The ability of a newly developed independent passive optical system OPTIMEX to measure the detonation front curvature is demonstrated on charges of pressed explosive A-IX-1 (RDX/ceresin-stearin mixture with 95/5 wt.%). The charges, with length to diameter ratios of from one to four, were prepared from cylinders with diameters of 21, 30, 40 and 50 mm pressed to a density of 1.66 g/cm3. Such charges detonate with a velocity of 8220 m/s. The detonation curvature was obtained using 8 optical fibers and the results were compared with photographs acquired by an ultra-high speed framing camera UHSi 12/24.