| A third, very-thick resist is then coated over the resist lenses and patterned to protect the diffractive elements. |
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| Optical configurations include plano, spherical, aspherical, toric, cylindrical, diffractive, and sometimes combinations of these. |
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| Superimposing such a diffractive structure on an aspheric surface yields an achromatic singlet whose spherical and chromatic aberrations are corrected. |
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| 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. |
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