Null-screen testing of the complementary freeform surfaces of an adjustable focus lens

A novel null-screen technique to test freeform optical surfaces for ophthalmic use is presented. We present an off-axis experimental setup to evaluate the surface shape of a two-element commercial adaptable focus lens based on the Alvarez principle. The advantages of the method are that it is fast, easy to implement, and reduces costs. This metrology technique is validated by testing both the convex and the concave complementary freeform elements of the spectacle under test. We also apply two methods to recover the surface shape of both elements: numerical integration and a custom probabilistic algorithm. Results are compared with a reference surface finding that for method 1, the RMS value in sagitta differences is 32.6 microns for the convex surface and 26.5 microns for the concave surface. Meanwhile, for method 2, the RMS value in sagitta differences found is 19.8 microns for the convex surface and 16.9 microns for the concave surface. Results show agreement in RMS in sagitta differences for both surfaces in each of the reconstruction methods employed.