Common Optical Terms

Here are the definitions of some common optical terms

Aberration: The deviation from a single focus of light rays emanating from one source.

Achromatic: Descriptive of a lens or lens system capable of refracting light without creating a rainbow effect.

Addition: The difference in plus power between the reading and distance portions of a multifocal lens.

Anti-Reflection Coating: A multi-layered coating that is applied in a vacuum deposition chamber. AR reduces reflections and when applied on a plastic lens that has front and back scratch resistant coating allows better adhesion and more scratch resistance.

Apex: Thinnest edge of a prism.

Aspheric: Descriptive of a lens surface having an infinite number of curves with different radii.

Astigmatism: A defect in an optical system whereby a point is imaged as a line or a pair of lines instead of a point.

AVN ( anti-visual noise): Chemically treated lenses that reduce visual stress from eyestrain, blurred vision, irritated eyes, headache and color perception changes, connected with VDT use.

Axis: (optical) The imaginary line passing through the optical center perpendicular to the lens surface. (Cylinder) The meridian of no power in a cylinder.

Balance Lens: A lens, typically requested for patients with no vision, of similar power and thickness of the lens for the eye with vision.

Base: Thickest edge of a prism.

Base Curve: A group curve, a curve used as a base for a series of powers. In the case of toric lenses, the curve of rotation.

Bi-Concave: Both lens surfaces, front and back, are concave.

Bi-Convex: Both lens surfaces, front and back, are convex.

Binocular: Pertaining to vision with both eyes.

Caliper: An instrument used to measure lens thickness, usually calibrated in 10ths of a millimeter.

Cataract: The condition of the eye in which the crystalline lens or lens capsule becomes opaque.

Center Of Rotation: The point near the center of the eye about which it rotates-approximately 13.0mm behind the cornea.

Chromatic Aberration: The convergence of the component colors of white light rays to different foci.

Coma: Spherical aberration of oblique pencils of light, so named because of the comet shaped image of a point formed by a lens exhibiting coma.

Convergent: Two or more rays proceeding inward toward a point.

Cutting line: The 180 degree meridian through the center of the lens from which the axis is measured.

DBL: The distance between lenses.

Decentration: The distance between the optical center of a lens and its geometric center.

Density: Degree of opaqueness in a lens.

Deviation: The change of direction of a ray of light as in passing through a prism.

Diffusion: The scattering of light.

Diopter: The unit of measurement of the refractive power of a lens. A lens whose focal length is one meter has a power of one diopter. A lens whose power is four diopters has a focal length of one fourth of a meter.

Dispersion: The separation of light into its component colors as happens when passing through a prism.

Distortion: The defect in a lens which causes a straight line to appear curved.

Divergent: Two or more rays proceeding outward from point one.

Edged: Lenses cut to shape and size of frame.

Equithin (cosmetic prism): The grinding of equal amounts of base down prism to equal the thickness of the top and bottom of the lens. It is especially effective on plus and high add prescriptions of all one piece multifocals, including progressives and executive style lenses.

Far Point: The point in space which is sharply imaged on the retina when the accommodation is relaxed. The far point of an emmetropic eye lies at infinity. As the eyeball rotates in its socket, the visual axis also rotates. The far point, being upon the visual axis, may occupy any position upon the far point surface.

Focus: the point to which the rays of a pencil of light converge or from which they appear to diverge. In plus lenses, the focus is a real focus; in minus lenses, it is a virtual focus.

Geometric Center: That point where the diagonals of a "boxed" lens cross.

Index Of Refraction: The ratio of the sine of the angle of incidence to the sine of the angle of refraction of a ray of light traversing the surface between two media; also the ratio of the speeds of light in two media.

Infinity: In optical science, the term "infinity" is used to denote a distance so great that rays of light from it appear to be parallel.

Inset: The horizontal distance from the 90 degree meridian of the distance optical center of a bifocal lens to the optical center of the segment.

Lap: A tool which has been cut to a prescribed surface curvature and is used for grinding and polishing lenses.

Mechanical Center: The point on a lens surface around which a lens is cut and edged.

Meridian: A line at right angles to the optical axis of an optical system.

Monochromatic: Composed of light of one color.

Monocular: Pertaining to vision with one eye.

Myo-Disc: Process done on very high minus lenses to reduce edge thickness.

Neutralization: The combining of two lenses of opposite powers so as to produce a resultant without power.

Oblique: Slanting, other than perpendicular or parallel.

Opaque: Impervious to light, not transparent.

Ophthalmic: Pertaining to the eye.

Optical Center: A theoretical point in a lens through which all secondary axes and the optical axis pass.

Oversize: Lens blanks larger than standard diameter.

Parallax: The apparent displacement of an object due to a change in the position of the observer's eye.

Pencil: A group of light rays emanating from a point, or converging toward a point.

Photo chromic: Glass and plastic lenses that change color in sun light.

Plane: Flat, without curvature.

Presbyopia: A condition wherein the accommodative power of the eye decreases with advancing age.

Prism: A wedge-shaped piece of glass/plastic having plano or curved sides. Also understood as a point in a lens outside of the optical center at which a radial displacement of a beam is produced.

Prism Diopter: The unit of measurement of the deviation of light by a prism. A prism which has a deviation of one prism diopter produces a deviation of one centimeter at a distance of one meter.

Pupillary Distance (PD): The distance between the centers of the pupils when in distance and reading vision positions.

Reflection: The throwing back of light incident upon a surface.

Refraction: The change in direction of light as it passes obliquely from one medium to another of different density.

Scratch Coat: A coating applied on the front or front and back surface of a plastic lens to provide scratch resistance.

Slab-Off (laboratory ground): Most effective when there is a vertical imbalance of two diopters or more at reading. Prism is ground in the lens of the highest minus or weakest plus, to neutralize vertical imbalance.

Slab-Off (reverse): Same benefits as laboratory ground slab off. The difference being it is made at the factory on the opposite lens. The weakest minus or strongest plus lens.

Sphere: A surface, every point of which is equidistant from a point within called the center.

Spherical Aberration: The defect in a lens due to its spherical surfaces, whereby rays of light incident upon the lens at different distances from the optical axis are not converged by the lens to a common focus.

Stock Lens: Single vision finished lenses cast by the lens manufacturer. Available in a limited power range and lens materials.

Transmittance (Transmission): The ratio (percent) of transmitted light to incident light for one part of the spectrum.

Transposition: Changing of expression of the powers of a lens without changing its refractive power.

Uncut: Laboratory surfaced lenses. Available in all lens styles, powers and materials. Not edged.

Vertex: The point at which the optical axis of a lens intersects the ocular surface.

Vertex Power: The refractive power of lens measured from its vertex to its principal focus. Vertex power is the significant factor in determining the power of a corrective lens.