Apparatus and method for measuring surface topography optically

An apparatus for determining surface topography of a structure, the apparatus comprising:

• a probe;
• an illumination unit configured to output a plurality of light beams;
• a light focusing assembly comprising an image space lens and an object space lens;
• a focus changing assembly located along an optical path between the image space lens and the object space lens, a portion of the focus changing assembly located at a back focal length of the object space lens and comprising a variable optical power element or a divergent and a convergent lens;
• a detector having an array of sensing elements configured to measure a characteristic of each of a plurality of light beams returning from the illuminated spots;
• and
• a processor coupled to the detector and configured to generate data representative of topography of the structure based on the measured characteristic of each of the plurality of light beams returning from the illuminated spots,
• wherein the light focusing assembly is configured to:
• receive each of the plurality of light beams;
• overlap the plurality of light beams within a focus changing assembly;
• and
• focus each of the plurality of light beams to a respective focal point external to the probe in order to generate illuminated spots on the structure, and
• wherein the focus changing assembly is configured to move the respective focal points along a direction of propagation of the plurality of light beams.