Optical measurement methods for refractive microlenses and arrays

There are several properties of microlenses which have to be characterized: paraxial parameters like the focal length and more complex properties like the surface quality or the wave aberrations. For arrays of microlenses the homogeneity of the lenses and the accuracy of the positioning may also be very important. Additionally, the surface of the microlenses can have spherical, aspherical or cylindrical shape depending on the application. Several measurement methods for nearly all of these parameters will be presented. A Twyman-Green interferometer is used for the measurement of the surface deviations of a microlens from an ideal spherical shape and for the measurement of the radius of curvature of the surface. A Mach-Zehnder interferometer measures the wave aberrations of microlenses and the focal length. With the help of grazing incidence interferometry all surfaces with cylindrical symmetry can be measured. The cross-section of the surfaces can be circular or non-circular. Diffractive optical elements are applied as beam shaper and reference elements. The testing of complete arrays of microlenses can be done with several methods. A Smartt test can be employed to measure the wave aberrations of an array of microlenses and a shearing device might be suitable to measure the uniformity of the focal lengths of the microlenses.