Light-Field Imaging Approaches Commercial Viability
Elemental Evolution of Digital-Signage Components
The strong market growth of digital signage over the past 10 years will be outshone by the expanded performance and applications made possible through
recent display and system advances.
by Gary Feather
DIGITAL SIGNAGE can be defined as a wide range of unique out-of-home experiences created with large-area display systems. These public displays can be used separately or tiled together to enable merged images, supporting any size and any resolution by extension. Digital-signage systems are generally commercial/industrial in nature and include both indoor and outdoor varieties. Large heterogeneous audiences experience these displays in many different venues for many different purposes (Fig. 1).
Fig. 1: LED digital signage is used to enhance the environment outside the elevators of the World Trade Center Freedom Tower.
The digital-signage market has grown impressively in recent years. According to a recent report from research firm IHS, shipments of public displays 26 in. or larger are forecast to approach 3 million units in 2015 and close to 4.5 million units in 2019. (For more discussion of the digital-signage market, see the article “New Directions for Digital Signage” in this issue.) While there are a number of technologies that have contributed to this growth, this article will focus on those technologies that are contributing to the evolution and future of direct-view digital signage in particular. (For the purposes of this article, “direct view” refers to digital signage that is not based on projection technology.)
As mentioned previously, the market for digital signage has been demonstrating profound growth, as the desires of companies to place digital imagery in front of a public audience grows on a global basis. Also, the exponential growth of and increased availability of high-quality HD content has created a need for more displays, larger displays, and displays with increased resolution. Segmentation in this market includes solutions for permanent digital-signage installations supporting sports (professional to high school), corporate (internal and public), transportation (airports, trains), casino gaming (statistics and entertainment), retail (advertising and promotion), and government and municipalities.
Group entertainment is another growth segment, as digital-signage image-quality performance now competes to replace or substitute for DLP and LCOS projection in digital-cinema applications. Over 1500 sq. ft. of three existing entertainment projection displays were replaced by Station Casinos at Red Rock Resort in Las Vegas, Nevada, this year with fine-pitch LED displays. Last year, Telstra installed a new LED HD theater in Australia. This wide range of applications (Fig. 2) requires a correspondingly wide range of technology advances and innovations in order to ensure competitiveness and ROI to the owner of the digital sign.
Fig. 2: Digital-signage solutions include, from top to bottom: a Holt Renfrew LED flat window display, an Estee Lauder LED and transparent display in Detroit, and an LED sign at a mall in Australia.
Key Trend: The Transition from Static Signage to Video Displays
One key requirement across all but street/ highway-level outdoor signage is the trend toward increasing image quality and video performance. Digital-signage application focus was initially on bright images promoting products and services. These were designed to capture many views and, subsequently, mindshare. Current displays have clear requirements for image quality, true 8-bit color, consistent color points, linear gray scale, higher contrast ratio, HD and UHD resolution (depending on size), and uniformity of image with no visible seams. Even a still image has increased viewer impact if the visual quality is more lifelike. And a large percentage of application spaces for digital signage are now showing video (except for roadside signs). All the market segments highlighted earlier require improved full-frame video quality. While video was initially a significant leap for the industry, this transition is well under way, as digital signs are providing compelling video in all market segments.
In the smallest sizes, early LCD signage has already met or exceeded the video and resolution requirements. LCD-backlight modifications have been able to support the brightness requirements. Tiling and uniformity over large areas are still challenges. The requirements for LED signage indoors and outdoors continue to become more demanding (Table 1).
Table 1: These requirements for outdoor and indoor LEDs will no doubt continue to increase.
||Delivered to Viewer
|Lifetimes to 70% bright
||Driven by LED/BLU
Digital-Signage Development Progress
Three major categories exist for the display modules that make up large direct-view digital signage:
• LCD flat panels (single panel or multiple panels tiled with seams) adapted for commercial/industrial environments.
• Custom-tiled surface-mounted-device (SMD) RGB LED displays; curved and flat displays (indoor and outdoor). Each tile (~0.5 ft.2 in area) is designed to create a seamless display product.
• Flat, seamless, and tiled module arrays of discrete LEDs (one for each color in a pixel) for outdoor usage.
The use of LCD flat panels takes advantage of economies of scale. While a customized ruggedized design is generally required by the digital-signage supplier, the provider is often able to use the flat-panel products already available on the market. Flat panels also have significant advantages in providing the greatest dpi density with existing panel manufacturing (Fig. 3). LEDs, for their part, provide fully adaptable pitch, resolution, and size. The past and current use of the LEDs (discrete and SMD) allows designs that can meet a customer’s exact requirements. However, every separate tiled LED display module must be custom designed and then built by manufacturers to satisfy the full seamless display solution. For all applications, a display system design can be either front or rear installable and serviceable (Table 2).
Fig. 3: Three major categories for display modules include discrete LEDs, SMD RGB LEDs, and LCD flat panels.
Table 2: Digital-signage panel types are listed along with some of the applications for which they are most often used.
||LCD Single Indoor
||LCD Tiled Indoor
||LCD Single Outdoor
||LED Outdoor Discrete
||LED SMD Outdoor Flat and Curved
||LED Indoor Flat and Curved
||POS, Retail Advertising, Food menus, Electronic directories
||TV Production Studios, Trade Shows, Corporate Lobbies
||Drive through window, Direction finding
||Large Roadside Billboards, Stadium scoreboards, Advertising
||Open Mezzanine areas, Video Entertainment
||Malls, Airports, Casinos, Corporate Corporate, Retail, Sports Stadium
Flat-panel solutions for large-area digital signage have been provided by both plasma and LCD for many decades. Emissive plasma panels showed great promise in the 1990–2010 time frame, as the size and the luminance (in nits) allowed for the largest single display solutions for indoors and outdoors. Plasma’s limited lifespan compared to that of LCD was a significant deficiency. With the monotonic technology and performance increases of LCD panels and the improvements in luminance and ruggedization, the LCD became a real competitor to plasma 5 years ago. From 2010 to today, the LCD has surpassed the plasma panel in a number of areas in both the consumer as well as the commercial market spaces. Those areas include efficacy, resolution cost, and lifetime. As a result, plasma panels as a digital-signage solution have been displaced.
The LCD panel is used in both indoor and outdoor markets. For the outdoor market in a single-panel system, the manufacturer can choose almost any size display that is currently in standard production. Current products support common single displays as large as 90 in., and high-brightness applications >6000 nits for outdoor to indoor common luminance solutions <1000 nits. These implementations can support power and cooling to assure luminance and resolution in keeping with the specified environmental application requirements. The designs are rugged for outdoor and interactive spaces. The methods and processes for meeting the environmental requirements have matured so that high-brightness, reliable, readable, and ruggedized LCDs can be deployed effectively.
For larger-area displays, the large LCD panels can be tiled. In this implementation, the single-panel requirements must be satisfied in addition to the LCD tile-element alignment, calibration, and implementation. Tiled LCDs support larger areas by incorporating an array of displays carefully aligned and calibrated to support a full-screen image. The results are effective, but the image tiling “lines” can be easily seen and are not acceptable in applications in which the customer is looking for a continuous (no lines/seamless) display solution. Current LCD technical approaches do not currently provide for a tiled seamless
display, due to both the core pixel layout and the difficulty in expanding the image portion to the very edge of the panel. If visible lines are acceptable, tiled LCDs can create very acceptable large-area displays.
Power requirements are a function of the total luminance requirements, but in general the LCD offers ~5 cd/W at 400 nits and half that at the highest luminance. These compelling HD and 4K displays satisfy nearly all resolution (and dpi) requirements. Resolution capabilities are usually greater than the viewing-distance
requirements, resulting in no limitations or design changes. While the panel lifetimes and quality for application are robust in basic performance, the implementation can have a major impact in limiting the lifetime and the quality of the display. For indoor displays, temperature variations across the display can create visual inconsistencies. For outdoor displays, dirt/dust, high temperatures, and UV from sunlight can cause life-limiting effects to the display.
Elemental Technology Drivers For Discrete and SMD LED Displays
The core building blocks for each of these digital-signage solutions have enabled the evolution of the market. Existing unsatisfied needs appear largely from new markets, enabling advantages for those companies with innovative solutions and leading to new market growth. Each element will be discussed to identify the changes over the past that are leading to future opportunities for both integrated flat panels as well as emissive discrete display solutions based on LEDs. These building blocks are:
• Display Elements and Pixels (size, luminance, design)
• Pixel Pitch (pixel-to-pixel spacing and characteristics for effective seamless tiling)
• Mechanical (weight lbs./ft.2, thickness)
• Power (W/ft.2) and Efficacy (nits/W)
• Resolution/Size Capabilities
The discrete outdoor LED package (Fig. 4) in red, green, or blue allows solutions from 50- to ~10-mm pixel pitch in the outdoors, which is now standard. However, closer viewing in outdoor situations demands a smaller pixel pitch. By employing SMD RGB technology, outdoor displays are now being installed at 8- and 6-mm pixel pitches in flat formats and in one Miami venue, at over 1000 ft.2 in a 4-mm pixel pitch on a curved display. The discrete pixel underpins the ability to leverage pixel patterns for better display imagery. Wide spacing and low fill factors provide a number of solutions for tiling and seamlessly creating nearly any size and any resolution. Maturation of the outdoor discrete LED has made possible solutions with wide color gamut and robust mechanical electrical assembly.
Fig. 4: Discrete red, green, and blue LEDs are deployed in an outdoor panel.
The discrete LED as a percentage of all outdoor usage will likely decline as more outdoor panels transition to tighter pixel pitches at 10 mm and lower. Outdoor SMD LED packages have an RGB in each SMD package. Sizes range from 4.5 to ~2.8 mm on a side. These packages are robust to provide package integrity even in the worst environments.
LED solutions for the indoor markets are generally those in which the user is much closer to the display. The technology uses surface-mounted devices, typically with a tighter pitch than 10 mm (per RGB SMD LED). To achieve this, the SMD LED assembly is packaged with one each of a single red, green, and blue LED together. The package size is a function of the die size and the thermal performance as well as the pitch requirement on the SMD-to-SMD spacing. SMD indoor packages are rectangular, with the largest geometries ~3.5 mm on a side. Solutions also exist at 3.0, 2.1, 1.5, and 1.0 mm with 0.8 and 0.6 mm emerging for the finest pixel pitches (1 and 0.9 mm).
Pixel Pitch, Resolution, and Size
LED digital-signage pixel pitches are selected to optimize the expected viewing distance with the greatest pixel pitch to create the optimal image. How much detail you can effectively perceive depends not on how many pixels there are (each with an RGB), but on the pixel density of display (pixels per inch), your visual acuity, and how far away you are from the screen. In addition, there needs to be a sufficient number of pixels on the smallest character or icons so that viewers can discern the character fonts and minimal lines in the input image for the various viewing distances. Factoring in the characteristics of the human eye (limited to no more than 1/60th of one degree), the standard display content and the operating environments, minimal viewing distances can be calculated. In general, the minimal viewing distances to mitigate pixilation in the image for various pixel pitches are ~1000 times the mm pixel spacing. Therefore, a 3-m pixel pitch is nominally best viewed at no closer than a viewing distance of ~3 m. Typical indoor displays will range up to a 10-mm pitch.
The LED indoor and outdoor digital-signage displays have already demonstrated dramatic capabilities in both resolution and total size (m2 of display). Indoor displays of 150 m2 and resolutions of over 11 million pixels have recently been installed in the US. As pixel pitch tightens, higher resolutions will be achieved in a number of emerging projects.
In outdoor LED displays, the development emphasis is currently on reducing weight that is caused by the large sizes and the sign structure. Outdoor displays must prioritize and apply a disproportionate amount of the display system’s weight to satisfy the environmental requirements and constraints of outdoor events such as wind, rain, extreme temperatures, etc. LED displays for indoor applications do not experience these challenges and are being continuously improved in size, weight, and depth. Typical industrial single-sided displays are currently in the 100-mm thickness range with all power and mounting and wiring included. Double-sided displays are usually two times thicker. Display weight is a major factor for competitiveness. Current indoor LED displays range from about 25–30 kg/m2. Outdoor displays require significantly more construction support for wind and weather and can range from 45 to 60 kg/m2. There exists significant headroom for future reductions in kg/m2 in both indoor and outdoor varieties. The use of polycarbonate materials for frames, lighter-weight power supplies, and thinner designs can reduce the mass.
Power and Efficacy
Total power consumed by an LED digital sign is a direct result of luminance, display contrast ratio, optical efficacy, pitch, power system, and image calibration losses. White levels are set at D65 (6500K) for consistency across various types of display viewing. Industry efficacy for power input to full-white light output (full end to end) has a wide range across the industry, based upon specific performance requirements. One example is a 6-mm pitch with the nominal contrast ratio at ~7 cd/W. High-contrast 4-mm displays can be at ~4 cd/W and fine-pitch 2-mm displays at ~2.5 cd/W. Future emphasis in this area is for increasing luminous efficacy. Solutions here relate to different distributed power and managing the drive circuits to minimize image off-state power.
LED displays, when assembled in adherence with application notes and operated in a fashion consistent with the manufacturer’s recommendations, have a lifetime equal to that of the LED. Common LED lifetimes are often specified with a 70% brightness number of 100,000 hours. Derating operating points will exceed that nominal lifetime. Most LED manufacturers follow JEITA (Japan Electronics and Information Technology Industries Association) and JEDEC (Joint Electron Device Engineering Council) standards for accelerated life testing and evaluation to predict relative lifetime advantages of new LED designs. Maintaining a low ambient temperature by properly managing the thermal system design can provide extended lifetimes. In addition, meeting rigorous requirements in die, package, mounting, bonding, sealing, and testing will assure that a device meets its quality requirements and will not be degraded by the environment.
5-Year Vision for Digital Signage
Large-area digital signage for LEDs is entering into a faster adoption and faster performance improvement rate than seen in the last 5 years. Improvements in each area are helping expand the market through new and improved offerings. LCDs have already met most performance requirements. Here is the future direction for these technologies:
• LCD Panels:
The application of LCD panels in digital signage will remain strong based on the technology’s firm foundation in the greater LCD panel infrastructure. Solutions are currently fully compliant and the innovation lies in increased reliability and potentially lower cost. More automated calibration will be a benefit to performance over time. Outdoor LCDs (high luminance) need to continue to improve in the areas of reliability and performance to meet the exacting requirements of harsher outdoor environments.
• LED Displays
Outdoor: The revolution in technology (increasing frame rates and bit depth) will focus on non-roadside applications. Outdoor LED-based displays are being used in more open public spaces at close proximity to the audience. As a result, expectations are for the current SMD 10-mm pixel-pitch category to grow rapidly, but, in parallel, for a much faster market growth in 8, 6, and 4 mm. Should the average public-space viewing distances decrease below 3 m, we can expect the pitches to go even smaller.
Shape and size are also differentiators. While in the past, the largest percentage of outdoor displays were rectilinear, expectations would be for concave and convex designs to be used in an increasing number of applications. Large-area seamless continuous displays will still require the same strict tiling requirements for uniformity; smaller pitches and curved will be addressed with successful approaches.
Success in improving efficacy will also continue. Current compelling efficacy numbers for outdoor installations will improve by 30% in the next 2 years and 50% in the next 5 years. Emphasis on the efficacy of the smaller outdoor pixel pitches has the greatest impact to market growth. The current size capabilities of outdoor displays are almost overwhelming, and few improvements are needed to maintain that size. Longer life and increased MTBF will continue at a measured pace, as most market requirements are being met. Improvements to the LED efficacy will impact the greatest percentage of gain.
Indoor: Indoor LED RGB SMD digital signage will show the greatest percentage of revenue growth of all categories in the next 5 years. Current discrete LED packages have already provided for prototype solutions that can be implemented down to 1-mm pitch. The RGB-packaged SMD LED innovations are a solid growth engine for indoor digital signage. Emerging solutions in LED microelectronic packaging and LED arrays in a module will continue to push growth in the finer pitch (<1.9 mm) categories and to support moving to submillimeter pitch (0.9 mm).
In addition, those making finer pitch (<1.9 mm) displays can now begin to consider organic devices such as OLEDs and other integrated solutions over discrete RGB LEDs. Organic devices will need to minimally maintain the best-of-class achieved efficacy and the luminous flux (cd/m2) while driving down manufacturing times and technology limiting inorganic LED costs. The challenges for digital signage with OLEDs are different from those of television. In addition, there are unique innovations in the interfaces and edges that will allow robust seamless solutions with consistent dimensional requirements approaching control to less than 100 µm.
An Ocean of Opportunity
Digital-signage growth remains significant for the foreseeable future. Increasing market demand, which is being met with an aggressive push to develop new products, are enabling improvements in all current technologies, especially those that are LED based. Many new applications are emerging based on these new innovations. The ever-widening availability of content is also driving the need for more displays. Digital-signage innovation is a blue ocean of opportunities in each segment, and many developers can contribute. •
Gary Feather is Chief Technology Officer at NanoLumens. He can be reached at GFeather@nanolumens.com.