very different (Fig. 5).

Fig. 5:  A typical outdoor digital-signage system comprises several elements in addition to the displays themselves, including media storage, a content delivery mechanism, and content mastering.

Wi-Fi systems that are ubiquitous indoors often behave very differently outdoors, and many require specific antennas and signal boosters to ensure reliable connection.  Effects of weather and temperature extremes on the signal and antenna materials, the difficulty in routing cabling to outdoor antennas, a combination of large open spaces and sometimes significant obstructions to radio-wave propagation, and the typically far greater distances between router and display outdoors all contribute to unique challenges in implementing reliable outdoor Wi-Fi connectivity.  Wired Ethernet, easily installable into a building’s drop ceiling plenum, can be nearly impossible to add to outdoor environments.  Permits, trenching, hardscape replacement, and durability requirements of cables for outdoor use may dwarf the cost of the actual display.

Cellular updates, often too complex or costly for indoor systems, can be the most reliable choice for outdoor installations.  Cellular data systems are designed for outdoor and mobile use, optimized for the multipath reflection and fading effects of outdoor obstructions, and easily cover wide areas.  Antenna selection and positioning are concerns, but strong cellular signals and carriers that are ubiquitous in urban and suburban areas make it easy to provide content updates without requiring infrastructure modifications or a dedicated Wi-Fi infrastructure.

Outdoor Displays: A Unique Animal

Delivering the right message and ensuring compelling or informative content is a primary concern for all digital-signage systems.  When incorporating smaller outdoor displays into a digital-signage system, location, hardware, sunlight readability, durability, and reliability are key components that must be considered.  While many outdoor display options are available today, they fit very different applications and there is no single solution that is optimal for all installations.  Ignoring the environment, display longevity, and maintenance concerns can result in an outdoor signage installation that can become dim and unreadable in a few months to a few years, or fail entirely.  Understanding the different environmental and maintenance requirements for an outdoor digital sign greatly improves the designer’s chances of selecting the right display and ensuring years of compelling, reliable messaging for outdoor display systems.

Delphi Display Systems’ signage is environmentally sealed, providing a more robust and maintenance-free solution for customers.  Many of its systems also incorporate ¼-wave polarizing filters for better visibility with polarized sunglasses.  Future challenges include innovations to significantly reduce the cost of outdoor signage systems, improve outdoor display energy efficiency and power management, incorporate fine-pitch LED displays and OLED technologies into robust outdoor systems, further improve the visual clarity and contrast ratio of outdoor display solutions, and provide innovative off-grid zero-energy display systems for outdoor applications where reliable power is cost prohibitive.  •


Benjamin Medvitz is the director of hardware engineering with Delphi Display Systems. He can be reached at bmedvitz@delphidisplay.com.