Submicrometric profilometry of nonrotationally symmetrical surfaces using the Ronchi test

Abstract

The Ronchi test is a widely used tool in the optical shop, because of its capability of measuring wide-field profiles of optical quality surfaces through its important ray slope dynamic range. Although many applications have been reported (intraocular lenses, capillary flow measurements, etc.), they are usually limited to obtaining profiles of rotationally symmetrical surfaces, either spherical or aspherical. An easy to set-up Ronchi test technique based in geometrical optics principles has been developed at the CD6, which allows accurate and repetitive wavefront measurements of the topography of both rotationally and non-rotationally symmetrical optical quality surfaces. In order to demonstrate its capabilities, the technique has been applied to the profilometry of the toroidal concave surfaces present in common astigmatic spectacle lenses. Profiles of two sample surfaces at two different distances from the ruling to the sample have been measured with micrometric resolution. As toroidal surfaces are not rotationally symmetrical, both samples have been positioned with its principal meridians following different orientations, in order to demonstrate that the technique performs adequately not only for a range of distances from the ruling to the sample but also regardless how the sample has been oriented. The accuracy of the measurements is demonstrated through the comparison of the radius of curvature values obtained using the proposed Ronchi test technique, with reference radius of curvature values obtained using a high precision radioscope. The technique is shown to measure sub-micrometric surface features on toroidal surfaces by use of a combination of self-developed and commercial software, allowing the observation of surface profile details with extensions of just some tenths of nanometers in depth.