The predicted primordial gravitational wave signature at degree-scales in the polarized part of the Cosmic Microwave Background radiation has spawned a number of dedicated experimental efforts. We will briefly describe one example: the balloon-borne Spider experiment, which collected its first data during a 16-day Antarctic flight in January 2015. Due to the smallness of the sought polarized signature, experiments such as Spider face stringent requirements on systematic error control. In this document, we focus on systematic errors related to the characterisation of the optical response. We describe the optical coupling to the polarized sky and introduce a numerical code that we have developed to help quantify the impact of such systematics. Finally, we apply the code to estimate the significance of several optical systematics in the context of the Spider experiment.