Encyclopedia … combined with a great Buyer's Guide!

Afocal Optical Systems

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

Alternative term: telescopic systems

German: afokale optische Systeme

Categories: general optics, vision, displays and imaging


How to cite the article; suggest additional literature

URL: https://www.rp-photonics.com/afocal_optical_systems.html

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 than 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.

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 then also parallel output rays are sent to the observing eye (accommodated to infinite distances), where they are finally focused to 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.

refractive telescopes
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 in order to obtain a larger mode radius in the laser crystal.

See also: ABCD matrix, geometrical optics, telescopes

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

Your question or comment:

Spam check:

  (Please enter the sum of thirteen and three in the form of digits!)

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.


Share this with your friends and colleagues, e.g. via social media:

These sharing buttons are implemented in a privacy-friendly way!

Code for Links on Other Websites

If you want to place a link to this article in some other resource (e.g. your website, social media, a discussion forum, Wikipedia), you can get the required code here.

HTML link on this article:

<a href="https://www.rp-photonics.com/afocal_optical_systems.html">
Article on Afocal optical systems</a>
in the <a href="https://www.rp-photonics.com/encyclopedia.html">
RP Photonics Encyclopedia</a>

With preview image (see the box just above):

<a href="https://www.rp-photonics.com/afocal_optical_systems.html">
<img src="https://www.rp-photonics.com/previews/afocal_optical_systems.png"
alt="article" style="width:400px"></a>

For Wikipedia, e.g. in the section "==External links==":

* [https://www.rp-photonics.com/afocal_optical_systems.html
article on 'Afocal optical systems' in the RP Photonics Encyclopedia]