Adding the Right Touch
LED-based lighting technology continues to transform the appliance industry as designers take advantage of the new opportunities it offers for brightness, color, sensing and control. Another trend among today’s designers is the increasing incorporation of touch technology into the displays and front panel graphic interfaces of kitchen appliances, printers, laptop PCs, home entertainment devices and an array of other products to enhance their functionality, ease of use, aesthetic appeal and brand recognition. Touch technology will extend the useful lifetime of control interfaces versus traditional mechanical buttons, switches and membrane-actuated devices.
Touch screen monitors on PCs have become more and more commonplace as their price has steadily dropped over the past decade, and the technology has rapidly spread from backlit LCD panels to the various graphic interfaces used to control appliances. These include on/off and function buttons, menus, keypads, directional symbols, rotary switches, sliders, company logos and any other graphic icons that provide information to the user and enable the user to get information or send commands to the appliance. Touch technology can also enhance the styling of the product and its perceived value to the consumer. It enables “dead front” capability where, when the device is not in use, the graphic icon backlights go out and the overlay “fades to black” until it is powered up again. This makes for a less intrusive and more pleasing appearance that is more in harmony with the overall aesthetic of the room.
There are three basic systems that are used to recognize a person’s touch:
- Surface acoustic wave
While each system has its merits, capacitive touch technology has become the most frequent design-in for touch-enabled appliances. Here, a layer that stores an electrical charge is placed on the overlay containing the graphic icons of the appliance or device. When a user touches the icon, some of the charge is transferred to his or her finger, so the charge on the capacitive layer decreases. This decrease is measured in circuits located on the PC board behind the overlay. The relative difference in charge is calculated to identify exactly where the touch event took place, and this information is relayed to the driver software. One advantage that the capacitive technology has over resistive is that it transmits almost 90 percent of the light from the backlight compared to only 75% for resistive; resulting in a brighter, clearer image.
Illuminating the Interfaces
These graphic interfaces need to be backlit. Up until recently, designers have had to dedicate an individual LED to illuminate each icon, button or symbol, and it has been difficult to achieve perfect uniformity among all of the backlights. An enabling technology that would solve the uniformity problem while reducing the number of LEDs required to backlight multiple graphic interfaces has been desirable.
The backlight that illuminates the graphic interfaces is placed between the overlay and the PC board/circuit sensor. This determines the appearance, brightness, clarity, color and overall aesthetic of the display interface. It also presents a design challenge to the appliance manufacturer.
Factors Influencing the Backlight
What makes for the optimum backlight? There are several factors to consider:
- Uniformity – Dispersion of Light
- Discrete Area Illumination Capability
- Number of LEDs Required / BOM
- Power Consumption
Edge Lighting is the Key
The use of low-profile side-firing white LEDs placed along the edge of an extremely thin light guide in concert with a pixel-based light extraction technology is the basis of the solution discussed here.
This light extraction technology harnesses and controls the LED light sources by spreading their light uniformly across the area to be illuminated, with no dark areas or hot spots. Hundreds of thousands of micro-optical elements (miniature reflective and refractive surfaces) are molded directly into the top and bottom of the light guide and the angle of the emitted light is optimized to maximize the efficiency of the LED backlight.
The light guides can be custom designed in a range of sizes with uniformities of >90% and brightnesses ranging from hundreds of cd/m2 to thousands, depending on the application requirements. The light can be directed to wherever is desired to illuminate only selected areas of an appliance overlay.
The LEDs are strategically placed along the edge of the light guide to disperse their light in the most efficient way possible. In previous designs, manufacturers had to dedicate an individual LED to each icon, button or symbol that needed illumination. Now, one LED can be used to effectively backlight multiple graphic icons, eliminating the need to use individual LEDs for each graphic interface or multiple LEDs for larger areas such as company logos. This keeps the number of LEDs required—and the bill of materials—to a minimum.
In touch enabled graphics applications, what the user sees is the front panel (Fig. 4, top), which is an overlay (usually plastic) that contains the application-specific graphics. Behind this is an ultra-thin (< 0.6 mm thick) light guide (Fig. 4, middle) with discrete optical areas molded into it for the most efficient and uniform dispersion of the light from the LEDs located on the PC board (Fig. 4, bottom), into which are embedded the LEDs and touch-sensor electronics. When assembled together the OEM has an exceptionally thin overall stack as shown in Fig 4A, with bright, uniform graphics and fewer LEDs.
The ultra-thin light guide shown in Fig. 4 uses only 2 LEDs to provide backlighting in exactly the places where illumination is required. When placed between the graphic overlay and the PC board /circuit sensor, its extreme thinness poses no obstacle to the signal going from any point on the overlay to the sensor element. The user can touch right through it with no loss of sensitivity. Depending on the application, these light guides can be designed as thin as 0.2 mm.
The operating current of the LEDs, along with the brightness desired and the number of LEDs used, will affect power consumption. The fewer the LEDs, the lower the overall power consumption. The side-firing LEDs typically used in appliance applications are low–current devices that operate at 25-30 mA to as low as 12 mA, and reducing the LED count from 3, 4 or more to 1 or 2 will proportionately reduce the power consumption, increasing the “green” component of the appliance and helping it to achieve the coveted Energy Star rating.
Benefits for the OEM
The graphic overlay can be glass, polycarbonate, polyester, acrylic or possibly some other non-conductive material. It is essential for the backlighting material to be compatible with the range of materials used in the overlays. Also, the thinner the backlight, the slimmer the assembly, as shown in Fig. 4A. And can it be injection molded? A thinner edge-lit light guide of compatible material that requires fewer LEDs and can be injection molded at high speeds simplifies the manufacturing process, lowers the BOM, and makes for a more cost-efficient assembly.
Touch technology is becoming increasingly ubiquitous as designers seek to enhance the performance, styling and visual appeal of a wide range of appliances as well as their company brand. As LEDs have established themselves as the backlight of choice for graphical user interfaces, ultra-slim edge-lit light guides that provide uniform illumination with fewer LEDs offer a cost-effective and more easily manufacturable alternative to previous backlighting approaches.