An optical subsystem of a scatterometer, the optical subsystem comprising: one or more light sources configured to produce light having different polarizations; a polarizing beam splitter (34;80) configured to separate the light into two different light beams having orthogonal and mutually exclusive polarizations; two or more first optical elements configured to direct the two different light beams to a wafer (44;92) thereby defining illumination paths; one or more second optical elements (50,52;94,96) configured to control which one of the two different light beams illuminates the wafer (44;92) during measurements; and a detection subsystem configured to separately detect two different scattered light beams resulting from illumination of the wafer (44;92) with the one of the two different light beams and to separately generate output responsive to the two different scattered light beams, wherein the two different scattered light beams have orthogonal and mutually exclusive polarizations, and wherein all optical surfaces of the optical subsystem used for the measurements are stationary during the measurements; characterized in that the two or more first optical elements comprise a first non-polarizing beam splitter (38;82) and an additional polarizing beam splitter (40;86), both positioned in the path of a first of the two different light beams, and a second non-polarizing beam splitter (48;84) and the additional polarizing beam splitter (40;86), both positioned in the path of a second of the two different light beams; and the optical subsystem further comprises an objective (42;90) configured to direct light from the additional polarizing beam (40;86) to the wafer (44;92) and to collect light scattered from the wafer (44;92), wherein the light scattered from the wafer (44;92) is separated into the two different scattered light beams by the additional polarizing beam splitter (40;86), and wherein the two different scattered light beams are separated from the illumination paths by the first non-polarizing beam splitter (38;82) and the second non-polarizing beam splitter (40;84). one or more light sources configured to produce light having different polarizations; a polarizing beam splitter (34;80) configured to separate the light into two different light beams having orthogonal and mutually exclusive polarizations; two or more first optical elements configured to direct the two different light beams to a wafer (44;92) thereby defining illumination paths; one or more second optical elements (50,52;94,96) configured to control which one of the two different light beams illuminates the wafer (44;92) during measurements; and a detection subsystem configured to separately detect two different scattered light beams resulting from illumination of the wafer (44;92) with the one of the two different light beams and to separately generate output responsive to the two different scattered light beams, wherein the two different scattered light beams have orthogonal and mutually exclusive polarizations, and wherein all optical surfaces of the optical subsystem used for the measurements are stationary during the measurements; characterized in that the two or more first optical elements comprise a first non-polarizing beam splitter (38;82) and an additional polarizing beam splitter (40;86), both positioned in the path of a first of the two different light beams, and a second non-polarizing beam splitter (48;84) and the additional polarizing beam splitter (40;86), both positioned in the path of a second of the two different light beams; and the optical subsystem further comprises an objective (42;90) configured to direct light from the additional polarizing beam (40;86) to the wafer (44;92) and to collect light scattered from the wafer (44;92), wherein the light scattered from the wafer (44;92) is separated into the two different scattered light beams by the additional polarizing beam splitter (40;86), and wherein the two different scattered light beams are separated from the illumination paths by the first non-polarizing beam splitter (38;82) and the second non-polarizing beam splitter (40;84).