The energy balance of the Arctic is of high importance to the climate system of our planet also outside the arctic. The sea ice cover as boundary layer between ocean and atmosphere plays a crucial role in the partitioning of incident energy between differend compartments of the climate system. While sea ice reflects most of the incident light, some part is penetrating through the ice. Extent, thickness and albedo of sea ice have been observed for many years. In contrast, there exist only few measurements of transmitted light, as acces under the ice is difficult. Few data exist especially on the spatial variability of transmitted light. This work presents the first measurements of transmitted light onboard a remotely operated vehicle (ROV) under sea ice in the central Arctic. Several profiles of transmitted light were measured during the cruise ARK-XXVI/3 (TransArc 2011) of the german research icebreaker Polarstern in 2011 (beginning in the eurasian basin, over the pole to the canadian basin and towards russian shelf seas). Two spectroradiometers with different angular response characteristics were used for the first time for simultaneous measurements. The amount of data enables statistically significant conclusions on a huge databasis and an approach to spatial variability of the light conditions under sea ice. Transmittances for different ice types could be derived and the variability within an ice type described. One of the main results of the work is that first year sea ice transmits three times more light than multi year sea ice. Spatial variability on single floes exceeds one order of magnitude, but is mainly influenced by surface properties. Due to the lack of a surface scattering layer melt ponds transmit considerably more light than the surrounding bare ice. From the ratio between the two different sensors we can deduce, that the scattering-coefficient of sea ice is dependent on direction and that the light field under the ice is not isotropic, as it was assumed earlier.