Counterfeiting is an important issue in unattended fingerprint identification. To help prevent this, we have proposed to look at the finger color changes during an input action. When a finger is pressed against an optical fingerprint sensor, it is gradually deformed and the color of the fingerprint images changes. This is due to the blood movements inside the finger. If the extent of this color change exceeds a certain threshold, the finger is judged to be real. We start by describing the spectral changes in light scattered by the pressed finger. As the pressure applied to the finger increases, the relative intensity of the red portion of the spectrum decreases. Second, we discuss a dual-LED imaging system based on scattered light detection. Here, two LEDs with peak emissions at 530nm and 630nm are mounted on the edge of a plastic plate serving as a light-guide. Light scattered by a finger on the light-guide is captured by a standard color camera. Using eight types of replicas and live fingers of 42 volunteers, we have acquired series of images during an input action. For each finger, three input actions have been recorded. Analysis of this database of 150 input trials shows that our imaging system can differentiate the live fingers and the replicas clearly. We have investigated signal extraction process for a reliable differentiation. The imaging system is adequately robust against temperature fluctuation in a finger and it can be used under illumination of 3050 lux.

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