Glossary of terms
Published: 2006-08-30
Aberration - A deviation from the normal or correct. In optics, the creation of a blurred or distorted image due to physical properties of the optical elements. The failure of a refracting surface or lens to bring all rays from an object point toward a desired image point. This can result in image blur.
Aberration, astigmatic - Undesirable, unequal refraction of an optical system in which an object point is imaged as line foci.
Aberration, chromatic - Unequal refraction of different wavelengths of light; commonly producing colored fringes about an image; particularly encountered with strong plus lenses, and lenses of high refractive material.
Aberration, coma - A defect in imaging objects off the optical axis in which there is a bright central area and a tail of lesser brightness
Aberration, spherical - An optical defect caused by peripheral and paraxial rays focusing at different points along the optical axis of a lens.
Accommodation - The dioptric adjustment of the crystalline lens of the eye to obtain maximal sharpness of retinal imagery for an object of regard; change in shape of the lens when focusing at different distances. A stretching or relaxing of the eye muscles, which causes a change in focal length of the crystalline lens, thereby producing clear images on the retina of objects that are relatively near the eye. Without the ability to accommodate, the image of the object would blur.
Addition
1. The difference in front vertex power between the reading or intermediate portion of a multifocal lens and its distance portion.
2. Another term for the bifocal reading segment.
Aberration, astigmatic - Undesirable, unequal refraction of an optical system in which an object point is imaged as line foci.
Aberration, chromatic - Unequal refraction of different wavelengths of light; commonly producing colored fringes about an image; particularly encountered with strong plus lenses, and lenses of high refractive material.
Aberration, coma - A defect in imaging objects off the optical axis in which there is a bright central area and a tail of lesser brightness
Aberration, spherical - An optical defect caused by peripheral and paraxial rays focusing at different points along the optical axis of a lens.
Accommodation - The dioptric adjustment of the crystalline lens of the eye to obtain maximal sharpness of retinal imagery for an object of regard; change in shape of the lens when focusing at different distances. A stretching or relaxing of the eye muscles, which causes a change in focal length of the crystalline lens, thereby producing clear images on the retina of objects that are relatively near the eye. Without the ability to accommodate, the image of the object would blur.
Addition
1. The difference in front vertex power between the reading or intermediate portion of a multifocal lens and its distance portion.
2. Another term for the bifocal reading segment.
Anti-Reflective Coating (A/R) - A single or multi-layered application of magnesium floride to a refractive surface designed to reduce by interference the amount of light normally reflected from the surface. Also, when applied to ophthalmic lenses, such coatings increase light transmittance and cause the medium to be less apparent, or causes the lens to "disappear." They reduce ghost images, and reduce eyestrain caused by the lighting commonly used office buildings and staring at computer screens for extended periods of time. They are also one of the best remedies for the strain caused by oncoming headlights.
Axis - A straight line perpendicular to both faces of a lens, along which path rays of light will pass through a lens without deviation, and without being deflected. It represents the locus of the lens at which the prism power is zero. It will intersect a spherical lens of a minus power at its thinnest point and a spherical lens of plus power at its thickest point.
Base Curve - The term refers to the curvature of the finished side of an ophthalmic lens.
Bevel apex - The point on the bevel of a lens.
Crystalline lens - That portion of the eye which further refracts the light and focuses it on the retina.
Decentration - The process of moving the major reference point to a place in the frame that is away from the geometric center of the frame.
Density - Measurement of weight based on a certain amount of material. The weight of a lens material is reflected as its density.
Diopter, prism - A unit of measurement used to express the angle of deviation of a ray of light by a prism or lens. In these units, prism power is measured in centimeters as the displacement of a light ray perpendicular to its line of incidence at a distance of one meter.
Distance, interpupillary - The linear distance between the fixation axes of the wearer's two eyes. It is commonly referred to as the distance between the centers of the pupillary openings with the eyes focused on a distant object.
Meridians, principal T - The two mutually perpendicular meridians of a sphero-cylinder lens or toric optical surface with minimum and maximum power.
Refraction - The bending of light rays caused by prisms and lenses. See power, refractive.
Author: Source: Dictionary of Ophthalmic Optics