Optical system in the context of Ray (optics)

In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. The aperture defines a bundle of rays from each point on an object that will come to a focus in the image plane.

An optical system typically has many structures that limit ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. These structures are called stops, and the aperture stop is the stop that primarily determines the cone of rays that an optical system accepts (see entrance pupil). As a result, it also determines the ray cone angle and brightness at the image point (see exit pupil). Optical systems are typically designed for a particular stop to be the aperture stop, but it is possible for different stops to serve as the aperture stop for objects at different distances. Some rays from object points away from the optical axis may clip on surfaces other than the aperture stop. This is called vignetting. The aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system.

In an optical system (generally a lens), the entrance pupil is the optical image of the physical aperture, as 'seen' through the optical elements in front of the stop. The corresponding image of the aperture stop as seen through the optical elements behind it is called the exit pupil. The entrance pupil defines the cone of rays that can enter and pass through the optical system. Rays that fall outside of the entrance pupil will not pass through the system.

If there is no lens in front of the aperture (as in a pinhole camera), the entrance pupil's location and size are identical to those of the aperture. Optical elements in front of the aperture will produce a magnified or diminished image of the aperture that is displaced from the aperture location. The entrance pupil is usually a virtual image: it lies behind the first optical surface of the system.

Stray light is light in an optical system which was not intended in the design. The light may be from the intended source, but follow paths other than intended, or it may be from a source other than that intended. This light will often set a working limit on the dynamic range of the system; it limits the signal-to-noise ratio or contrast ratio, by limiting how dark the system can be. Ocular straylight is stray light in the human eye.

An image intensifier or image intensifier tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low-light conditions, such as at night, to facilitate visual imaging of low-light processes, such as fluorescence of materials in X-rays or gamma rays (X-ray image intensifier), or for conversion of non-visible light sources, such as near-infrared or short wave infrared to visible. They operate by converting photons of light into electrons, amplifying the electrons (usually with a microchannel plate), and then converting the amplified electrons back into photons for viewing. They are used in devices such as night-vision goggles.

A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. Sights are equipped with a referencing pattern (reticle) mounted in a focally appropriate position in its optical system to provide an accurate point of aim. Telescopic sights are classified in terms of the optical magnification (power) and the objective lens diameter.

The first experiments directed to give shooters optical aiming aids go back to the early 17th century. For centuries, different optical aiming aids and primitive predecessors of telescopic sights were created that had practical or performance limitations. Most early telescopic sights were fixed-power and were in essence specially designed viewing telescopes. Telescopic sights with variable magnifications appeared later, and were varied by manually adjusting a zoom mechanism behind the erector lenses. Other types of scopes include prism sights and low-power variable optics.

Optomechanics is the manufacture and maintenance of optical parts and devices. This includes the design and manufacture of hardware used to hold and align elements in optical systems, such as:

• Optical tables, breadboards, and rails
• Mirror mounts
• Optical mounts
• Translation stages
• Rotary stage
• Optical fiber aligners
• Pedestals and posts
• Micrometers, screws and screw sets

Optomechanics also covers the methods used to design and package compact and rugged optical trains, and the manufacture and maintenance of fiber optic materials

In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It is the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. Therefore, the directivity of a hypothetical isotropic radiator, a source of electromagnetic waves which radiates the same power in all directions, is 1, or 0 dBi.

An antenna's directivity is greater than its gain by an efficiency factor, radiation efficiency. Directivity is an important measure because many antennas and optical systems are designed to radiate electromagnetic waves in a single direction or over a narrow-angle. By the principle of reciprocity, the directivity of an antenna when receiving is equal to its directivity when transmitting.

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".

In optics, a relay lens is a lens or a group of lenses that receives the image from the objective lens and relays it to the eyepiece. Relay lenses are found in refracting telescopes, endoscopes, and periscopes to optically manipulate the light path, extend the length of the whole optical system, and usually serve the purpose of inverting the image. They may be made of one or more conventional lenses or achromatic doublets, or a long cylindrical gradient-index of refraction lens (a GRIN lens).

Relay lenses operate by producing intermediate planes of focus. For example, in a SLR camera the zoom lens produces an image plane where the image sensor or photographic film would usually go. If you place another lens with focal length f at the distance 2f from that image plane and then put an image sensor at 2f beyond that lens, that lens will relay the first image to the second image with 1:1 magnification (see thin lens formula showing that with object distance ${\displaystyle s=2f}$ from the lens, the image distance from the lens is calculated to ${\displaystyle s'=2f}$). Ideally, this second image is the mirror image of the first image, so you could put an image sensor there and record the mirrored first image. If a longer distance is needed, this can be repeated. In practice, the lens will be an achromatic doublet.

Folded optics is an optical system in which the beam is bent in a way to make the optical path much longer than the size of the system. This allows the resulting focal length of the objective to be greater than the physical length of the optical device. Prismatic binoculars are a well-known example. An early conventional film camera (35 mm) was designed by Tessina that used the concept of folded optics.

Fold mirrors are used to direct infrared light within the optical path of the James Webb Space Telescope. These optical fold mirrors are not to be confused with the observatory's deployable primary mirrors, which are folded inward to fit the telescope within the launch vehicle's payload fairing; when deployed, these segments are part of the three-mirror anastigmat design's primary element and don't serve as fold mirrors in the optical sense.