| A third, very-thick resist is then coated over the resist lenses and patterned to protect the diffractive elements. |
| Few security devices are as versatile or provide as many benefits as the hologram, which form part of a wider group of diffractive elements known as Optical Variable Devices. |
| Superimposing such a diffractive structure on an aspheric surface yields an achromatic singlet whose spherical and chromatic aberrations are corrected. |
| Optical configurations include plano, spherical, aspherical, toric, cylindrical, diffractive, and sometimes combinations of these. |
| These diffractive spectra have been examined with great skill by Draper and Langley. |
