Method and apparatus for determining a height of a number of spatial positions on a sample

An apparatus for determining a height of a number of spatial positions on a sample, defining a height map of a surface, the apparatus comprising:

• a light source adapted to generate a light beam having a spatial periodic pattern which is periodic in a direction perpendicular to an optical axis and which moves in a direction in which the pattern is periodic;
• an optical detector adapted to convert received light into electrical signals;
• optical means for directing light from the light source to the surface and for directing the light reflected by the surface to the optical detector;
• and
• a processing unit adapted to control scanning means to perform a scanning action, to control movement of the pattern, to receive the signals from the optical detector, and to convert the signal received from the optical detector into a height map,
• wherein the processing unit is adapted to make only a single image in any scanning position and to control the scanning speed to have a predetermined relation to a phase of projected periodic pattern;
• the processing unit is adapted to determine, for each spatial position of he detector, an amplitude of the signal detected during the scanning and to determine a scanning location where the amplitude is maximal;
• and
• an optical path of the light directed by the optical means to the surface is parallel to an optical path of the light reflected by the surface to the optical means.