Glossary: O

The angle at which an off-axis mirror directs and focuses incident collimated light.

Describes the amount of energy that can pass through an optical material. It is directly related to the transmittance of the material, which is the ability of light to propagate through a given medium. A large OD means more blocking.

A type of optic comprised of a core surrounded by a relatively thin cladding layer of lower index material. This geometry causes internal reflection of light inside the fiber and defines the maximum angles accepted by the fiber. An optical fiber emits into a cone of equal angle, not as a beam of light. Coherent fiber bundles can relay images from one end to the other; whereas, incoherent bundles are used for propagation of light alone.

A precisely polished flat surface, used as a reference against which the flatness of an unknown surface may be compared.

A useful tool that allows optical designers to determine various values without having to completely ray trace a system. It is obtained by comparing two rays within a system at any axial point. It is constant for any two rays at every point in the system. In other words, if the invariant for a set of two rays is known, ray trace one of the rays and then scale that by the invariant to find the second.

For a perfect optical system, the optical path, or distance, from an object point to a corresponding image point will be equal for all rays. In near-perfect systems, slight differences will exist between rays resulting in an optical path difference, usually expressed in fractions of the wavelength being analyzed.

A system in which a supplied signal causes the deflection of the electron beam in a cathode-ray tube, thus forming a visible trace on the phosphor screen of the tube, allowing for examination of signal characteristics.

The power measured at the exit of a laser cavity, commonly expressed as minimum output or maximum output. The class designation of a laser is based in part upon this characteristic.