Digital displays are everywhere, from consumer electronics and industrial terminals to retail kiosks and digital signage. The falling cost of TFT-LCDs and other displays have enabled many innovative applications, but designers are pushing beyond the restrictions of flat, 4:3 or 16:9 rectangular form factors. New technologies permitting custom display sizes, curved surfaces and even flexible displays are opening new creative avenues. Taking the next step by adding interactive capabilities, however, is more problematic due to limitations which exist with most touchscreen technologies.
Flexible Display Media
Rear Projection Film (RPF) is an optically coated polymer sheet, which is used in combination with a digital projector to replicate the image projected onto the ‘pixels’ of the coating. Varieties of digital projectors are available, and may be used in conjunction with a local or networked computer to control the delivery of content to a single RPF display or to a large number of displays. These can be installed throughout a local, regional or even national chain of stores, for example, to present common or individually tailored content. The content can also be changed to present offers and promotions at the most appropriate times during the day.
Among the practical advantages of rear projection, there is no shadowing of the image if individuals or a group gather in front of the screen. RPF is capable of delivering high image quality and accurate color reproduction and has been proven in a number of projects including merchandising displays and street-level advertising. Imaginative uses have included guitar-shaped screens presenting rock videos in entertainment venues, and in some cases, entire storefronts have been converted to giant video screens for special promotions.
Designers of digital signage see RPF as an attractive alternative to using a standard-sized TFT-LCD or Plasma Display Panel (PDP) in retail window applications. These types of displays have become popular with real-estate, fashion and financial service businesses, as they allow promotional information to be communicated to passing customers at any time of day or night. However, the displays are heavy and usually require a space consuming, strong mounting to hold the screen in position at suitable height for public viewing. Furthermore, ultra large panels remain prohibitively expensive for all but the most affluent retailer. In contrast, lightweight RPFs can be adhered directly to the inside of the window and fixing a small digital projector behind the film is relatively easy and inexpensive. The projector is usually attached at ceiling level; hence it occupies no floor space and the use of the latest wide angle, short focal length projectors mean that it is unlikely that the image will be obscured if people walk behind the screen inside the store.
Moreover, the flexible RPF can be rolled for dispatch by the supplier, and packaged in a cardboard tube to be carried at low-cost by an ordinary courier or postal service. This reduces the cost of transportation and also avoids the ever present risk of breakage which occurs when moving a large LCD or PDP.
In addition to enabling graphic designers to cost effectively deploy a large, customized form factor, RPF also provides the freedom to move beyond a one dimensional display. One example is the Legacy Gallery presentation at the University of Pittsburgh, which comprises cylindrical pillars each eight feet in diameter and 10 feet tall. The pillars feature several digital displays presenting illustrated profiles describing the history of the university (see figure 1). Using RPF allowed the designers to wrap almost the entire circumference of each pillar with images, whilst maintaining a design sympathetic to the architectural style of the historic hall in which the units are permanently installed. By use of the appropriate projector technology, the images are maintained in the correct proportions, compensating for the curvature of the screens.
Emerging Display Technology
The emerging Organic LED (OLED) technology also provides exciting opportunities for the display users. Monochrome or full-color displays can be made by printing organic materials onto a substrate. Directing an electric current to the pixels causes light emission through the process of electro luminescence – and unlike conventional LCDs, without need of a backlight.
OLEDs are capable of being produced in high volumes and in increasingly large sizes. Unlike an LCD, no filter is required for color correction, which combined with the absence of backlighting, contributes to a lighter, thinner overall display. The technology and production processes are also not limited to rigid substrate materials, and therefore developers are excited about the opportunity to create flexible OLED displays.
Compared with established display technologies, OLEDs can offer faster response times, lower power consumption, wider viewing angles and brighter, better colors with higher contrast ratios. Product designers are already taking advantage of the extremely low profile of OLED displays in numerous miniaturized consumer products such as cell phones, personal media players and portable televisions.
As OLED technology begins to mature, and costs start to fall, designers will enjoy extra freedom to add small displays to products such as home appliances or industrial equipment. Unlike a small LCD, the OLED will require no aperture for mounting, and may be simply adhered to a flat or curved surface. New products in development include foldable, daily-refreshable electronic newspapers, wearable displays and walls or partitions that double as computer screens.
The Next Step: Interactivity
The natural next step for these enhanced display technologies is to add interactive capabilities allowing the system behind the display to detect and record users’ responses to the information displayed. This would be particularly beneficial in applications such as retail displays, allowing customers to immediately request information or place orders via the screen at the point of sale. Where the screen is located behind the store window, this could allow store owners to maximize the sales performance by engaging with passers-by and allowing customers to browse, interact and even order outside the store’s opening hours.
Adding a touchscreen to a display is an established and understood method of turning a one-way image into a two-way experience. Many types of equipment already use touchscreens to promote speed and ease of use, and to invite customer interaction. Examples include self-service kiosks, ticketing terminals and industrial computers, as well as consumer products such as desktop and laptop PCs and smart phones.
There are several well known touchscreen technologies. Among these, multi-wire resistive overlays comprise of two conductive layers separated by an air gap and rely on the user’s physical touch to push the membranes into contact, completing a circuit and allowing the touchscreen controller to calculate position. Conventional surface capacitive sensing also employs an extremely thin ITO conductive coating on the entire front surface of the screen. The coating is energized and uses the change in current drawn to transducers mounted in the corners of the screen to triangulate the touch point. Other widely used touchscreen technologies include infrared (IR) and Surface Acoustic Wave (SAW) types. These technologies use transmitters and receivers positioned around the edge of the screen to set up a light beam or acoustic wave across the screen surface, and are able to calculate a touch point by analyzing disturbances in the field.
Each touchscreen technology has its own set of strengths and weaknesses in relation to a given application. Resistive touchscreens, for example, are produced in large numbers at relatively low cost and have been widely used in personal portable electronic devices such as PDAs that are usually operated in benign environments by the owner. However, the overlay has a finite lifetime and can become unreliable or unusable if worn or scratched. To choose the optimum touchscreen, designers must always consider aspects of the operating environment and application such as weather conditions, moisture or dirt and dust, the required lifetime, ambient light levels, and the potential for accidental or deliberate damage by the intended users. New opportunities and applications expected to be created by flexible displays such as RPF and OLEDs will add further criteria that touch system designers should consider.
Barriers to Flexible Touchscreens
Most touch sensing technologies, including resistive or capacitive overlays, or IR/SAW screens, require a frame or bulky bezel to encapsulate and house the necessary edge mounted electronics required to detect a front surface touch. Designers wishing to take advantage of the form factor and flexibility benefits offered by RPF or OLED technologies must carefully consider whether this limitation is consistent with the overall design aesthetic required. As made fashionable by the iPhone, users and designers alike now seem to prefer a sleek, frameless, smooth-fronted design to displays, whether touch interactive or not.
In addition to the bezel consideration, it is also notable that touch technologies reliant upon front-face touch detection which can be used successfully with rigid one-dimensional screens, may not be compatible with curved display designs.
Adding touch interactivity to a retail window display presents an even greater challenge, not least through the exposure of the touch sensor and its ancillary electronics to weather, dirt and damage. Therefore, to achieve a satisfactory solution in this application, a touch technology capable of sensing touch through the window is desirable, thereby allowing the touchscreen and its components to be positioned safely inside the store.
Projected Capacitive Technology (PCT)
The principle of projected capacitive touch detection is well established and is now becoming widely used in modern, personal electronic devices such as smart phones and notebook computers. PCT is a unique type of projected capacitive touch sensing. It was originally developed in the late 1990s to answer the demand for touch sensing in the harshest environments and with larger displays, such as those found in outdoor ATMs, industrial computing, and interactive public terminals for ticketing and information display. Unlike resistive, IR and surface-capacitive sensors, the PCT sensor has no front-face-active components. Instead, an overlapping array of 10-micron-diameter copper capacitors is deposited as an XY grid. At approximately one quarter the diameter of a human hair, the tracks are near invisible and have no impact on light transmitted from a powered display.
Figure 2 shows an enlarged cross section of this sensing array, which generates a capacitive field sensitive enough to detect touch through up to 20mm of glass. The connections to the grid are terminated to one edge of the touchscreen, and then attached to an IC-based controller running proprietary firmware which is designed to scan each micro-fine capacitor to identify minute changes in capacitance caused by the proximity of a finger. Even changes caused by a gloved finger may be detected. By increasing the number of capacitors, touchscreens can be built to cater for screen sizes ranging from 5 inches up to 82 inches, in an almost limitless variety of form factors, and without the need for special tooling or masks.
In touchscreens intended for use with conventional flat fronted TFT- LCDs or similar displays, the sensing array is embedded within a laminated panel. The outer layer is usually glass, but can also be polycarbonate or acrylic. Depending on the application, anti-glare, anti-reflective or strengthened glass treatments may be specified. Because the sensing array is located behind the front panel, it is also protected against hazards such as scratching or chemical spills, and may be sealed to a high level such as required to pass IP67 or NEMA 4x enclosure standards.
These proven strengths of PCT have enabled the benefits of touchscreen interactivity to reach a wide range of applications previously deemed impractical for touch interactivity. These include public environments, demanding industrial areas, sterile environments, and outdoor, all weather locations, such as petrol station forecourt payment terminals.
With its ability to be constructed using a variety of rigid or flexible substrates in a wide range of sizes, PCT also now provides designers with the means to add interactivity with RPF. It is also expected to address some anticipated applications for OLED displays.
The Legacy Gallery at Pittsburgh University described earlier is, in fact, one of the first applications to demonstrate how PCT can be used to add interactive features to a large curved display. The individual curved display panels making up each pillar have a toughened glass outer surface containing an embedded PCT sensor, to which the RPF is applied. With a number of projectors mounted within each pillar, bright, clear and interactive images allow visitors to the Legacy Gallery to navigate information containing over 1700 images that recount Pittsburgh University’s 220-year history.
Touch for a Flexible Future
To solve the highly specific challenges associated with adding touch to RPF through-window displays in retail applications, flexible PCT sensors have been developed and combined with RPF to create a through-glass touchscreen display as a single flexible foil. An adhesive front face means it can be quickly and easily applied to the inside of a shop window. Figure 3 shows the ZYPROFILM flexible PCT/RPF laminate, which has a flexible tail providing the connection to the touchscreen controller. Power to the sensor and the necessary linkage to a computer generating the images and software content are managed by a USB or Serial connection. Figure 4 shows how the digital projector and combined RPF/PCT laminate interact to present a large, touch-sensitive, computer-generated display.
The techniques developed to create this type of interactive rear projection foil may also be applicable to OLED displays and may be used in the future to deliver further innovation in user-interface designs. It can be envisaged that a PCT foil may be combined with a flexible OLED to create a rollable or curved interactive display, such as an electronic newspaper. With designers able to move from concept to reality with minimal compromise, users of the next-generation of interactive electronic displays have a lot to look forward to.