Canon has developed what it claims is the world’s first Multi-Layer Diffractive Optical Element for camera lenses. A prototype Canon EF 400mm f/4 DO IS USM lens incorporating this element was exhibited at Photokina 2000 in Cologne, Germany and a commercial version of the lens is to be marketed during the first half of 2001.
Diffractive optical elements have a diffraction grating that alters the path light travels through diffraction. Such elements are already incorporated in such industrial instruments as spectroscopes and in signal-reading optical systems found in CD and DVD players.
Diffractive optical elements, however, have not been employed in camera lenses due to the tendency of natural (white) light to produce superfluous diffracted light upon entering the lens, resulting in flare that degrades image quality.
Canon’s Multi-Layer Diffractive Optical Element features a multi-layer construction comprising two single-layer diffractive optical elements with opposing concentric circular diffraction gratings. When incident light enters the Multi-Layer Diffractive Optical Element, superfluous diffracted light is not produced and almost all of the light is used for the image. The achievement allows the diffractive optical element to be incorporated in a camera lens.
The most significant characteristic of the diffractive optical element is that the positions where the wavelengths combine to form an image are reversed from those of a refractive optical element.
By combining a Multi-Layer Diffractive Optical Element and a refractive optical element within the same optical system, chromatic aberration (color smearing), which adversely affects image quality, can be corrected even more effectively than with a fluorite element. Also, by adjusting the pitch (spacing) of the diffraction grating, the diffractive optical element makes possible the same optical characteristics as a ground and polished aspherical surface, which effectively corrects for spherical and other aberrations.
Canon will continue its research and development efforts with the aim of incorporating the Multi-Layer Diffractive Optical Element in a variety of products, such as interchangeable SLR camera lenses, digital camera lenses, HMDs (Head-Mounted Displays), LCD projector lenses, and other imaging equipment.
More at http://www.canon.com/do-info/index.html