Patent ID: 7388943

Claim:
A method of making a gap between first and second objects have a predetermined value, said method comprising the steps of: a first introduction step of introducing light diagonally into an entry window on the first object; a first detection step of detecting a first total intensity of light from a first exit window on the first object with respect to each of a plurality of values of the gap, the first exit window being at such a position that light would enter through the first object, reflect off the second object, and then enter the first exit window if the gap has the predetermined value; a second introduction step of introducing light diagonally into the entry window on the first object from a direction different from that of the light introduced in the first introduction step; a second detection step of detecting a second total intensity of light from a second exit window on the first object with respect to each of a plurality of values of the gap, the second exit window being at such a position that light would enter through the entry window of the first object, reflect off the second object, and then enter the second exit window if the gap has the predetermined value; an obtaining step of obtaining a position of the second object, with respect to a direction of the gap, where the gap has the predetermined value based on a plurality of total intensities of light detected in said first detection step and a plurality of total intensities in said second detection step; and a position step of positioning the second object at the position obtained in said obtaining step.