Patent Number: 
Section: claims

1. 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 to a first 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 entry window of 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 into a second entry window on the first object;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 second 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; anda positioning step of positioning the second object at the position obtained in said obtaining step,wherein said first and second introduction steps respectively introduce light to a diffraction grating, as the first and second entry windows, common to said first detection step and said second detection step. 2. A method according to claim 1, wherein said first detection step detects a total intensity of light from a first diffraction grating in the first exit window, and said second detection step detects a total intensity of light from a second diffraction grating in the second exit window. 3. A method according to claim 1, wherein said first detection step includes a step of: varying a position of the second object with respect to the direction of the gap while light is introduced to the first entry window on the first object and a total intensity of the light from the first exit window on the first object is detected. 4. A method according to claim 1, wherein said second detection step includes a step of varying a position of the second object with respect to the direction of the gap while light is introduced to the second entry window on the first object and a total intensity of the light from the second exit window on the first object is detected. 5. An apparatus for making a gap between first and second objects, said apparatus comprising:a light introducing unit configured to introduce light to a first entry window formed in the first object;a first detection unit configured to detect a total intensity of light from a first exit window formed in 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 entry window of the first object, reflect off the second object, and then enter the first exit window if the gap has a predetermined value;a second detection unit configured to detect a total intensity of light from a second exit window formed in 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 a second 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; anda positioning mechanism configured to obtain 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 by said first detection unit and a plurality of total intensities of light detected by said second detection unit, and to position the second object at the obtained position,wherein said light introducing unit is configured to introduce light to a diffraction grating, as the first and second entry windows, common to said first detection unit and said second detection unit. 6. An apparatus according to claim 5, wherein said first detection unit is configured to detect a total intensity of light from a first diffraction grating in said first exit window, and said second detection unit is configured to detect a total intensity of light from a second diffraction grating in the second exit window.