The Instituto de Astrofísica de Canarias (IAC), Spain, together with the Spanish company Tecdis Displays Ibérica, S.A., are developing voltage tunable optical retarders using liquid crystals as phase retarding medium. The ROCLIs are built for being used in the Imaging Magnetograph eXperiment (IMaX), which is one of the instruments aboard of the SUNRISE balloon mission (details about IMaX are described in a different paper in this session). A big advantage of using voltage tuned retarder plates is that no mechanisms are needed, which reduces significantly failure risk, weight, power and cost, aspects of particular importance in the SUNRISE balloon mission and for many future space borne applications. A set of prototypes has already been fabricated by Tecdis S.A. and is being characterized in the IAC laboratories. The purpose of these prototypes is to evaluate and demonstrate conceptually the suitability of the chosen liquid crystal for our use in IMaX. First results are very promising. In this paper we will present a full technical description of the ROCLIs for IMaX together with the laboratory test and verification results.