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Summary form only given. A new microscope-based spectrometric instrument has been designed for measuring the critical dimensions of dense line and space targets on patterned silicon wafers. The repeating structure of the target behaves as a one dimensional diffraction grating. Light, focussed at normal incidence on the wafer, is diffracted symmetrically into positive and negative orders with the O/sup th/ order beam being reflected back through the objective lens. The arrangement of the optics prevents all but the O/sup th/ order beam from being collected for detection. At normal incidence, the beam is a superposition of two linearly polarized components with some phase difference, /spl Delta/. Light polarized in the direction of the grating lines is called TE polarized light, and light polarized perpendicular to the lines is called TM polarized light. Any or all of three spectra can be measured: (1) TE reflectance, (2) TM reflectance, (3) phase difference. Measurement results from both photolithography and etch applications are presented.