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.