Ophthalmic Lens Terms Flashcards
It is the ratio of the refractivity in air to the main dispersion
Abbe Number
The dioptric power added to the distance prescription to be used for a fixed distance such as for reading. Commonly the dioptric power of the bifocal segment.
Addition (ADD)
A non-spherical 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.
Aspheric
Glass containing mostly barium oxides. Used primarily for the reading segments in bifocal lenses
Barium Glass
A manufacturer’s marked or nominal surface of a semi-finished spherical lens or the marked minimum surface power of a semi-finished toric lens. A semi-finished lens of a given base curve may be part of a manufacturer’s corrected curve design series.
Base Curve
A term used to distinguish the form of curved lenses.
Bent Lens
The V-shaped edge ground on the periphery of a lens to hold the lens in the frame.
Bevel
A lens in which both surfaces are concave.
Biconcave
A lens in which both surfaces are convex.
Biconvex
A spectacle lens in which one area has more dioptric power than the other.
Bifocal
The basic lens substrate, to which segment or different refractive power may be added to produce a multifocal lens.
Blank
The term used to describe the size of a lens, measured by its diameter in millimeters.
Blank Size
The thickness of a lens at the major reference point.
Center Thickness
A lens containing a sphere and clyinder.
Compound Lens
surface that curves inside. Hollow and curved, such as the inside of a disc.
ConcaveA
surface that cuves outside. Elevated and rounded, as the outside of a disc.
Convex
A lens that has been designed to reduce spherical power errors for the conditions of intended use over a specified portion of the field of view. The use of different performance criteria may lead designers to different curvatures for a given prescription. A group of lenses covering a range of prescriptions and based on the same performance criteria is referred to as a corrected curve series.
Corrective Curves
The highest curve on either the front or the back surface of any cylindrical lens.
Cross Curve
An ophthalmic lens in which one of its surfaces is cylindrical.
Cylinder Lens
The X-axis of a lens through the geometric center of the lens. The 180’ line .
Datum Line
The placement of the optical center in which it does not coincide with the geometric center of a lens.
Decentration
A unit measurement (plus or minus) used to express the focusing power of a lens. It is expressed as the reciprocal of the focusing distance given in meters. The diopter is also used to express the curvature of surfacing tools and the refracting power of curved surfaces.
Diopter
Refers primarily to two times the longest radius of a frame’s eyewire in millimeters.
E.D. (Effective Diameter)
A lens whose periphery has been ground (flat, rimless, or beveled) to a specified size and shape
Edged
A glass with a high refractive index, containing lead.
Flint Glass
The reciprocal of the dioptric power. The distance from the optical center of the back surface of the lens and its principal focus.
Focal Length
An instrument for determining vertex power, axis location, optical center, and major reference point location and prism power given on an ophthalmic lens.
Focimeter/Lensometer
The distance from the temporal edge of one eye rim to the nasal edge of the other eye rim.
Frame P.D.
A point lying midway of all meridians on a lens.
Geometric Center
A refractive lead-free glass containing a lead substitute called titanate.
Hi-Lite Glass
The ratio of the velocity of light in one medium to the velocity of light in a given medium.
Index of Refraction
The setting of the bifocal segment to enable the customer to read through the segment optical center.
Inset
That area in a trifocal lens or blank which has been designed to correct vision ranges intermediate to distant and near objects. Refers to an intermediate viewing distance of approximately 30 inches.
Intermediate
A lens containing two pieces of glass separated by a thin sheet of plastic.
Laminated Glass
A cast iron tool used in the grinding and polishing of lens surfaces.
Lap
A transparent medium bounded by two geometric surfaces, one of which is curved.
Lens
An instrument designed to measure the sagitta of a lens curve and usually calibrated to express the measurement in dioptric tool surface power.
Lens Clock (Measure)
A strong powered lens which has a central area usually 30 to 40mm, ground to the prescription and attached to a Plano carrier.
Lenticular Lens
A transparent mineral used in anti-reflection coating of glass lenses.
Magnesium Flouride
The property of some optical lenses or systems of projecting a real inverted image of larger area than the object.
Magnification
That point on a lens at which specified distance prescription requirements shall apply (commonly but imprecisely referred to as the optical center).
Major Reference Point (MRP)
The power resulting from the algebraic combination of the front and back surfaces resulting from surfacing tools calibrated for an index 1.530
Marked Power
A substance that transmits light.
Medium
Crescent shaped lens. One side convex, the other concave.
Meniscus Lens
The lind of intersection of a surface with a plane +/-2’ to that surface at a specified point. When applied to a lens, it may also be defined as a plane that contains the optical axis.
Meridian
The refractive or surface power of a lens measured in a specified meridian.
Meridional Power
To reduce the apparent dimensions (The opposite of magnify).
Minify
A lens with two curvatures on the back surface.
Minus Cylinder
A lens that diverges light and has a negative dioptric power. It is thinner at the center than at the edge.
Minus Lens
A lens designed to provide correction for wo or more viewing ranges, e.g. bifocal, trifocal and progressive
Multifocal
Index of refraction. A measure of the ability of lens material to refract or bend a ray of light of given wave legth
‘n’
Toward the nose
Nasal
The nearest point at which the eye can see clearly when employing its full accommodation.
Near Point
The power of a lens resulting from the algebraic combination of the front and back surfaces.
Nominal Power
Right Eye
OD (Oculus Dexter)
Left Eye
OS (Oculus Sinister)
One of the intersection points of the optical axis of a lens with the lens surfaces. (When the image of a focimeter target is centered on the cross hairs, the optical center of the lens being measured is also at this point. It is the point at which the lens has no prism power unless specified.
Optical Center
Both Eyes
OU (Oculus Uterque)
A graphic illustration showing the dioptric curvature in one meridian of a lens and the dioptric curvature in the other meridian 90’ away. Also referred to as a lens cross.
Optical Cross
The abbreviation used to denote interpupillary distance signifying the distance in millimeters between the pupils.
P.D.
Zero dioptric power
Plano
A lens which has no focus power.
Plano Lens
A lens that converges light to a real focus and has a positive dioptric power. It is thicker at the center than at the edge.
Plus Lens
The formula for lenses required by patient as prescribed by the examiner.
Prescription
A lens designed to provide correction for more than one viewing range in which the power changes continuously rather than discretely.
Progressive Addition Lens
A wedge shaped piece of glass or transparent material having plane or curved sides with a base, apex and apical angle.
Prism
A unit of measurement used to express the angle of deviation of a ray of light by a prism or lens. Prism power, in these units, is measured as the displacement of a ray. One prism diopter produces a deviation of 1cm at a distance of 1meter.
Prism Diopter
One half the diameter of a circle that would be created if the surface curvature were drawn all the way around.
Radius of Curvature
A specified area of a multifocal lens having a different power from the major portion. This may also refer to the actual piece of material added to the lens in the case of a fused or cemented multifocal lens.
Segment
A lens with one focus
Single Vision
The distance from the top of the multifocal segment to the bottom in a finished pair of eyewear.
Segment Height
The placing of two different curvatures on the front surface of a lens to eliminate vertical imbalance at the reading level. Also known as bi-centric grinding.
Slab-Off
A lens that has the same power in all meridians. Such a lens may have rotationally symmetrical aspheric surfaces.
Sphere
The lowest dioptric curve on the back surface of a lens.
Sphere Curve
A lens having two curvatures at right angles to each other on the same surface.
Toric
The passing of light waves through one medium into another.
Transmission
Pertaining to a medium having of transmitting light so that object can be seen through it.
Transparent
The mathematical switching of one cylinder form to another, either plus to minus to plus.
Transposition
A lens with three focusing points: distance, intermediate and near.
Trifocal
A lens with finished optical surfaces on both sides but which has been edged for mounting in a frame.
Uncut
