indicates the measured position as the intersection point between the optical axis of lens 46 and the detector plane 48 this indication of the position of the examined surface with respect to the reference plane is unaffected by the surface patterns on the examined surface since the radiation spot 42 is located within the range of the depth of focus of both lens 41 and 44 the indication of the position on detector plane 48 is unaffected by tilting of the surface of the wafer w the images 47 on the position detector 48 of the radiation spot 42 on the examined surface wafer 43 are magnified by the factor m as follows where f41 and f44 are the focal lenths of the lenses 41 44 respective and are equal to each other and f46 is the focal length of lens 46 the main radiation loss of the optical components in the detection system is due to low collection of radiation from the measured surface and the transmission and reflection of radiation out of the system by the beam splitters 34 and 33 the amplitude collection efficiency is equal to where t33 t44 are the amplitude transmissions of the beam splitters 33 34 r33 r34 are their amplitude reflectivities respectively and r42 is the total collection efficiency of radiation at the position detector plane 48 collected from the measured surface over the area of the radiation spot 42 amplitude reflectivity r42 depends on the optical properties of the measured surface such as reflection absorption scattering and diffractions it also depends on the collection solid angle of the.