DEVICE FOR CHECKING VALIDITY OF HOLOGRAMS Russian patent published in 1998 - IPC

FIELD: holograms and diffraction patterns for authentication of securities, articles which carry hologram with hidden data. SUBSTANCE: device has serial optical circuit of coherent light source, collimating optical system, frame for holding hologram, phase mask, Fourier lens and diffuse screen. Phase mask is designed as periodic raster of domain lenses. Back focal plane of domain lenses is matched with front main plane of Fourier lens. Maximal permissible shift of hologram with respect to device is equal to , where f_o is focal length of Fourier lens, d_s is size of diffuse screen, r is radius of raster domain lenses, n refraction index of raster. This design of phase mask results in invariance with respect to hologram shift in +/-x tolerant range when image is viewed. EFFECT: increased functional capabilities. 3 dwg