afocal optical systems (original) (raw)

Author: the photonics expert (RP)

Definition: optical systems which output parallel rays for parallel input rays

Alternative term: telescopic systems

Categories: general optics, vision, displays and imaging

• imaging systems
  • afocal optical systems
  • cameras
  • hyperspectral imaging devices
  • infrared viewers
  • laser microscopy systems
  • LIDAR devices
  • microscopes
  • multispectral imaging systems
  • night vision devices
  • optical coherence tomography systems
  • telescopes
  • thermal imaging cameras
  • (more topics)

Related: ABCD matrixgeometrical opticstelescopes

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

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Contents

What are Afocal Systems?

Based on geometrical optics, an afocal optical system is defined as a system which outputs parallel light rays in cases with parallel input rays. Concerning the ABCD matrix of the system, this implies that the matrix component ($C$) is zero. Further, we have ($A \cdot D = 1$), assuming that the refractive index is the same on the input and output side. Such a system has no focal length, no focal points, no nodal points and no principal planes.

Other optical systems, not being afocal, are called focal or sometimes non-afocal.

The simplest example is a system with no focusing elements, but only free space, or space filled with a homogeneous optical medium. Here, we have ($A = D = 1$) in addition to ($C$) = 0.

Example: Telescope

The probably most prominent example is that of a telescope in its most basic configuration — a combination of two focal components (e.g. lenses). Such a telescope can be used for viewing distant objects, sending approximately parallel rays to the instrument, and the resulting also parallel output rays are sent to the observing eye (accommodated to infinite distances), where they are finally focused on the retina. Figure 1 shows two common realizations of refractive telescopes. Other realizations are based on curved mirrors or on prisms, e.g. in the form of anamorphic prism pairs.

Figure 1: Basic setups of refractive telescopes of (a) Keplerian and (b) Gallilean type.

Such an afocal telescope can not only be used as an optical addendum to the eye, but also in combination with a photo camera or an infrared viewer, for example. It then provides some amount of magnification, and at the same time there is a reduction of the field of view.

The term telescopic systems is often used for afocal systems, as the telescope is the classical example.

The magnification of a telescope equals the matrix component ($D$). The image on the retina of the viewing eye, which is generated by an object at a large distance, is enlarged by that factor.

Beam expanders are also afocal systems. Considered in the context of wave optics, such a beam expander converts a collimated input laser beam into a collimated output beam with increased beam radius. By turning it around, one can also have the beam radius decreased. In some cases, such a beam expander is used within a laser resonator, for example to obtain a larger mode radius in the laser crystal.

Frequently Asked Questions

What is an afocal optical system?

An afocal optical system is a system that outputs parallel light rays when the input rays are also parallel. Consequently, it has no focal length, focal points, or principal planes.

What is the defining characteristic of an afocal system in the ABCD matrix formalism?

For an afocal system, the C component of its ABCD matrix is zero. If the refractive index is the same on the input and output side, it also holds that the product ($A \cdot D = 1$).

What are common examples of afocal systems?

The most prominent examples are telescopes, used for viewing distant objects, and beam expanders, which are used to change the diameter of a collimated laser beam.

How is the magnification of an afocal telescope determined?

The angular magnification of an afocal telescope is equal to the ($D$) component of its ABCD matrix.

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