Freeform microlens arrays surface description

Microlens arrays are used in a variety of different applications for sensing and projection. Master level imprint technology has boosted its use but usually needs expensive masters in its technology chain. All applications ask for high throughput. The conventional reflow technology to fabricate microlens arrays is limited in round shape lenses what also limits the fill-factor. A high fill-factor based mastering process, for large area master fabrication using machining and step-and-repeat, is today still very expensive. For many applications which do not need imaging, an alternative could be the use of square or hexagonal footprint reflow structures. Such structures are easy to produce with excellent quality over large areas but have a surface shape sometime difficult to predict. In our contribution, we explore the potential of their use. The difficulty with square footprint reflowed objects is that they do not exhibit rotational symmetry and are in optical terms freeforms. Hence, they can have complicated shapes that often need high-order polynomial descriptions for accurate modelling. We present our findings to fabricate and model such shapes, based on the use of Zernike polynomials to describe a suitable surface. This results in a generic model with a reduced parameter set and convenient control of the quality and effects of such structures when comparing the nominal shape with its deviation. It will be very useful to trace changes within a conventional imprint fabrication process. Finally, we can design square footprint shapes for applications such as projectors used in headlamps.