Abstract
The null-screen method has been used to test aspheric surfaces. This geometrical method measures the slope of the test surface and by a numerical integration procedure the shape of the test surface can be obtained. Now, in order to test the surfaces of a variable focal length lens (VFLL), we propose use a conical null-screen. We present the formulae to design the null-screen in such a way that the image on the CCD is a perfect array of spots; departures from this geometry are observed as deformation of the surface. The VFLL is designed in such a way that under conditions of mechanical equilibrium both surfaces are spherical; however, its shape can be easily modified mechanically changing its radius of curvature. In order to analyze the shape of the surfaces of the VFLL at different radius of curvature, we evaluate its form using a conical null-screen. This procedure allows study the deformations of the surface.
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