Touch Technology Primer

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2 Touch Technology Primer Consumer expectations for new high-end interfaces are pushing point of transaction device manufacturers to integrate intelligence and design together, but at a cost that allows the product to be widely accepted. One of the most soughtafter interface technologies are touch screens. From the checkout lines at your favorite grocery to your next-generation smart phones, touch screen technology is fast becoming the de facto standard interface. The use for touch screens has also moved quickly to more outdoor applications, such as self-service kiosks, adding another layer of complexity in product design considerations. There are many touch technologies available, so determining which touch technology provides the most benefits for your particular outdoor application can be confusing. This white paper offers some guidelines and suggestions on the different touch technologies and how touch screens can be intelligently integrated into your outdoor product s display and interface solution. A Brief Look at the Different Touch Technologies There are many different technologies available for capturing the user s touch input. In most cases, an overlay is used over the LCD to protect the fragile top polarizer and LCD structure. This overlay adds additional surfaces that generate reflections and haze and lower the performance of the display system. Some of the sensors also have thin indium tin oxide (ITO) transparent conductive layers within their structure. These ITO layers have a very high index of refraction and also add internal reflections. In outdoor applications, these additional internal reflections will significantly reduce the quality of the displayed image, so care must be taken to manage the extra surfaces and the reflectance introduced by the touch sensor. Currently, there are five major categories of touch technology available: resistive, capacitive, surface acoustic wave (SAW), bending wave and infrared; each using different properties to deliver touch actuation. Resistive A resistive touch sensor is composed of several layers. The most important are two transparent conductive (ITO) layers separated by thin spacer dots. When any object, like a finger or stylus touches this kind of touch panel, the conductive layers contact at that touch point. The touch controller can accurately determine where the conductive layers contacted, and pass these touch coordinates on to the operating system. 2 White Electronic Designs Corporation Phoenix AZ

3 Resistive Touch A resistive touch sensor can consist of four, five and eight wire designs. Four wire designs are the lowest cost, but often require frequent recalibration. Five wire designs are more expensive, but add substantially improved durability and calibration stability. Eight wire designs are derived from four wire systems, but have the highest calibration stability and accuracy. Because of their construction, resistive touch sensors can be actuated with virtually anything. The actuator can be a bare finger or gloved hand. A conductive or non-conductive stylus as well as the corner of a credit card can be used. The actuator can be hard or soft. Resistive touch sensors are not affected by outside elements such as dust or water and are commonly used in indoor POS (point of sale) applications. Resistive touch screen panels are generally more affordable than other touch screens but typically have the lowest optical performance. Standard resistive touch screens transmit only about 75% of the display luminance, and that coupled with their high reflectance of incident light usually makes them a poor choice for outdoor applications. Enhanced resistive touch screens are now available which incorporate a circular polarizer light trap or custom optical coatings to significantly reduce the sensor s reflectance. This provides more satisfying performance in outdoor applications. These sensors do come at a higher cost, and have some other drawbacks so it is wise to consider other technologies before selecting an enhanced resistive screen. Surface Capacitive A surface capacitive touch sensor is coated with a conductive material, typically indium tin oxide (ITO). This produces a continuous electric field across the touch sensor. When the sensor s normal capacitance field (its reference state) is altered by another capacitance field, i.e., someone s finger, electronic circuits on the sensor measure the resultant distortion of the reference field and send the information about the event to the controller for mathematical processing. 3 White Electronic Designs Corporation Phoenix AZ

4 Capacitive Touch Capacitive sensors can either be touched with a bare finger or with a conductive device being held by a bare hand. As the touch sensor is laminated with glass on the front (up to a thickness of 25mm), the touch screen will still operate even when the glass is scratched or damaged. Additionally, the glass touch sensor typically has a very hard surface, which is quite durable. Strengthened glass can also be used to increase the sensor s resistance to shock and impact. Additionally, surface capacitive sensors can have high optical clarity, although the total reflectance of the display system must still be managed carefully. This is because the ITO layer will introduce reflectance, reducing the viewability of the display system in bright environments. Many surface capacitive sensors incorporate a back shield to reduce EMI noise from the display and improve touch performance. This back shield will further degrade the optical performance, particularly in outdoor applications. Projected Capacitive Projected capacitive touch sensors use a grid of transparent conductors or fine wires. A capacitive field is projected from this grid, sensing a bare or gloved finger that enters into the field. With this approach, a touch event can be triggered with very light actuation force, or even while hovering above the screen. This technology can also register more than one touch event at a time (i.e.: multitouch). The conductive grid is generally unobtrusive, but can be seen at some viewing angles. Since the bulk of the sensor is bare glass, the optical performance can be very good. This technology can be tricky to integrate however, and if not properly packaged into the display system calibration and stability issues will ensue. 4 White Electronic Designs Corporation Phoenix AZ

5 Surface Acoustic Wave (SAW) Surface Acoustic Wave technology uses ultrasonic waves that pass over the touch sensor. When the sensor is touched, a portion of the acoustic energy is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. SAW touch sensors can be used with gloves on, but can be affected adversely by the outside elements (rain, snow & etc.). Contaminants on the surface can also interfere with the functionality of the touch screen, making them less suitable for applications where exposure to debris or use with dirty fingers or gloves is the norm. SAW is also somewhat difficult to seal effectively so it generally is not recommended for NEMA4/IP 65 (or higher) applications. The optical performance of SAW sensors is excellent, as only glass is between the viewer and the display. Transmission can be 92% or higher, with excellent clarity and low reflectance. This low reflectance and high tranmissivity enable good performance in high brightness environments, even direct sunlight. SAW is fairly easy to integrate into a display system, but care must still be taken in the design and assembly of the system to provide reliable operation and high yield in manufacturing. SAW touch screens typically are available for display sizes from 10.4" to 32". Infrared An infrared touch sensor employs one of two very different methodologies. One method uses thermally induced changes of the surface resistance. This method is sometimes slow and requires warm hands. More commonly, arrays of vertical and horizontal sensors are used that detect the interruption of a modulated, invisible infrared light beam near the surface of the screen. IR touch sensors can have very durable surfaces and are used in many defense and aerospace applications that require a touch panel display. IR touch screens can be affected by outside heat sources swamping the IR sensors, and often do not have enough fine resolution to permit the use of small touch targets. IR touch screens typically employ a construction that houses the sensors and electronics in a wide frame that fixes over the display bezel. Often this frame is larger than the host LCD bezel, so this technology can require more physical space than desired in the final system design. One good advantage of IR frames is that they can be used on large display systems (>30") and still provide very good performance at a reasonable cost. The optical performance of IR touch sensors is excellent since no overlay is typically required. However, it is usually unwise to leave the polarizer on the LCD 5 White Electronic Designs Corporation Phoenix AZ

6 exposed since it can be easily damaged, so with IR sensors a protective front filter is often used. The selection and design of this front filter is not as trivial as it might seem at first, and should be done carefully to complement the display system and application requirements. Bending Wave A bending wave touch sensors are relative new technologies that effectively listen for a touch event on the display through acoustics. Piezo transducers are located in the corners of the transparent glass substrate, listening for the touch events. This technology is called Dispersive Signal Technology (DST) from 3M Touch Systems and Acoustic Pulse Recognition (APR) from Elo Touchsystems. Much like infrared and SAW sensors, bending wave touch sensors have excellent optical performance. They are scratch resistant and will function when the front surface is dirty or scratched. Like the resistive touch sensors, almost anything can be used to actuate the touch, provided that sufficient force is used. At this time both of these technologies are new, proprietary, and currently being rolled out in only a limited number of sizes. Both technologies command a price premium and while very promising, do have integration issues that prevent their use by inexperienced touch system designers, so they are not offered currently as standard alone touch sensors for integration by OEMs. So Which Is Right For What Application? The touch sensor interface is an increasingly popular manner of interacting with many electronic devices. Commodity touch sensors (like resistive) are generally not acceptable for applications requiring greater optical performance, higher reliability or operation in harsh environments. Because the touch sensor is typically the outer surface, it must be optimized to support the overall optical, environmental and mechanical performance requirements. Unfortunately, no one touch technology is right for every application. Therefore, it is important to work with a partner that understands the trade-offs associated with each touch technology. There are trade-offs in touch capability, optical characteristics, environmental performance, reliability, mechanical size, lead-time, manufacturability, maintenance, and price. The varied nature of applications and their requirements must be considered when integrating touch into different environmental conditions. Figure 1 is a matrix that compares different operational and application requirements with the available touch technologies and methodologies. This matrix can be used a guide as you are comparing your design and operational requirements that include the integration of touch technology. 6 White Electronic Designs Corporation Phoenix AZ

7 Figure 1 4 Wire Resistive 5 Wire Resistive Surface Capacitive Projected Capacitive Infrared Surface Acoustic Wave Acoustic Pulse Recognition Optical Performance Haze / Clarity Transmission Reflectance Color Purity Touch Performance Speed Best Good Fair Poor Resolution Multi-Touch Drag Palm Rejection Calibration / Stability Immunity to Contaminants Input Performance Signature Capture Gloved Hand Pen / Credit Card Edge Handwriting Recognition Environmental Performance Vandal Resistance Durability Calibration / Stability Chemical Resistance Shock / Vibe Impact Resistance Altitude Scratch Resistance Dust / Water (NEMA 4 / IP65) Mechanical Inactive Area (edge size) Edge Sealing Small size displays < 8.4 Medium size displays " Large displays 20"- 60" Cost with Controller 7 White Electronic Designs Corporation Phoenix AZ

8 Touch Screens in Outdoor Applications Many factors converge when attempting to create an integrated touch display solution that is legible in conditions of high ambient illumination, which can be either direct sunlight or bright interior lighting. One of the first aspects to consider is where and how the display will be mounted and oriented with respect to the illumination source. The amount and angle of the ambient light that strikes the display are key factors that must be considered while developing the touch panel display solution. Figure 2 Surface Reflections Touch Sensor Membrane Top ITO Bottom ITO Touch Sensor Substrate Air Gap or Max-Vu Optical Bond Front Polarizer LCD Panel Display Reflectance and Contrast Ratio In high ambient conditions the light striking the display surface degrades the perceived contrast ratio of the displayed image, because as the ambient illumination falls on the display it is reflected by the display surface, the display s internal layers, and from the surrounding area. An AMLCD itself typically has a reflectance from 4% to 6%. If other optical elements are installed over the display (such as a resistive touch switch or protective window) the total reflected light can easily climb above 20%. When there is a high amount of ambient light from the environment, the light output from the display backlight is quickly overpowered by the reflected light and the displayed image is obscured. To overcome high ambient conditions it is just not as simple as adding a more powerful backlight. While adding a more powerful backlight can certainly help to overcome high brightness environments, it also adds a good deal of complexity, cost and thermal load to the display system. A more effective and desirable approach is to design a system that first reduces the reflected light as much a possible, and then has sufficient backlight luminance to deliver an image that is clearly legible but does not exceed the target product cost or thermal budget. In this approach the integrated touch display can be much simpler and more cost effective, providing a touch panel solution that is truly satisfying. 8 White Electronic Designs Corporation Phoenix AZ

9 Passive Enhancements After analyzing the application requirements it is frequently found that applying passive optical enhancements will reduce reflections and enhance the system s performance to a level that is very satisfactory. Passive enhancements used are advanced optical films, optical filters, and optical bonding. White Electronic Designs utilizes passive enhancements such as our Max-Vu technology to arrive at an outdoor touch panel display solution that is simple, cost-effective and robust. Max-Vu With Max-Vu Without Max-Vu Max-Vu is our proprietary optical bonding process that attaches a filter (glass or synthetic) or touch sensor to the front of the display with an optically matched adhesive. This reduces internal reflections and ensures that the gap between the display and filter cannot degrade due to fogging, or accumulate dust and debris. This process ensures that the image leaving the integrated touch display will exit the front filter with the minimum loss and much higher effective contrast ratio throughout the product s lifetime. Additional benefits to the Max-Vu optical bonding process are improved thermal coupling and impact resistance. With Max-Vu it is also possible to bond vandal resistant, anti-reflective filters in conjunction with appropriate touch sensors. We can enhance standard resistive touch panels by optically bonding this sensor onto the display, index matching the top of the LCD and the bottom of the touch sensor and eliminating two optical interfaces. We can also coat the top of the sensor with an anti-reflective (AR) coating or index-matching (IM) film, substantially reducing the reflectance off of the top surface of the sensor. Such an enhanced Max-Vu With Max-Vu Without Max-Vu 9 White Electronic Designs Corporation Phoenix AZ

10 display system would cut in half the reflectance of the display system and substantially increase the backlight transmission (brightness), arriving at a very cost competitive solution for many applications. Solutions for Your Outdoor Touch Screen Applications White Electronic Designs works with first class materials and technology to develop touch systems that meet the stringent requirements of demanding applications. We can offer quick turn design that ensures that the AMLCD enhanced with a touch sensor will be optimized for your application. We understand that the goal is to deliver an integrated touch screen display system that provides bright, compelling images at a very competitive price E. University Dr Phoenix, AZ White Electronic Designs Corporation Phoenix AZ Touch Technology Primer 01/08 Rev. 1 WP0004