the subject invention relates to a wafer stage , such as may be used in optical wafer metrology instruments . the stage contains a wafer - chuck that can be connected to translation stages for the purpose of clamping and translating the wafer so that a plurality of sites on the wafer surface may be measured . the chuck includes a holder for mounting a reference sample . the holder is movable between a retracted position where the reference sample is held below the chuck surface and an extended position , such as where the surface of the reference sample is co - planar with the wafer surface . therefore the holder may be installed within the area of the chuck that is utilized for wafer clamping . by this arrangement , the size of the wafer translation system can be reduced minimizing the stage travel and enabling increased spatial resolution , increased wafer throughput and reduced capital equipment and operating costs .