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SFH4555 Datasheet(PDF) 8 Page - OSRAM GmbH |
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SFH4555 Datasheet(HTML) 8 Page - OSRAM GmbH |
8 / 14 page August 13, 2010 page 8 of 14 Using visible LED sources (e.g. monitor backlighting) in IR touchscreen solutions is recommended, as these LEDs have no IR content. In contrast most conventional (non- LED based) light sources emit also in the IR spectral range. E.g. sunlight and incandescent bulbs contain components of equal or even higher amplitudes in the visible as well as IR wavelengths range1. For applications where intense incandescent or halogen illumination is expected, some additional electrical-domain effort is advised to enhance the signal-to-noise ratio. There are several steps with increasing complexity to counterfight the IR noise topic. The implementation depends on the signal compared to the IR noise level. The simplest version is the operation of the emitter and detector in a pulsed and synchronized operation. An AC-coupling of the detector signal might efficiently filter out the present DC-components of the ambient light. If the IR background noise becomes more dominant, a more complex implementation is necessary. This measure compares two subsequent measurements. The first, called the reference (without IR illumination) is compared with the second, the signal (with IR illumination). Based on the difference signal the touch event can be extracted. The most demanding solution is the inclusion of a lock-in amplifier type circuitry, which demands a modulated emitter signal. In this case the IRED is modulated with a carrier frequency and the signal is detected through a frequency synchronous receiver, either in a homodyne or heterodyne structure. Alternatively, digital signal processing allows a direct detection by employing computational algorithms (e.g. Fourier-type). Such a system can be made immune even to severe IR ambient light (noise). The selection of the right modulation frequency and the implementation of a narrow band-/lowpass filter are key elements for a high signal-to-noise ratio. Similar considerations should be undertaken for camera/line scan systems. In this case, it is recommended as a first measure to insert a narrow optical bandpass filter (matching the IRED wavelength) in front of the camera. This increases significantly the signal-to- noise ratio. 4.4 Scalability Issues Camera-based and projector-based solutions have the advantage of easy scalability. An increase in resolution is commonly achieved by utilizing a sensor with higher resolution. Additionally, the optical power from the emitters might be increased to keep the signal-to-noise ratio stable. In a more general sense, every free space beam tends to broaden over distance. This leads to a decrease of the irradiance over distance. The half-angle value is an appropriate approximation concerning the broadening and propagation of light in free space in the so called far field regime2. Mathematically the irradiance E e(r) in the far field drops with the basic relationship E e(r) ~ 1 /r2. As a conclusion: A doubling in distance r reduces the irradiance by a factor of four. Note that the irradiance depends on the distance and also on the angle (normal to the surface of the emitter). The irradiance E e is related to the radiant intensity I e (stated in the data sheet of the optical source) by the equation I e = Ee·r 2. ___________________________________________ 2 The transition distance between the near- and far field depends, among other issues, on the package type. For non- lensed types the transition is usually within the first centimeter. Devices with a domed lens and narrow angle might have a transition distance of up to 25 cm. The main difference between near- and far field lies in the shape and distribution of the radiation characteristics. For proper modelling OSRAM provides raytrace files if accurate information is necessary. An important issue for practical applications might be that the detector should be located within the far field. By using non-lensed emitters this is ensured for almost all applications. ___________________________________________ 1 IR noise may contain many frequency components from various light sources: - DC (e.g. in sunlight, incandescent or halogen lamps) - Mains frequency (50/60 Hz) and driver caused harmonics (e.g. in incandescent and halogen lamps) - Frequencies depending on the driver/power supply (e.g. up to kHz range in fluorescent lamps) |
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