We present a new method for determining the electrically induced twin angle alpha of a (100) bulk single crystal of barium titanate (BaTiO3) using a nondestructive optical technique based on Fraunhofer diffraction. The technique required two steps that were performed simultaneously. First, we analyzed the diffracted light intensity captured with a line camera. Second, we measured the size of the diffracting element by analyzing images of crystal's surface taken with a CCD camera. The value obtained for the twin angle is 0.67 degrees +/- 0.05 degrees, which compares favorably with the theoretical value of 0.63 degrees.