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The Cooke triplet is a photographic lens design designed and patented in 1893 by Dennis Taylor who was employed as chief engineer by Cooke of York. It was the first lens system that allows elimination of most of the optical distortion or aberration at the outer edge of lenses. A Cooke triplet comprises a negative flint glass element in the centre with a crown glass element on each side. In this design, the sum of all the curvatures times indices of refraction can be zero, so that the field of focus is flat. In other words, the negative lens can be as strong as the outer two combined, when one measures in diopters. Yet the lens will converge light, because the rays strike the middle element close to the optic axis. The curvature of field is determined by the sum of the diopters, but the focal length is not.
It was at that time a major advancement in lens design. The triplet design was made obsolete by later designs on high end cameras, but remained widely used up to this day on cheap cameras.
Despite the fact that the Cooke design was patented in 1893 it seems that the use of achromatic triplet designs in astronomy appeared as early as 1765. The 1911 encyclopedia Britanica wrote:
- The triple object-glass, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter, son of, John Dollond, and many excellent telescopes of this kind were made by him.
A similar design is used in the strong focusing synchrotron, invented first by Nicholas Christofilos in 1949, but his work was not known in the U.S., where parallel development took place.
See also: Chromatic aberration
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