Analysis of laser speckle patterns from fingertips

The trend in human-machine interface technology is heading towards optical solutions for tracking and movement detection. Especially, interactive touch screens and pads, in which the movement of the user’s fingertips is detected and tracked, are of great commercial interest. The applications range from mobile phones to laptops and PDA´s. However, the dynamics of scattered light from live tissue must be taken into account when designing optical sensor systems for tracking e.g. fingertips in touch-applications. Especially, when using coherent light sources, the statistics of the speckle-pattern originating from the scattering structure is of critical importance for the sensor performance and has to be understood in details. We investigate theoretically and experimentally the characteristics of the dynamics of backscattered speckle pattern generated by a human fingertip and address the effects of an intermediate optically flat interface, between the fingertip and the illuminating light source.