Ophthalmic Lens Terms

Question 1

It is the ratio of the refractivity in air to the main dispersion

Answer 1

Abbe Number

Question 2

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.

Answer 2

Addition (ADD)

Question 3

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.

Answer 3

Aspheric

Question 4

Glass containing mostly barium oxides. Used primarily for the reading segments in bifocal lenses

Answer 4

Barium Glass

Question 5

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.

Answer 5

Base Curve

Question 6

A term used to distinguish the form of curved lenses.

Answer 6

Bent Lens

Question 7

The V-shaped edge ground on the periphery of a lens to hold the lens in the frame.

Answer 7

Bevel

Question 8

A lens in which both surfaces are concave.

Answer 8

Biconcave

Question 9

A lens in which both surfaces are convex.

Answer 9

Biconvex

Question 10

A spectacle lens in which one area has more dioptric power than the other.

Answer 10

Bifocal

Question 11

The basic lens substrate, to which segment or different refractive power may be added to produce a multifocal lens.

Answer 11

Blank

Question 12

The term used to describe the size of a lens, measured by its diameter in millimeters.

Answer 12

Blank Size

Question 13

The thickness of a lens at the major reference point.

Answer 13

Center Thickness

Question 14

A lens containing a sphere and clyinder.

Answer 14

Compound Lens

Question 15

surface that curves inside. Hollow and curved, such as the inside of a disc.

Answer 15

ConcaveA

Question 16

surface that cuves outside. Elevated and rounded, as the outside of a disc.

Answer 16

Convex

Question 17

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.

Answer 17

Corrective Curves

Question 18

The highest curve on either the front or the back surface of any cylindrical lens.

Answer 18

Cross Curve

Question 19

An ophthalmic lens in which one of its surfaces is cylindrical.

Answer 19

Cylinder Lens

Question 20

The X-axis of a lens through the geometric center of the lens. The 180’ line .

Answer 20

Datum Line

Question 21

The placement of the optical center in which it does not coincide with the geometric center of a lens.

Answer 21

Decentration

Question 22

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.

Answer 22

Diopter

Question 23

Refers primarily to two times the longest radius of a frame’s eyewire in millimeters.

Answer 23

E.D. (Effective Diameter)

Question 24

A lens whose periphery has been ground (flat, rimless, or beveled) to a specified size and shape

Answer 24

Edged

Question 25

A glass with a high refractive index, containing lead.

Answer 25

Flint Glass

Question 26

The reciprocal of the dioptric power. The distance from the optical center of the back surface of the lens and its principal focus.

Answer 26

Focal Length

Question 27

An instrument for determining vertex power, axis location, optical center, and major reference point location and prism power given on an ophthalmic lens.

Answer 27

Focimeter/Lensometer

Question 28

The distance from the temporal edge of one eye rim to the nasal edge of the other eye rim.

Answer 28

Frame P.D.

Question 29

A point lying midway of all meridians on a lens.

Answer 29

Geometric Center

Question 30

A refractive lead-free glass containing a lead substitute called titanate.

Answer 30

Hi-Lite Glass

Question 31

The ratio of the velocity of light in one medium to the velocity of light in a given medium.

Answer 31

Index of Refraction

Question 32

The setting of the bifocal segment to enable the customer to read through the segment optical center.

Answer 32

Inset

Question 33

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.

Answer 33

Intermediate

Question 34

A lens containing two pieces of glass separated by a thin sheet of plastic.

Answer 34

Laminated Glass

Question 35

A cast iron tool used in the grinding and polishing of lens surfaces.

Answer 35

Lap

Question 36

A transparent medium bounded by two geometric surfaces, one of which is curved.

Answer 36

Lens

Question 37

An instrument designed to measure the sagitta of a lens curve and usually calibrated to express the measurement in dioptric tool surface power.

Answer 37

Lens Clock (Measure)

Question 38

A strong powered lens which has a central area usually 30 to 40mm, ground to the prescription and attached to a Plano carrier.

Answer 38

Lenticular Lens

Question 39

A transparent mineral used in anti-reflection coating of glass lenses.

Answer 39

Magnesium Flouride

Question 40

The property of some optical lenses or systems of projecting a real inverted image of larger area than the object.

Answer 40

Magnification

Question 41

That point on a lens at which specified distance prescription requirements shall apply (commonly but imprecisely referred to as the optical center).

Answer 41

Major Reference Point (MRP)

Question 42

The power resulting from the algebraic combination of the front and back surfaces resulting from surfacing tools calibrated for an index 1.530

Answer 42

Marked Power

Question 43

A substance that transmits light.

Answer 43

Medium

Question 44

Crescent shaped lens. One side convex, the other concave.

Answer 44

Meniscus Lens

Question 45

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.

Answer 45

Meridian

Question 46

The refractive or surface power of a lens measured in a specified meridian.

Answer 46

Meridional Power

Question 47

To reduce the apparent dimensions (The opposite of magnify).

Answer 47

Minify

Question 48

A lens with two curvatures on the back surface.

Answer 48

Minus Cylinder

Question 49

A lens that diverges light and has a negative dioptric power. It is thinner at the center than at the edge.

Answer 49

Minus Lens

Question 50

A lens designed to provide correction for wo or more viewing ranges, e.g. bifocal, trifocal and progressive

Answer 50

Multifocal

Question 51

Index of refraction. A measure of the ability of lens material to refract or bend a ray of light of given wave legth

Answer 51

'n'

Question 52

Toward the nose

Answer 52

Nasal

Question 53

The nearest point at which the eye can see clearly when employing its full accommodation.

Answer 53

Near Point

Question 54

The power of a lens resulting from the algebraic combination of the front and back surfaces.

Answer 54

Nominal Power

Question 55

Right Eye

Answer 55

OD (Oculus Dexter)

Question 56

Left Eye

Answer 56

OS (Oculus Sinister)

Question 57

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.

Answer 57

Optical Center

Question 58

Both Eyes

Answer 58

OU (Oculus Uterque)

Question 59

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.

Answer 59

Optical Cross

Question 60

The abbreviation used to denote interpupillary distance signifying the distance in millimeters between the pupils.

Answer 60

P.D.

Question 61

Zero dioptric power

Answer 61

Plano

Question 62

A lens which has no focus power.

Answer 62

Plano Lens

Question 63

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.

Answer 63

Plus Lens

Question 64

The formula for lenses required by patient as prescribed by the examiner.

Answer 64

Prescription

Question 65

A lens designed to provide correction for more than one viewing range in which the power changes continuously rather than discretely.

Answer 65

Progressive Addition Lens

Question 66

A wedge shaped piece of glass or transparent material having plane or curved sides with a base, apex and apical angle.

Answer 66

Prism

Question 67

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.

Answer 67

Prism Diopter

Question 68

One half the diameter of a circle that would be created if the surface curvature were drawn all the way around.

Answer 68

Radius of Curvature

Question 69

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.

Answer 69

Segment

Question 70

A lens with one focus

Answer 70

Single Vision

Question 71

The distance from the top of the multifocal segment to the bottom in a finished pair of eyewear.

Answer 71

Segment Height

Question 72

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.

Answer 72

Slab-Off

Question 73

A lens that has the same power in all meridians. Such a lens may have rotationally symmetrical aspheric surfaces.

Answer 73

Sphere

Question 74

The lowest dioptric curve on the back surface of a lens.

Answer 74

Sphere Curve

Question 75

A lens having two curvatures at right angles to each other on the same surface.

Answer 75

Toric

Question 76

The passing of light waves through one medium into another.

Answer 76

Transmission

Question 77

Pertaining to a medium having of transmitting light so that object can be seen through it.

Answer 77

Transparent

Question 78

The mathematical switching of one cylinder form to another, either plus to minus to plus.

Answer 78

Transposition

Question 79

A lens with three focusing points: distance, intermediate and near.

Answer 79

Trifocal

Question 80

A lens with finished optical surfaces on both sides but which has been edged for mounting in a frame.

Answer 80

Uncut