The deviation from a single focus
of light rays emanating from one source.
American Board of Opticianry
Descriptive of a lens or lens
system capable of refracting light without creating a rainbow
The difference in plus power between the reading and
distance portions of a multifocal lens.
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.
Thinnest edge of a prism.
Descriptive of a lens surface having an
infinite number of curves with different radii.
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.
(optical) The imaginary line passing
through the optical center perpendicular to the lens surface. (Cylinder)
The meridian of no power in a cylinder.
A lens, typically requested for patients
with no vision, of similar power and thickness of the lens for the eye
Thickest edge of a prism.
A group curve, a curve used as a base for a series
of powers. In the case of toric lenses, the curve of rotation.
Both lens surfaces, front and back, are
Both lens surfaces, front and back, are
Pertaining to vision with both eyes.
An instrument used to measure lens
thickness, usually calibrated in 10ths of a millimeter.
The condition of the eye in which the
crystalline lens or lens capsule becomes opaque.
Continuing Education credit
Center Of Rotation:
The point near the center of the
eye about which it rotates-approximately 13.0mm behind the
The convergence of the component colors of
white light rays to different foci.
Spherical aberration of oblique pencils of
light, so named because of the comet shaped image of a point formed by a
lens exhibiting coma.
Two or more rays proceeding inward toward a
The 180 degree meridian through the center
of the lens from which the axis is measured.
The distance between lenses.
The distance between the optical center of
a lens and its geometric center.
Degree of opaqueness in a lens.
The change of direction of a ray of light
as in passing through a prism.
The scattering of light.
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
The separation of light into its component
colors as happens when passing through a prism.
The defect in a lens which causes a
straight line to appear curved.
Two or more rays proceeding outward from
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
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.
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.
That point where the diagonals of a "boxed"
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.
In optical science, the term "infinity" is
used to denote a distance so great that rays of light from it appear to
The horizontal distance from the 90 degree
meridian of the distance optical center of a bifocal lens to the optical
center of the segment.
A tool which has been cut to a prescribed
surface curvature and is used for grinding and polishing lenses.
The point on a lens surface around which a
lens is cut and edged.
A line at right angles to the optical axis
of an optical system.
Composed of light of one color.
Pertaining to vision with one eye.
Process done on very high minus lenses to
reduce edge thickness.
The combining of two lenses of opposite
powers so as to produce a resultant without power.
Slanting, other than perpendicular or
Impervious to light, not transparent.
Pertaining to the eye.
A theoretical point in a lens through which
all secondary axes and the optical axis pass.
Lens blanks larger than standard diameter.
The apparent displacement of an object due
to a change in the position of the observer's eye.
A group of light rays emanating from a
point, or converging toward a point.
Glass and plastic lenses that change color
in sun light.
Flat, without curvature.
A condition wherein the accommodative power
of the eye decreases with advancing age.
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.
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.
The throwing back of light incident upon a
The change in direction of light as it
passes obliquely from one medium to another of different density.
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
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.
A surface, every point of which is
equidistant from a point within called the center.
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
Single vision finished lenses cast by the
lens manufacturer. Available in a limited power range and lens
The ratio (percent) of transmitted light to
incident light for one part of the spectrum.
Changing of expression of the powers of a
lens without changing its refractive power.
Laboratory surfaced lenses. Available in
all lens styles, powers and materials. Not edged.
The point at which the optical axis of a
lens intersects the ocular surface.
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.