A crystal with a 3-D photonic band gap is a powerful tool to control light, with applications for new types of solar cells, sensors and miniature lasers. Inside a man-made crystal like this, a range of light wavelengths is forbidden. Until now, the characteristic wavelength region is determined by using theoretical models. These idealized models have clear shortcomings. Researchers of the University of Twente (MESA+) have now developed a fully experimental method of determining the band gap, literally making the unseen visible. They present their results in Optics Express, the journal of the Optical Society of America.