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SFH309FA Datasheet(PDF) 4 Page - OSRAM GmbH |
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SFH309FA Datasheet(HTML) 4 Page - OSRAM GmbH |
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4 / 14 page  August 13, 2010 page 4 of 14 2.4 Projector-based Touchscreens Another group of systems are based on a projector concept. Due to the setup their main application is in large screens for overview or presentation purposes. The principle of such a technique is presented in Fig. 4. Usually the visible image is projected from the backside onto a diffuse screen. One or several IR sensitive cameras are mounted behind the screen to monitor the reflected IR image of the screen. To illuminate the screen with IR radiation there are various options. One makes use of diffuse illumination (DI) from IR-sources behind the screen. If a stylus or finger touches the screen, a reflection occurs and the IR camera detects the bright spot. 2.5 Projector-based with FTIR Fig. 5 presents a similar version which works on the principle of frustrated total internal reflection (FTIR). This setup uses the waveguide properties of e.g. the acrylic glass as a part of the screen to distribute the IR radiation. Usually a pressure sensitive polymer layer is added on top to display the projected image, as acrylic glass is almost transparent to the visible image. IR light is coupled into the acrylic waveguide usually by several IREDs located on all sides of the screen. The light is captured inside the waveguide by total internal reflection. If pressure or a touch is applied on the polymer/acrylic surface due to a stylus or an object (e.g. finger) light is coupled by FTIR into the polymer (or into the finger if no polymer is used), from where it is scattered and remitted towards the IR sensitive camera located in the rear part of the screen. This technique is desired for applications where IR emission through the screen should be avoided, e.g. in touch screens used in TV studios to avoid interferences or saturation of TV camera pictures by IR light. It should be mentioned that FTIR combined with camera sensors is also used in the biometrics industry, most notably in fingerprint scanning applications. 2.6 In-Cell Optical Sensing The in-cell optical sensing principle is an integrated solution. Inside each pixel cell in a LCD display there is typically a phototransistor integrated. The principle works without a designated light source. In a bright environment the phototransistor sees the shadow of the finger tip, whereas in a dark or dim lit ambience the reflections of the backlight generates the signal. The absence of an active illumination is also the drawback of this principle, especially a black screen in dark environments. ε γ Fig. 5: Principle of a FTIR-based touch detection system. Different IRED coupling options are sketched. Fig. 4: Principle of projector-based touchscreen realization. The depicted system works with diffuse illumination (DI). |
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