Skin Strain Measurement Technique
Step 1: Fingerprint Image Acquisition
The experimental platform exploits the principle of frustrated total internal reflection (FTIR) to obtain high-contrast fingerprint images. This technique is often used in biometric fingerprint sensors. A typical FTIR fingerprint sensor uses a light source, a prism and a camera as shown in Figure 1. In the absence of contact, light is reflected at the hypotenuse face of the prism towards the camera. The contact between fingerprint ridges and the surface causes light to be scattered. This results in a high-contrast pattern of black ridges over a white background.
Figure 1. Typical prism-based fingerprint sensor.
The experimental platform is illustrated in Figure 2. Unlike conventional fingerprint sensors, this apparatus includes a removable contact surface that may have simple geometrical features such as bumps and holes. A diffuser is also required at the entry face of the prism to insure that reflections are maintained along a wide range of non-flat surfaces.
Figure 2. Experimental platform.
Geometric distortions are corrected by imaging a precise calibration grid printed on a sheet of transparency film. The pattern resulting from the application of the grid on the contact surface is analyzed to correct the perspective projection, unroll the contact surface, and measure pixel size. The intensity of fingerprint valleys is also normalized to compensate for illumination non-uniformity. Figure 3 shows a typical fingerprint image acquired with this platform.
Figure 3: Typical fingerprint image.