  We present the Focus-Induced Photoresponse (FIP) technique, a novel approach
to optical distance measurement. It takes advantage of a widely-observed
phenomenon in photodetector devices: a nonlinear, irradiance-dependent
photoresponse. This means that the output from a sensor is dependent on the
total number of photons incident and the size of the area in which they fall.
With a certain arrangement of sensor and lens, this phenomenon will cause the
output of the sensor to change based on how far in or out of focus an object
is. We call this the FIP effect. Here we demonstrate how to use the FIP effect
for distance measurements. We show that this technique works with different
sensor materials, device types, as well as visible and near infrared light. In
principle, any sensor exhibiting a photoresponse that depends nonlinearly on
irradiance could be used with the FIP technique. It is our belief that the FIP
technique can become an important method for measuring distance.
