A review of some methods and their optical principles for measuring the corneal topography are presented in this paper. The concepts of principal curvatures and the ambiguity concerning the axial curvature of surfaces without symmetry of revolution are analyzed. These methods are divided into three groups according to the following optical principles: (1) specular reflection, which includes the Placido disk system, interferometry, and moiré deflectometry; (2) diffuse reflection, which includes moiré fringes, rasterstereography, and Fourier Transform Profilometry; and (3) scattered light, which includes the slitlamp system. We avoided describing the details of commercial instruments, only their working principles.

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We propose to use a mask with a nonredundant array (NRA) of multiple apertures to measure spatial coherence in two dimensions. The spatial distribution of the apertures in the mask is made in such a way that we obtain a quasi-uniform sampling in the coherence domain. The spatial coherence is obtained by Fourier transform of the interferogram generated by the mask when it is illuminated by the light field under analysis.