Abstract: This optical device has, in the rear of the projectile, a retro-reflector fitted out with a polarizer and, at the projectile firing station, a light source whose beam illuminates the rear of the projectile, and a light flux analyzer deducing the roll angle of the projectile from the direction of polarization of the light flux reflected by this projectile, wherein the polarizer is a polarizer with refraction index discontinuity positioned on the rear of the projectile, before the retro-reflector, with an angle of inclination between the direction normal to its plane of index variation and the longitudinal axis of the projectile chosen to be greater than the Brewster angle, and wherein the light source is offset laterally with respect to firing axis of the projectile, these two measurements giving rise to a modulation of intensity of the reflected light beam as a function of the roll angle plus or minus 2.pi.