Methods and apparatus, including computer program products, implementing and using techniques for collecting optical data pertaining to one or more characteristics of a sample. A light beam of a first frequency is scanned onto a sample surface using one or more illumination optical elements. Light of a second frequency is collected from a scan line on the sample surface using one or more collection optical elements. None of the one or more collection optical elements are included among the one or more illumination optical elements. The collected light is transmitted to a detector.