A dialyte lens (sometimes called a ''dialyt'') is a compound

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

design that corrects

optical aberration In optics, aberration is a property of optical systems, such as Lens (optics), lenses and mirrors, that causes the ''image'' created by the optical system to not be a faithful reproduction of the ''object'' being observed. Aberrations cause the i ...

s where the lens elements are widely air-spaced. The design is used to save on the amount of glass used for specific elements or where elements can not be cemented because they have dissimilar curvatures. The word ''dialyte'' means "parted", "loose" or "separated" in

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Design

In its simplest form, a ''dialyte'' can be formed by separating the elements in a cemented achromatic doublet of positive and negative lenses, although the powers of the individual elements must be increased to compensate.

Applications

Telescopes

The idea of widely separating the color correcting elements of a lens dates back to W. F. Hamilton's 1814

catadioptric A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses ( dioptrics) and curved mirrors ( catoptrics). Catadioptric combinations are used in focusing systems such as searchlig ...

Hamiltonian telescope and Alexander Rogers' 1828 proposals for a ''dialytic refractor''. The goal was to combine a large crown glass

objective Objective may refer to: * Objectivity, the quality of being confirmed independently of a mind. * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pron ...

with a much smaller

flint glass Flint glass is optical glass that has relatively high refractive index and low Abbe number (high dispersion). Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractiv ...

downstream to make an

achromatic lens An achromatic lens or achromat is a lens (optics), lens that is designed to limit the effects of chromatic aberration, chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into ...

since flint glass at that time was very expensive. Dialyte designs were also used in the Schupmann medial telescope designed by in German optician Ludwig Schupmann near the end of the 19th century, in John Wall's 1999 "''Zerochromat''" retrofocally corrected dialytic refractor and the Russian made "''TAL Apolar125''" telescope which uses 6 elements arranged in three widely separated groups.

Photography

File:Celor lens (Wix).svg, ''Dagor Type B'' (later ''Celor''/''Syntor'') File:Warmisham US1361207A (Aviar, 1917).svg, Taylor, Taylor & Hobson ''Aviar'' (Warmisham, 1917) There are many types of dialyte camera lenses. One popular design is perfectly symmetric, which provides good correction for many aberrations. This consists of two air-spaced achromatic doublets arranged back-to-back around a central stop, or four air spaced lens elements in total: the outer pair is biconvex and the inner pair is biconcave; one example is the ''Celor''. The Swiss mathematician Emil von Höegh, who had designed the popular ''Dagor''

anastigmat An anastigmat or anastigmatic lens is a photographic lens completely corrected for the three main optical aberrations: spherical aberration, coma (optics), coma, and Astigmatism (optical systems), astigmatism. Early lenses often included the wor ...

lens for Goerz in 1892, continued to refine that design, resulting in the Goerz ''Dagor Type B'' lens of 1899, later renamed to ''Celor'' and ''Syntor''. The ''Aviar'' lens ( Taylor Hobson) designed by Arthur Warmisham (1917) is similar but is considered to have a different origin, from the splitting of the central biconcave element of the

Cooke triplet The ''Cooke triplet'' is a photographic lens designed and patented in 1893 by Dennis Taylor who was employed as chief engineer by T. Cooke & Sons of York. It was the first lens system that allowed the elimination of most of the optical distort ...

. The resulting two biconcave elements are closer together than in the Dialyte/ Celor design.

Enlarging

Since the aberrations remain constant over a wide range of object distances, and is favourable for fairly wide apertures, this design proved useful for enlarging lenses.

References