Achromatic lens

Lens that is designed to limit the effects of chromatic and spherical aberration

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"Achromat" redirects here. For the form of color blindness, see achromatopsia

Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths. An achromatic doublet brings red and blue light to the same focus, which exemplifies the concept of an achromatic lens. In an achromatic lens, specifically two wavelengths are focused together, typically red and blue. Achromatic lenses are engineered to minimize both chromatic and spherical aberration effects. These lenses are specially refined to converge two distinct wavelengths, commonly red and blue, onto the same focal plane. Consequently, wavelengths between these two achieve improved focus compared to those attained with a standard lens. The predominant form of achromat is the achromatic doublet, consisting of two distinct lenses fabricated from glasses with varying levels of dispersion. Typically, one lens constitutes a negative (concave) element crafted from high-dispersion flint glass like F2, while the other encompasses a positive (convex) element of lower-dispersion crown glass such as BK7. These lens components are positioned adjacent to one another, frequently cemented together, and shaped precisely so that the chromatic aberration produced by one is neutralized by that of the other. In typical designs, the positive power of the crown lens does not fully counterbalance the negative power of the flint lens. Together, they generate a weak positive lens that effectively aligns two disparate wavelengths toward a mutual focal point. Variations known as negative doublets, where the concave element takes precedence, are also manufactured.

History

Theoretical discussions regarding the possibility of correcting chromatic aberration emerged in the 18th century, following Newton's assertion that such corrections were unattainable (refer to History of the telescope). An English barrister and amateur optician named Chester Moore Hall is often credited with the creation of the first achromatic doublet. Hall, aiming to keep his work under wraps, hired two different opticians, Edward Scarlett and James Mann, for the production of crown and flint lenses. These craftsmen, in turn, subcontracted the work to George Bass, who recognized that the two components were intended for the same purpose and discerned the achromatic properties upon assembling them. Although Hall utilized his invention to construct the pioneering achromatic telescope, it did not gain widespread recognition at that time. In the late 18th century, Bass disclosed Hall's designs to John Dollond, who acknowledged their significance and successfully replicated the design. Dollond eventually applied for and received a patent for the technology, which resulted in intense disputes with other opticians over production rights to achromatic doublets. Later on, Dollond's son, Peter, enhanced the achromat, creating the apochromat.

Types

Various types of achromats have been developed, differing in the shape of the lens elements and the optical properties of their glass, particularly in terms of their optical dispersion or Abbe number.

For the following descriptions, R signifies the sphere radii that determine the refracting lens surfaces. By convention, R1 indicates the initial lens surface counted from the object. A doublet lens features four surfaces with radii R1 through R4. Surfaces with positive radii curve away from the object (R1 positive is a convex surface), while negative radii curve towards it (R1 negative is concave).

Littrow doublet

Employs a crown glass lens with equiconvex properties and a corresponding curved second flint glass lens. The back of the flint glass lens remains flat, facilitating imaging between specific lens radii.

Fraunhofer doublet (Fraunhofer objective)

Features a first lens with positive refractive power and a second with a negative one, enabling superior performance in compact configurations.

Clark doublet

Initially following the Fraunhofer design, early Clark lenses evolved to integrate Littrow principles to minimize ghosting effects by adjusting specific lens parameters.

Oil-spaced doublet

Utilizes oil between the two lens elements to mitigate ghosting instances and enhance light transmission while compensating for manufacturing variations.

Steinheil doublet

Created by Carl August von Steinheil, this design represents a departure from the conventional Fraunhofer doublet by employing a negative lens first, necessitating more rigorous curvature adjustments.

Dialyte

Historically devised with broader airspaces to accommodate smaller flint glass elements, these lenses are noteworthy for their unique manufacturing challenges.

Design

The primary design process for an achromat encompasses two main factors: the total optical power of the doublet and the selection of the glass types to utilize. This choice contributes to the mean refractive index and the Abbe number necessary for effective color correction. Optically, a functional achromat must adhere to several mathematical equations governing its behavior.

Optical aberrations beyond chromatic errors are prevalent across all lenses, necessitating further refinements for an improved imaging quality. Contemporary design techniques prioritize minimizing non-color-related optical discrepancies.

Further color correction

More sophisticated lens designs can refine image clarity further by accurately aligning additional wavelengths. However, these require utilizing premium materials, precise shaping, and spacing of lens combinations, which escalates costs.

• Apochromatic lenses: Integrate three wavelengths into a unified focal point.
• Superachromatic lenses: Align four wavelengths and necessitate advanced manufacturing techniques and materials.

In highly advanced systems, several simple lenses may be incorporated to achieve optimal color focus, all while navigating challenges associated with rising production costs and minimized returns in image quality enhancements.

See also

References

• Achromatic lenses at Wikimedia Commons

What Makes Achromatic Lenses Important

Achromatic lenses are crucial as they are capable of focusing various colors at a common point, thus enabling users to achieve fully focused images. In contrast with uncorrected singlet lenses, they yield clearer images that enhance observational accuracy.

Since their inception, achromatic lenses have been transformative in imaging methodologies. Despite ongoing advancements in lens quality, they remain instrumental in both scientific and general optical applications. Key benefits of the achromatic combination of lenses include:

1. Enhanced image quality: They significantly diminish color fringing, boosting brightness and clarity, particularly in multi-color imaging scenarios.
2. Efficient light transmission: Their coaxial performance is maintained even with enlarged aperture sizes, allowing the use of the entire transparent area.
3. Cost-effective production: Although improvements are possible, achromatic lenses offer substantial correction capabilities, making them an economical selection for clear white light image acquisition.

Achieving the intended color correction necessitates the collaboration of multiple types of optical glass, each exhibiting varied dispersion characteristics. Typically, this involves the integration of concave lenses with high dispersion and convex elements exhibiting lower dispersion levels, arranged to ensure the deformations of one lens effectively counterbalance those of the other. This configuration results in an achromatic doublet lens, recognized as the prevalent form of achromatic lens, although triplet variations also exist.