Further Analysis of TIR sensor technology

The source of light is generally from LED's (light emitting diodes) that are located on the left side of the prism. This light then reflects off of a diffuse reflective coating that is located on the right side, causing the upper region of the prism (the platen) to light up. If there is no finger placed on the platen, the light will reflect off the platen due to total internal reflectance. There is an imaging system in the lower left side, which will create the image of the top horizontal platen. So when a finger is placed on the platen, the light will cross the valley’s of the fingerprint and cause TIR at these “valley” like locations. So the image that is processed will have dark features where there is contact with the ridges, and lighter features where there is none.