Starshades are a leading technology to enable the detection and spectroscopic characterization of Earth-like exoplanets. We report on optical experiments of sub-scale starshades that advance critical starlight suppression technologies in preparation for the next generation of space telescopes. These experiments were conducted at the Princeton starshade testbed, an 80-m long enclosure testing 1/1000’th scale starshades at a flight-like Fresnel number. We demonstrate ^{10  −  10} contrast at the starshade’s geometric inner working angle (IWA) across 10% of the visible spectrum, with an average contrast at the IWA of 2  ×  10^−  10 and contrast floor of 2  ×  10^−  11. In addition to these high-contrast demonstrations, we validate diffraction models to better than 35% accuracy through tests of intentionally flawed starshades. Overall, this suite of experiments reveals a deviation from scalar diffraction theory due to light propagating through narrow gaps between the starshade petals. We provide a model that accurately captures this effect at contrast levels below 10^−  10. The results of these experiments demonstrate that there are no optical impediments to building a starshade that provides sufficient contrast to detect Earth-like exoplanets. This work also sets an upper limit on the effect of unknowns in the diffraction model used to predict starshade performance and set tolerances on the starshade manufacture.