Smart phones, tablets and other electronic gadgets have become an integral part of our daily life. To maintain the steady development to even faster and smaller devices, it is desirable to replace their slow electrical data interconnects with fast optical connections. Photonic crystals are an ideal tool for such a purpose as they can guide and bend light on a nanometer scale. Surprisingly, researchers have so far been unable to truly probe inside these crystals how the light intensity is distributed - this, however, is a requirement for being able to accurately characterize the local density of electromagnetic states inside the crystal which is the key for controlling the interaction of light with matter. Researchers have now demonstrated a new concept to measure the intensity distribution of light inside photonic crystals. This method, for the first time, allows researchers to map the absolute strength of an electromagnetic field component inside a photonic crystal.