The Luminescent Solar Concentrators (LSC) are innovative devices whose main application is the realization of transparent photovoltaic systems that are particularly interesting in the field of building integrated photovoltaics. Despite the great potentialities of LSC photovoltaic systems, up to now only a few real world applications exist and most of the scientific work about LSCs concerns only luminescent dyes and small size devices (generally 5×5 cm2). The purpose of this work is to investigate the optical behavior of a large (50x50 cm2) LSC slab that can closely represent the devices used in building integrated applications like roofs or windows. The output spectrum of the emitted light is measured while the LSC is illuminated in different positions with a round spot. With this setup it is possible to collect information about the contribution of radiative and non-radiative absorption to the performance of the system.