Glossary of Terms
Below you will find the various types of lenses available today, their descriptions and additional useful information that will assist you in understanding the optical industry.
[ A | B | C | D | E | F | G | H | I | L | M | O |
P | R | S | T | V | W | Z ]
A
Aberration
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 also results in curvature in the image of a straight line. It may be inherent in the lens design or may result from an error in processing.
Accommodation
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
- The difference in front vertex power between the reading or intermediate portion of a multifocal lens and its distance portion.
- Another term for the bifocal reading segment. In this case, the addition is a simple plus lens placed on top of a major distance lens.
Anti-Reflection Lenses
These lenses are coated with A/R to help eliminate reflections on the lens surface and reduce ghost images.
Anti-Reflection lenses also help 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.
Aperture
The opening in an ophthalmic frame into which a lens is inserted. Aperture dimensions (in millimeters) do not include the depth of the bevel groove. That principal meridian which contains only the spherical power component of a spherocylinder lens.
Axis, optical
The straight line perpendicular to both faces of a lens along whose path a ray will pass without being deflected. 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. If the lens has prism power, the optical axis may lie outside the lens.
B
B-dimension
A measurement of the Boxing System which measures the distance between horizontal tangents to the bevel of a lens.
Backorder
An order placed to fill a definite order from a dispenser. Also called short order, or special order.
Base
The finished side of a semi-finished blank. The term refers to the curvature of the finished side. See Curve, Base
Bevel apex
The point on the bevel of a lens.
Bifocal Lenses
Bifocal Lenses are useful in affording the patient 2 vision corrections. One for distance and the other reading.
Bin card
A piece of paper or card stock on which the inventory, sales and ordering of frames is recorded in the laboratory.
Blank, major
The basic lens substrate to which segments of different refractive power may be added to produce a multifocal lens.
Blank, molded
A blank that is unfinished on both sides when it arrives from the factory. It is used to grind lenses for non-standard prescriptions or prescriptions that are particularly strong.
Boxing System
A system of measurement used to define various prescription requirements relative to lens and frame dimensions.
Bridge
The supportive structural member connecting the two eyes of an ophthalmic frame front.
Bridge, keyhole
A bridge design for a front that does not permit continuous contact between the nose
and the front in the area of the nasal crest.
Bridge, saddle
A bridge design for a front that permits continuous contact between the nose and the front in the area of the nasal crest.
Bridge, size
The shortest horizontal distance between lenses (DBL), measured in millimeters.
C
Center, geometric
The intersection of the horizontal
and vertical centerlines of a box that circumscribes the lens shape.
Center, optical
One of the intersection points of
the optical axis of a lens with the lens surfaces. It is the point at
which the lens has NO prism power. Pure cylinder lenses have no
well-defined center point since they have no power, and therefore no
prism, in one direction.
Cornea
That portion of the eye through which
light rays first enter and are bent or refracted. Corneal reflection
Method of measuring the distance
from the pupil, using light reflected from the cornea to the center of
the nose.
CRP
Abbreviation for corneal reflection
pupillometer, the instrument used to measure corneal reflection. Crystalline lens
That portion of the eye which
further refracts the light and focuses it on the retina.
Curve, base
A manufacturer's marked or
nominal surface power of a semi-finished spherical lens or the marked
minimum surface power of a semi-finished toric lens. A semi-finished or
finished lens of a given base curve may be a part of a manufacturer's
corrected curve design series.
Curve, cross
The maximum surface power of a toric
surface (90 degrees from the base curve meridian).
Custom Lenses
Custom lenses are cataract lenses that allow
patients with very bad vision get most of their sight back along with slab off
lenses that help patients with vertical imbalance see much clearer without
double vision effect. This type of correction is called a slab-off or bicentric
grind.
D
DBC
Abbreviation for Distance Between
Centers.
DBL
Abbreviation for Distance Between
Lenses.
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. Diameter, effective
A linear measurement, expressed in
millimeters, equal to twice the longest radius of an ophthalmic lens
measured from its geometric center to the apex of its edge.
Dimension
A measurement of the Boxing System
which measures the distance between vertical tangents to the bevel of a
lens.
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.
Distance between centers (DBC)
The horizontal linear distance between
geometric centers of the two eyes of a frame front.
Distance between lenses (DBL)
The minimum horizontal distance
between lenses, as measured at the apices of their bevels.
E
ED
Abbreviation for effective diameter.
Effective diameter
Defines the minimum lens that will fit a
frame when the geometric center of the lens is exactly centered in the
frame.
Eye
See Eyewire.
Eye, emmetropic
Another name for the normal eye, one
that does not need corrective lenses.
Eye, hyperopic
Farsighted eye, cannot focus on
objects that are close up. This eye
may be thought of as optically shorter than it should
be.
Eye, left
The aperture in front of the wearer's
left eye, as worn.
Eye, myopic
Nearsighted eye, unable to focus
well on items that are in the distance.
Eye, right
The aperture in front of the wearer's
right eye, as worn.
Eye size
See Lens size.
Eyeglasses
A term commonly used to describe an
ophthalmic frame with lenses inserted.
Eyewire
The component of an ophthalmic frame
front which encircles one lens. Also called an eye.
F
Focimeter
An instrument used to determine
vertex power, axis location, optical center, and major reference point
location and prism power at a given point on an ophthalmic lens.
Former
See Pattern, lens.
Frame (ophthalmic or spectacle)
A device for holding ophthalmic lenses
in the proper position on the head in front of the eyes. A frame
typically consists of a front that holds the lenses, and a pair of
temples (earpieces) that secure the unit to the head.
Frame, combination
A frame whose front consists of a
metal chassis with attached trim parts (sometimes known as top rims).
These trim parts are typically plastic, aluminum or other metal, and are
attached to the top portion of the chassis. These top rims may serve
functional or cosmetic purposes, or both
Frame, dress ophthalmic
A frame for prescription or corrective
lenses, intended for ordinary use in correcting or improving vision.
Such a frame is not intended for occupational or safety use.
Frame, rimless
A type of frame that provides no, or
only partial, peripheral support for the lenses.
Frame, zyl
A frame made from cellulose acetate.
Front
A component of an ophthalmic frame,
typically consisting of a bridge and eyewires.
G
GC
Abbreviation for geometric center.
Glazed
Assembled with appropriate ophthalmic
lenses.
Groove, eyewire T
The recessed area of an eyewire in which
the lens edge is seated, also called the lens groove.
H
Hinges
Part of the hardware of the frame.
The hinges attach to both the temple and front of the frame. High index lenses
Lenses made with a higher refractive material than CR-39 plastic or glass. High Index Lenses are thinner than lower index lenses.
I
IPD
Abbreviation for interpupillary
distance.
Index of refraction
A measure of the ability of a lens
material to refract a ray of light of a given wavelength. This is
usually stated for the wavelength of the helium d-line (587.56 nanometers).
The higher the index, the more the refractive power of the lens. For
the ophthalmic glasses most commonly in use, n=1.5230. The index of
refraction for allyl resin, a plastic most commonly used for
eyeglasses, is typically 1.4975.
Intermediate
That area in a trifocal lens or blank
which has been designed to correct vision at ranges intermediate to
distant and near objects.
L
Lens, bifocal
A lens designed to provide
correction for two viewing ranges.
Lens, concave
See Lens, minus
Lens, converging
See Lens, plus
Lens, convex
See Lens, plus
Lens, corrected curve
A lens that has been designed to
reduce peripheral power errors for the conditions of intended use over a
specified portion of the field of view.
Lens, cylinder
A special case of the spherocylinder
lens in which one of the principal meridians has zero refractive power.
Lens, crystalline
That portion of the eye which
further refracts the light and focuses it on the retina.
Lens, diverging
See Lens, minus
Lens, edged
A lens whose periphery has been
ground (flat, beveled or grooved) to a specific size and shape.
Lens, fused
A multifocal lens made from three or
more pieces of glass which are thermally sealed together. On fused
multifocal lenses, curve changes or ledges are invisible.
Lens, laminated
A lens constructed as a sandwich of
multiple layers of glass or plastic, or both, bonded together to form
a single unit.
Lens, lenticular
A lens, usually of a strong
refractive power, in which the prescribed power is provided over only a
limited central region of the lens, called the lenticular portion. The
remainder of the lens is called the carrier and provides no refractive
correction but gives dimension to the lens for mounting.
Lens, minus
A lens having negative dioptric
power. It is thinner at the center than at the edge.
Lens, multifocal
A lens designed for two or more
viewing ranges, for example, bifocal or trifocal lenses.
Lens, one-piece multifocal
A multifocal lens or a blank fabricated
from a single piece of glass or plastic.
Lens, pattern
A cam, or template, used in lens
edging equipment to generate the correct peripheral shape and geometric
center location. Also called a lens former.
Lens, photochromic
A lens that darkens in response to
the ultraviolet component of sunlight.
Lens, plano
A lens which has zero refractive
power.
Lens, plus
A lens that has positive refractive
power. It is thicker at the center than at the edge.
Lens, progressive power
A lens that is designed to provide
correction for more than one viewing range in which the power changes
continuously rather than discretely.
Lens, semi-finished
A lens that has only one surface
finished.
Lens, single-vision
A lens designed to provide
correction for a single viewing distance.
Lens, spherical
A lens that has the same refractive
power in all meridians. Such a lens may have rotationally symmetrical
aspheric (reflective) surfaces.
Lens, sphero-cylinder
A lens that has different refractive
power in the two principal meridians. It is often referred to as an astigmatic
or toric lens and sometimes incorrectly referred to as a cylinder
lens.
Lens, stock
A lens supplied by a manufacturer
with both surfaces finished and a specific back vertex power or powers.
Such a lens has yet to be edged to a specific shape. Also known as
factory finished, uncut.
Lens, toric
A lens which has two distinct
curvatures, at right angles (90 degrees) to each other. See Lens,
sphero-cylinder.
Lens, uncut
A lens with finished optical
surfaces on both sides but not edged for mounting in a frame.
Lens measure
An instrument that is used to measure
surface curvature. Also called sagitta gauge, or lens clock.
Lens size
The horizontal box dimension
(A-dimension) of a finished lens. Also called eye size.
Line, geometric center
A horizontal line running through
the geometric center of a lens. Also called normal mounting line.
M
MPD
Abbreviation for monocular pupillary
distance.
Major reference point
The point on a lens at which the specified
distance prescription requirements shall apply (commonly but
imprecisely referred to as the optical center).
Meridian
The line of intersection of a surface
with a plane perpendicular to that surface at a specified point. When
applied to a lens, it also may be defined as a plane that contains the
optical axis.
Meridians, principal T
The two mutually perpendicular meridians
of a sphero-cylinder lens or toric optical surface with minimum and
maximum power.
O
OD
Latin abbreviation meaning right
eye.
OU
Latin abbreviation meaning both eyes.
OS
Latin abbreviation meaning left eye.
Order form, prescription
Order form that originates with the
customer. It gives the name of the doctor, the name of the patient,
and a description of the desired product.
Order form, stock
Order form that is sent by the dispenser
to the laboratory. The form is used to replenish the dispenser's
stock.
P
PD
Abbreviation for interpupillary
distance.
Packing slip
The slip of paper found inside the
cartons which come from the manufacturer. The packing slip may list a description
of the item, the quantity ordered, quantity shipped, and the code
number as well as the name and address of the manufacturer.
Photochromic Lenses
Photochromic lenses change from light to dark
through changing levels of sunlight. "Transitions 3" are the
state of the art in the Photochromic category of lenses. When worn indoors
they are virtually clear, when worn outside they change to a dark color.
Polarized Lenses
Polarized Lenses block out virtually all Ultra
Violet Rays. These lenses help to eliminate haze and glare, while
increasing visibility. Colors appear more vibrant while others are
subdivided to give the wearer true view without the irritating sun.
Excellent for driving!
Polycarbonate Lenses
Polycarbonate Lenses are the most durable of
all lenses. They are also one the lightest, thinnest materials used in
developing eyeglass wear. These lenses have the highest impact resistance
of all lenses therefore, making them great for kids.
Power, cylinder
The difference (plus or minus)
between the powers measured in the two principal meridians of a lens.
Progressive Lenses
Progressive lenses are state of the art boasting an infinite number of
corrections. No-line bifocals have been round for over 20 years with major
improvements taking place yearly. The progressive multifocals have a distance
viewing area in the upper area of the lens, down to
where the near correction begins. Major benefits of the lens is the lack of
image jump on the segline. Power, marked surface
The nominal curve of a semi-finished
lens marked in diopters, as expressed by the manufacturer. The
difference between marked and actual tool curve of the surface
represents the manufacturer's compensation for that base curve.
This compensation allows standard tooling to be utilized over a range of
prescriptions with little or no further compensation required by the
laboratory to produce accurate vertex powers.
Power, meridional
The refractive or surface power of a
lens measured in a specified meridian.
Power, nominal
See power, marked surface.
Power, prism T
The deviation of a light ray produced by
a prism or by the prismatic component in a lens, expressed in prism
diopters. Power, refractive
The ability of a lens or an optical
surface to produce a change in the convergence or divergence of a beam
of light, usually expressed in diopters.
Power, sphere
In a spherical lens, the dioptric
power of the lens. In a sphero-cylinder lens, the sphere power is
located in the cylinder axis meridian.
Power, surface refractive (R)
The refractive power of a lens
surface having index of refraction (n) is a measure of its ability to
refract light, and is expressed in diopters. The expression relating (R)
to (S) and (n) is: R=[(n-1)S]/0.530 Since common ophthalmic materials do
not have indices of refraction equal to 1.530, there is not a one-to-one
correspondence between surface tool power (S) and surface refractive
power (R). For example, common ophthalmic glass has an index of
refraction of 1.5230. Therefore, a one-diopter surface tool will produce
a surface refractive power (R) of 0.987 diopter. Surface refractive
power is also called true power.
Power, surface tool (S)
The actual radius of curvature of a
tool or the surface it produces. By common usage in the United States, a
tool with a radius of curvature of 530mm will produce a surface tool
power (S) of one diopter. When (r) is the radius of curvature in
millimeters, then S = 0.530/(0.001r).
Power, vertex
The inverse of the distance,
expressed in meters, from the lens vertex to the corresponding focal
point. This is expressed in diopters. In a prescription, the spherical
component of power and cylindrical component are always expressed in
terms of rear (or back) vertex power. Focimeters are designed to measure
vertex power directly.
Pupillary distance, binocular
The measurement between the patient's
pupils, expressed in millimeters. Also called IPD.
Pupillary distance, monocular
The measurement from the center of
the nose to the pupil. Also called MPD.
R
Reading Glasses
Reading Glasses have lenses with stronger
powers because they incorporate both the distance and near powers to
concentrate power for easy reading for the wearer. Refraction
The bending of light rays caused by
prisms and lenses. See power, refractive.
S
Sagitta
See vertex depth.
Segment
A specified area of a multifocal
lens having different power from the major portion. This also may refer
to the actual piece of material added to the lens in the case of a fused
or cemented multifocal lens. Also called the addition.
Short order
See backorder.
Spectacle
An ophthalmic device consisting of two
ophthalmic lenses and a supporting frame to position and retain the
lenses in proper optical alignment with the eyes.
Sun Lenses
Sun Lenses are lenses that have a mirror
coating and are usually very dark in appearance. These lenses help reduce
light transmission and come in many colors such as yellow, blue, mirror
etc.
Surface, aspheric
A nonspherical surface curvature
commonly used to improve optical performance, particularly for high refractive
powers. Such curvatures are often derived from the oblique intersection
of a plane and a conical surface, and are referred to as
"conoids" or "conic" sections.
Surface, plano
A flat surface having zero surface
power, or an infinite radius of curvature.
Surface, spheric
A curved surface having the same
radius of curvature in all meridians.
Surface, toric
A surface in the form of a torus
having different powers in two principal meridians. The shape may be
visualized as that of a section cut from a doughnut or from a
football-shaped surface.
T
Temple, library
The style of temple that has almost
no bend over the ear; it was originally designed for ease of removal.
Temple, riding bow
A style of temple which bends around
to hug the ear. It is particularly useful to people whose jobs are very
active, or for small children. Temple, skull
A style of temple that has a slight
bend which allows the frame to fit easily over the ear, and to hug the
head lightly.
Temple, spatula
Another term for skull temple; also
called paddle temple.
Temple screws
Tiny screws which connect the front
and temple halves of the hinges.
Thickness, center
The thickness of a lens at the major
reference point.
Transitions Lenses
The leader in plastic photochromic lenses to the industry. Transitions
continue to develop the technology for changing lenses. Transposition
Changing the powers of a
sphero-cylinder lens or astigmatic prescription from one cylinder form
to the other (- to + or + to -). Trifocal Lenses
Trifocal lenses are useful in affording the patient 3 vision corrections. One for distance, intermediate (arm length) and reading.
V
Varilux Lenses
The leader in progressive line-free bifocals for over 20 years. Varilux
continues to lead this category and this year introduces the Panamic
lens which will give the patient an even more enhanced vision correction. Vertex depth
The depth of the surface curve on a lens
measured over a specific diameter. Also called the sagitta.
W
Wave
A local ripple-like irregularity in
a lens surface.
Z
Zyl
Cellulose acetate, used to make frames.
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