A measuring technique based on Ronchi test measurements has been developed in the Center for the Development of Sensors Instrumentation and Systems (CD6) in the Technical University of Catalonia, which allows the observation of submicrometric surface profiles on reflective concave surfaces. Being a grating technique based on a geometrical approach, it does not require any vibration-isolated environment, and it adequately performs under a range of known positions of the sample and regardless of its orientation, becoming a very robust surfacing technique. In order to achieve high sampling densities together with submicrometric accuracy, some enhancements were incorporated to the classical Ronchi test setup, including the use of CCD detectors, laser sources, multiple ronchigram acquisition and computing power. A multiple acquisition procedure has been used which allows to tailor the number of acquired data points to densities of up to 50mm-2.
As an example, the technique has been applied to the measurement of three-dimensional topographies of spherical ophthalmic surfaces, although the technique is able to successfully manage non-rotationally symmetrical surfaces. Surface profiles of two pairs of spherical lenses identical to the standards of the ophthalmic industry have been measured showing how, although all lenses satisfactorily passed quality control procedures, in some cases their surface finishing was quite different, showing alterations along their manufacturing process which might lead to future quality control failures. The ability of the described Ronchi technique to measure submicrometric surface features, and to apply them to detect surface deviations in the manufacturing process, will be demonstrated.