In one aspect, the invention relates to an apparatus for monitoring thickness
variations of an object having first and second opposing surfaces. The apparatus
includes a first source positioned to project a first fringe pattern at a first
location on the first surface and a second source positioned to project a second
fringe pattern at a first location on the second surface. The apparatus further
includes a first detector positioned to detect the first fringe pattern at the
first location on the first surface, the first detector generating a first signal
in response to the first fringe pattern at the first location. The apparatus also
includes a second detector positioned to detect the second fringe pattern at the
first location on the second surface, the second detector generating a second signal
in response to the second fringe pattern at the first location. The apparatus further
includes a processor adapted to calculate variations in the thickness of the object
in response to the first and the second signals.