Optical solutions are increasingly popular for displays. Unlike traditional touchscreens, optical screens can detect any type of pointer or stylus and gloved fingers as they aren’t reliant on the conductivity of these objects. Optical designs are also not sensitive to scratches and can be used for any size screen. In most cases, the optical components are mounted in a frame around the display, so existing displays can be easily upgraded. Optical touchscreens used to be considered too expensive, too large, and too sensitive to ambient light. However, IREDs now provide the basis for cost-effective, low-profile touchscreens, lowering the price and erasing size requirements. The sensitivity to ambient light can be overcome by appropriate designs.
Commonly used technologies for optical touchscreens all benefit from highly efficient, thin-film chip technology, which provides the basis for compact IREDs with high optical output. A wide range of packages is available for all design options, from narrow-angle emitters for light grids to high-power emitters for illuminating large displays. IREDS offer a wavelength of 850 nm and light that is barely visible to the naked eye, but easily registered by the detectors.
Light Grids
The simplest solution for optical touchscreens is a light grid created by rows of infrared emitters and detectors placed opposite one another. The components are mounted in a low-profile bezel around the screen, just a few millimeters deep. A finger or stylus blocks the light beams, causing the detector signal to attenuate at the appropriate point (Figure 1). This design can be used as a multi-touch version if emitters and detectors are switched sequentially and signals are evaluated.
Important factors for selecting an emitter are the size of the component, its optical output, and its radiant intensity. High radiant intensity is synonymous with an intense narrow-angle beam. High radiant intensities enable large screen diagonals to be covered. Narrow beam angles, coupled with narrow detection angles on the detectors, ensure that, even on large displays, the beams from the individual emitters do not hit more than one sensor. In some applications, particularly ones with strong halogen lighting, daylight filters can reduce the influence of ambient light on the detectors.
A light grid design can be easily scaled up to larger screen diagonals. Compared with non-optical technologies, this scaling involves less expense because the functional components are mounted in the frame around the display. The scaling factor in the case of optical touchscreens is dependent on the circumference. In all other technologies, the scaling factor is dependent on the square of the display. Remember, light output reduces in proportion to the square of the distance from the detector. This, in turn, leads to a poorer signal-to-noise ratio for the touch signal, and it may be necessary to adjust the emitter current accordingly.
Camera-Based Solution
A setup with line sensors needs far fewer components than light grids. High-power IREDs flood the display with infrared light from two corners, and detectors — optically separated to prevent crosstalk — only receive a signal when objects on the display reflect the infrared beams. In most cases, the sensors are line scanners. The precise position and size of the finger or stylus is calculated by evaluating both signals using a procedure similar to triangulation. This design has the potential to produce touchscreens with a much higher resolution than other technologies. It can be scaled up to larger screens without the need for additional components, as long as the emitters produce enough light.
In a slightly modified version, light guides are mounted around the display and fed with light by IREDs at the corners. Light exits the light guide at certain intervals along its length, creating a curtain of light over the display. The line sensors register the shadows cast by objects on the display.
While IREDs with a wide beam angle are suitable for illuminating from the corners, the choice of emitters for the light guide version depends on the design of the light guide. IREDs with a narrow beam angle and flat surface are ideal for injecting light into light guides. To free the touch signal from ambient light influences, measure without infrared illumination and then with infrared illumination, and calculate the difference between the two signals.
Large Displays Solutions
Large projection panels and consoles are mostly backlit with infrared light. A finger or stylus on the display reflects the light to one or more cameras. Alternatively, infrared light can be injected into the glass of the display. The IRED emits light into the glass so the beams are totally reflected at the top and bottom surfaces. When an object touches the surface, the light escapes and scatters so the signal at the detectors is changing. The simplest way to eliminate ambient light effects in projection systems is to place bandpass filters in front of the camera sensor. These two versions of optical touchscreens do not require a bezel and therefore give designers greater flexibility.
