Transient measurement of thin liquid films using a Shack–Hartmann sensor

A Shack–Hartmann (SH) wavefront sensor was used to measure the thickness of thin liquid films of n–octane on silicon substrates. A SH device consists of an array of microlenses spaced from an image sensor by their effective focal length. A planar wavefront imposed on a SH device would produce uniformly spaced foci on the sensor for each lenslet. A distorted wavefront would then shift the foci by a calculable amount. Typically used for optical system alignment and calibration, the SH device was coupled with magnifying optics to determine the distortion of a planar wavefront source upon refraction through a film and reflection by a polished substrate. Geometrical optics, in the form of ray transfer matrices, were used to determine the location and orientation of rays emanating from the film surface given their positions and angles recorded by the SH device. The SH foci movements were translated into sample coordinates and film slopes, which were subsequently integrated to produce the film profile.