Ultra-high-contrast low-leakage-light optical touch device structures using light scattering and total internal reflection concepts

Ultra-high-contrast low-leakage-light optical touch sensor architectures arranged in transmissive and reflective configurations are proposed. The key idea is based on a combination of total internal reflection (TIR) and light scattering concepts. The TIR phenomenon confines the optical beam to stay inside the lightguide and creates the touching region where light can be scattered due to the change in surface composition from some fat and water on the fingertip. Other key features include low activating force requirement, tunable sensitivity, and ease of implementation. Experimental proof of concept using a visible laser diode and an acrylic Dove prism arranged in the transmissive configuration shows a measured very high optical contrast ratio of 12.2 dB for a 0.5-N activating force with only a small 1.47% scattering light. In addition, ambient light cannot disturb the operation of this optical touch device due to the deployment of the electronic noise rejection scheme that allows only the corresponding electrical signal to be analyzed. A switching response time in milliseconds is also investigated which is limited by the time constant in the electronic control box and the mechanical movement of the fingertip. This optical touch device has been tested 20 times in sequence under both a typical illumination of 342 lx and an external 1-mW visible laser diode without showing device disoperation. (C) 2005 Elsevier B.V. All rights reserved.