A method and apparatus for viewing or authenticating a diffractive device and a diffractive security device (1) are provided in which a first diffractive relief structure (200) is responsive to a first wavelength of visible monochromatic light, a second diffractive relief structure (200) is at least partially interlaced with the first diffractive relief structure (100) and responsive to a second wavelength of visible monochromatic light, and a third diffractive relief structure (400) is at least partially interlaced with the first and second diffractive relief structures (100, 200) and responsive to a third wavelength of visible monochromatic light. Under illumination by the first, second and third wavelengths of monochromatic light, the first diffractive relief structure (200) produces a first partial image of a first colour in a reconstruction plane, the second diffractive relief structure (300) produces a second partial image of a second colour in the reconstruction plane and the third diffractive relief structure (400) produces a third partial image of a third colour in the reconstruction plane, and the first, second and third partial images at least partially overlap in the reconstruction plane to form a multicoloured image. The diffractive security device may be used in a security document and the device may be authenticated by comparing the multicoloured image to a reference image.