Cooke-Triplet
The Cooke triplet has a dreilinsiges lens that corrects the chromatic aberration of two wave lengths and, in addition to the astigmatism. Triplet is engl. for triple, triple or even Dreilinser. The Cooke triplet is a classic lens design. It was first described in 1893 by Harold Dennis Taylor and, after his employer, the company Thomas Cooke & Son, named. However, Ernst Abbe reported in 1890 a patent for a triplet structure at.
Design and Function
The name alludes to the composition of three free-standing lenses. Usually there is a triplet of two outer collecting members ( of soda lime glass ) and an inner dispersion member ( flint glass). The combination of the first two lenses results in a very long focal length. This long focal length is shortened by a third lens further away again. The chromatic aberration is corrected for two wavelengths. The Cooke triplet is therefore an achromatic lens system.
The aperture of a triplet is almost always placed in the vicinity of the central negative lens. In projection lenses, the diverging lens is often so small that without an aperture gives the correct light intensity. Mostly a portion of the lens surface is even blackened the edges to reduce light intensity.
In historical perspective glasses have been used with ever higher refractive index for the two outer Kronglaslinsen. This will let light intensity, increase angle and correction clearly. The first triplets had a luminous intensity of 1:6,8. About 1:4.5 could be increased in the late 1930s to 1:3.5. After the Second World War triplets were used with a speed of 1:2.8, because now new glasses with improved optical properties were available. Thus, the outer Kronglaslinsen were initially made of glass places like SK16 (n = 1.62), and later from LaK8 (n = 1.71 ), and finally from LaFN21 (n = 1.79 ). Especially now containing lanthanum crown glasses are at a very high refractive index used in a relatively low dispersion ( for example, the Schott Glass LaFN21 ).
The construction of the triplets but is also suitable for optimal choice of glass types not suitable for large picture angle (see wide-angle lens ) because - as with larger light intensities - the aberrations can be brought under control anymore. The coma and astigmatism, however, are at a moderate opening width (initial intensity ) - ie today to 1:2.8 usually - usually already well corrected. Lenses by type of Cooke triplets were therefore (besides lenses of a different design ) also marketed as " anastigmats ". One example is the Novotrinast - Anastigmat, which serves as a projection lens of the Liesegang " E8 " episcopes.
For derived types of lenses (see below) with more than three lenses are single lenses often replaced by cemented elements ( see, eg, Tessar and Heliar ).
Optical lens details
By Gross et al. (2008) has a Cooke triplet of thin lenses eight degrees of freedom for the correction of aberrations: Each of the three spherical lenses has two radii. Furthermore, there are two lens distances. These degrees of freedom are sufficient to eliminate the five monochromatic aberrations of the third order and the two most important chromatic aberrations as well as set the desired focal length. A lens system without the curvature of field can thus be achieved but only with a suitable choice of the types of glass.
There also exist according to Gross et al. (2008) a set of design guidelines that lead to a usable triplet.
1, the refractive power of the rear lens should be about 20% higher than that of the front lens.
2 The power of the front lens should be about one and a half times the total refractive power of the triplets have.
3 Are all aberrations completely eliminated, resulting in a system with large zone errors. A compromise solution would be more appropriate.
4 The reduction of the field curvature requires two positive outer lenses made from high index crown glass and a low refractive index, negative flint glass lens in the middle.
Applications
Zeiss Triotar on a Rollei rangefinder camera
The Domiplan, a standard lens by Meyer -Optik
The triplet quickly developed to the standard design of precision optics, especially with inexpensive cameras. For decades, possessed almost every compact camera viewfinder via a triplet lens with a fixed focal length. Examples of classic triplet lenses are
- " Triotar " by Carl Zeiss, Jena
- " Cassar " by C. A. Steinheil & Söhne, Munich
- " Trioplan " and later " Domiplan " Meyer -Optik, Görlitz
Widely used is the use of Cooke triplets as a lens for projection of slides. Well-known examples are the German tradition manufacturer Patrinast of Ed. Liesegang oHG, the Stellar Stäble of the plant, the Projar of ISCO Göttingen, the Diaplan of Meyer- Optik ( Goerlitz ), the Maginon of Will, Wetzlar, or Diarectim of Rathenower Optical drives. The most common light intensity is 1:2.8 at a focal length of 85-90 mm.
Projection lens Stellar ( Stäble plant) 1:2.8 / 85
Projection lens Maginon (Will, Wetzlar ) 1:2.8 / 100
Projection lens Diarectim the VEB Rathenower Optical Works for epidiascope
The projection of opaque originals by episcopes and epidiascopes is also done mostly by lenses on the type of Cooke triplets. The lenses are sometimes very large. The built- in E8 Episkop Novotrinast - Anastigmat of Ed. Liesegang oHG has a lens diameter of 10 cm ( light intensity 1:3.6 ), for example, at a focal length of 330 mm. The intended use for extra projection tasks " Meganast - Anastigmat " of the same company, there teaches 1:5,6 / 1000 mm lenses of almost 20 cm in diameter with a weight of about 7 kg.
Derived types of lenses
Tessar (Zeiss, 1902)
The bright Primo plan of Meyer -Optik, Görlitz
The fünflinsige Heliar was developed by Voigtländer in 1900. The classical Heliar were each replaced by an element from each of two cemented lenses, both the front and the rear lens. A derived type of lens is also developed in 1902 with Zeiss Tessar. Here there is only the rear element composed of two cemented lenses. The Primo plan of Meyer -Optik was a fünflinsige, bright Tripletvariante.
As a further, highly derived types the Ernostar and the Sonnar Zeiss apply.