A method of detecting the position of a surface to be examined along the height direction of the surface orthogonal to the surface, comprising the steps of:
- projecting two beams of light which interfere with each other so that they are converged adjacent to the surface to be examined;
- collecting light reflections of the two beams from the surface;
- generating a first interference light from the two reflected beams;
- optoelectrically converting the first interference light to detection signals containing data about the first interference light;
- and
- detecting the position of the surface along the height direction from a lateral displacement of interference fringes generated between the two reflected beams which is calculated from the detection signals and corresponding reference signals.