apochromatic optics (original) (raw)

Definition: optical elements or systems designed to minimize chromatic aberrations to a greater extent than standard achromatic optics

Category: general optics

• optical elements
  • achromatic optics
    * apochromatic optics

Related: achromatic opticschromatic aberrations

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DOI: 10.61835/8w1 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What Are Apochromatic Optics?

Apochromatic optics are optical elements or systems used in high-end instruments to minimize chromatic aberrations to a greater extent than standard achromatic optics. While ordinary achromatic optics correct axial chromatic aberration at two wavelengths (typically in the red and blue regions), apochromatic optics achieve this correction at three distinct wavelengths (commonly red, green, and blue). For beam focusing, for example, this leads to a single focal position for three different colors and typically reduces chromatic aberrations across a wider range of wavelengths compared to achromatic or non-corrected optics.

In so-called super-achromats, the correction may extend to four wavelengths. As a result, apochromatic systems offer exceptionally low chromatic aberrations over broad spectral ranges.

Apochromatic designs also help reduce transverse chromatic aberrations — color fringes at image edges — although minimizing axial aberrations does not automatically eliminate transverse effects.

In some cases, apochromatic optics also improve performance in other respects. For example, apochromatic objectives can offer a better light gathering power, since optical apertures can be wider without leading to excessive chromatic effects.

Apochromatic Lenses

Ordinary achromatic lenses are constructed from two types of optical glasses. In contrast, apochromatic lenses require three different glass types, providing the additional degrees of freedom necessary for superior chromatic correction. Commonly used triplet lens designs include a crown glass, a flint glass and an ED (extra-low dispersion) fluoride glass. The precise thickness and curvature of each component are determined using numerical methods.

Apochromatic lens elements are often cemented together for stability, although air-spaced designs are also in use.

Apochromatic Systems

Apochromatic correction can be applied not only to individual lenses but also to more complex optical systems, including:

• Microscope objectives are multi-element objectives designed for precise color correction.
• Apochromatic refractor telescopes often have three lens elements.
• Apochromatic relay optics are used in applications such as endoscopy, where faithful color reproduction is essential.
• Similarly, there are apochromatic condensers, beam expanders and fiber launch systems.

Combining refractive optics with diffractive optical elements can also be used to obtain apochromatic performance.

Applications of Apochromats

Apochromats are used in high-end optical systems, delivering excellent imaging quality over a wide spectral range, e.g. throughout the visible region. Typical uses include:

• In some microscopes, special oil immersion apochromats are used, which combine high numerical aperture for high image resolution with excellent chromatic properties.
• Astronomical refractive telescopes can use apochromatic designs, preventing chromatic halos around stars. (Note the reflective telescope types avoid chromatic aberrations altogether.)
• Apochromatic photographic lenses are used in high-end photo cameras for professional-quality imaging.

Less common is the use with lasers at multiple wavelengths, e.g. laser-based RGB sources.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What are apochromatic optics?

Apochromatic optics are systems designed to correct chromatic aberrations at three distinct wavelengths (e.g., red, green, and blue). This provides superior performance over standard achromatic optics, which only correct for two wavelengths, by bringing more colors to a single focal point.

How do apochromatic lenses differ from achromatic ones in their construction?

While achromatic lenses typically use two types of optical glass, apochromatic lenses are constructed from three different glass types, such as a combination of crown, flint, and extra-low dispersion (ED) glass. This allows for a higher degree of chromatic correction.

What are the main advantages of using apochromatic optics?

The primary advantage is exceptionally low chromatic aberration over a broad spectral range, resulting in sharper images with faithful color reproduction. They can also allow for wider optical apertures, improving light-gathering power without introducing significant color distortions.

In which applications are apochromatic optics typically used?

Apochromatic optics are used in high-end instruments where precise color fidelity is essential. Common applications include professional photographic lenses, high-performance microscope objectives, and astronomical refractor telescopes for preventing chromatic halos around stars.

What is a super-achromat?

A super-achromat offers an even higher level of color correction than an apochromat. It is designed to correct for chromatic aberration at four different wavelengths, providing extremely low aberration over a very wide spectral range.

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