Achromatic lens

A lens is called as an achromatic or achromatic, when the change of the back focal length at the wavelength for a wavelength disappears.

Because of the dispersion of optical materials shows each individual lens the so-called longitudinal chromatic aberration. In this case, the refractive index decreases continuously from red to blue to the focal length of the lens from. By combining two lenses of glasses with different steep refractive index profile, see Abbe number can be a reversal in the average length reached by the wavelength.

If the two lenses are thin and have a small distance from each other, can also be the lateral chromatic aberration, which causes particular at the edge of the image field disturbing color fringing at the edges of the observed objects are largely corrected.

Invention in the middle of the 18th century

The first achromatic doublet lenses were designed about 1733 by the English amateur optician Chester Moor Hall. Hall wanted to keep his work secret and hired two different companies with the manufacture of crown glass and flint glass lenses ( Edward Scarlett and James Mann ). However, this gave both the same optician to sub-contract the actual manufacturing, George Bass. He recognized that both lenses for the same customers were. After the two lenses attached to each other, he recognized the achromatic properties. Hall itself, however, did not recognize the carrying length of his invention and remained largely unknown.

In the late 1750s, Bass mentioned to the invention Hallsche the French-born Englishman John Dollond, who recognized their potential and could reproduce their production principle. Dollond submitted a patent that was granted to him in 1758. Around 1760 he built the first good, equipped with achromatic lenses telescopes. The lenses were clear case connected to each other at their contact surface ( cemented ), thereby interfering reflections remain low. Previously used it to Canada balsam. The apertures ( lens diameter ) were initially limited to 2 to 3 inches, but provided over one -lens lenses that you had to stop down heavily for a sharp image, represents a huge step forward

By 1875, it already achieved openings of 25 inches ( for example, the large refractor of the Vienna University Observatory ), and up to the turn of the century 40 inches ( Yerkes Observatory ). But already encountered in such a one-meter lenses, the deflection of the lens to its limits, so you went back over to smaller lenses or mirrors on telescopes.

Developments

The achromatic corrected the primary spectrum, as is referred to the uncorrected color error between the two design wavelengths. A measure of the residual color error is called secondary spectrum.

From the original design of the Hall-/Dollond-Achromaten different variants were derived. So Joseph von Fraunhofer added the two lenses are not directly related, but left between them a narrow air gap ( " Fraunhofer doublet " or " Fraunhofer - Achromat "). The air gap can be varied to a certain extent in the width and allows mainly to give the opposite lens surfaces have a different radius. An achromatic lens with narrow air gap is also called Dialyt. With the obtained degrees of freedom can be corrected the dependency of the spherical aberration of the wavelength.

For smaller telescopes with the previously common focal ratio of about f/15 "slow" hardly occur disturbing color fringes in the observation on. For large achromatic refractor, which would have to reach at low image blurring caused by atmospheric turbulence in principle much higher resolutions, but the secondary spectrum at higher magnifications occurs again produced strong disturbing. The reason is that the absolute color residual error ( from the glass data dependent) in arc seconds for a given aperture ratio constant, in contrast to the diffraction- limited resolution does not decrease with increasing lens diameter.

Another variant of the achromatic doublet was marketed by CA Steinheil & Sons as " Steinheil Lens ". The arrangement of the lenses is slightly different than the standard achromat. First, frame -side member is a negative flint glass lens followed by a positive lens of crown glass. As with the Fraunhofer achromat separates a narrow air gap, the two lenses. The inner radii of the lenses are more curved than the Fraunhofer lens. To avoid error zone, an area is retouched by hand. This requires that the position of the lenses to each other can not be changed.

In the photograph were used as lenses from the beginning Achromat ( official date of invention: 1839). For portrait photography, the optically better Petzvalobjektiv was developed at around 1850. From about 1870 two slightly curved meniscus -shaped achromatic lenses were combined to Aplanat. Aplanats were formerly also for projectors and are also still used today for high-quality loupes. It followed as the next important invention, the sharper, 3 -element Cooke triplet. Today's photographic lenses often contain 5 to 10 lenses.

For further claims, especially in microscopy, so-called Apochromats have been developed which typically consist of three lenses.