means 30 is located under the sealed enclosure 36 at the next step the supporting means 30 accurately moves the wafer w by a pre determined distance u03b4x 2 and locates the wafer under the optical window 31 of the positioning channel 300 as shown schematically in fig8 a and 8 b if the wafer w is not aligned pre alignment can be performed according to any known method such as that which is disclosed in u s application ser no 09 097 298 final positioning can be conducted according to any known method such as the method disclosed in u s pat no 5 682 242 based on patterned features alternatively the whole positioning step can be conducted without rotating the wafer according to the method disclosed in u s pat no 5 682 242 during the positioning step the optical head 34 may move along the optical window 31 followed by or simultaneously rotation of the wafer w by rotation means 33 in order to locate under the optical window 31 any point of the wafer w when positioning is completed and the predetermined site defined during recipe preparation is found measuring commences if the spectral range used for measuring is similar to that used for positioning measuring is.