Apparatus for measuring a small gap using a Savart plate

A flying height tester that can measure a space between a slider and a disk, comprising:

• a transparent disk;
• a motor that rotates said transparent disk;
• a loader which can place the slider adjacent to said transparent disk so that the slider and said transparent disk are separated by a space;
• a light source that reflects an unpolarized light beam from the slider and said transparent disk at a normal incidence;
• a birefringent element that splits the reflected light beam into a ordinary beam and an extraordinary beam;
• a mechanism which can create a relative movement between the ordinary and the extraordinary beams;
• a photodetector which detects the ordinary and extraordinary beams;
• a controller that is coupled to said photodetector and which can compute the space between the slider and said transparent disk from the detected ordinary and extraordinary beams.

A method for determining a space between a first surface and a second surface, comprising;

• a) reflecting an unpolarized light beam from the first and second surfaces at a normal incidence;
• b) splitting the reflected light into an ordinary beam and an extraordinary beam;
• c) retarding the ordinary and extraordinary beams relative to each other;
• d) detecting the ordinary and extraordinary beams;
• and,
• e) computing the space from the detected ordinary and extraordinary beams.