Abstract: The determination of an apparent distance between an object and a surface is disclosed. One disclosed embodiment comprises illuminating the object with a spatially inhomogeneous irradiance from the surface, an intensity variation in the spatially inhomogeneous irradiance in a plane parallel to the surface responsive to a distance between the plane and the surface. Next, an image of the object is acquired while the object is illuminated by the spatially inhomogeneous irradiance, and the apparent distance of the object from the surface is determined based on a brightness contrast in the image of the object due to the spatially inhomogeneous irradiance.