Optical Terms

Front 1

Index of Refraction

Back 1

A ratio that enables the comparison of the density of various materials. It is found by dividing the speed of light through the material.

Front 2

Abbe Value

Back 2

An index that measures the optical clarity of a lens material. This is measured by the degree of dispersion induced by the lens material as the wearer looks away from the optical center of the lens. The higher the value the greater the optical clarity of the material.

Front 3

Abberation

Back 3

Deviation from a single focus of light rays emanating from one source.

Front 4

Accomodation

Back 4

The ability of the eye to adjust focus for varying distances.

Front 5

Achromatic

Back 5

Descriptive of a lens capable of refracting light without creating a rainbow affect.

Front 6

Acuity

Back 6

Visual expression of sharpness of vision (20/20 distance, J.1 near)

Front 7

Addition

Back 7

Difference in plus power between the reading and distance portions of a multifocal lens.

Front 8

Amblyopia

Back 8

Loss of vision without any apparent disease of the eye.

Front 9

Ametropia

Back 9

A refractive error in which the eye, when in a state of rest, does not focus the image of an object upon the retina; invludes hyperopia, myopia, and astigmatism.

Front 10

Aniseikonia

Back 10

A condition in which the image of an object as seen by one eye differs so much in size and shape from that seen by the other eye that the two images cannot be fused into a single impression.

Front 11

Anisometropia

Back 11

A condition in which the refractive error of one eye significantly differs from that of the other.

Front 12

Anterior chamber

Back 12

Front cavity or compartment of the eye, located between the cornea and crystalline lens. Contains the aqueous humor.

Front 13

Apex

Back 13

Thinest edge of a prism.

Front 14

Aphakia

Back 14

An absence of the crystalline lens of the eye (pseuodophakia: having a false lens)

Front 15

Aspheric

Back 15

Not spherical; a lens surface having a number of curves with different radii.

Front 16

Astigmatism

Back 16

A defect of the eye, of a lens or of an image formed by either, the curvature of the refracting surface or surfaces being different in different planes. This results in the focal lengths also being different in different planes (the curvature of the cornea in the case of the eye).

Front 17

Axis

Back 17

The meridian of no cylinder power in a spherocylinder lens.

Front 18

Base

Back 18

Thickest edge of a prism.

Front 19

Base curve

Back 19

The curve used as a base of a series of powers; the degree of curvature of the front surface of a lens.

Front 20

Beam

Back 20

A group of parallel rays of light.

Front 21

Binocular

Back 21

pertaining to vision with both eyes.

Front 22

Caliper

Back 22

Instrument used to measure thickness, callibrated in fifths and tenths of a millimeter.

Front 23

Canthus

Back 23

The angle at either end of the slit between the eyelids.

Front 24

Cataract

Back 24

A condition when the crystalline lens of the eye becomes opaque.
-Congenital cataract: one, which originates at birth.
-Hyper-mature: the lens has become either solid and shrunken or soft and liquid.
-Incipient: any cataract in its early stages.
-Mature: the lens is completely opaque.
-Senile: a hard opacity of the lens occurring in the aged.
-Traumatic: cataract following an injury.

Front 25

Choroid

Back 25

Vascular (blood-rich) layer of the eye; its function is to nourish the other parts of the eye, primarily the retina.

Front 26

Chromatic Aberration

Back 26

Distortion of an optical image produced by the dispersion of light passing through a lens and generally characterized by blurred, multicolored edges.

Front 27

Ciliary Body

Back 27

The thick rim of choroid to which the crystalline lens is attached.

Front 28

Compound hyperopic astigmatism

Back 28

The refractive error which results in two points of focus falling behind the retina.
Example: +1.00 +2.00 x 090

Front 29

Cones

Back 29

Light sensitive cells in the retina responsible for color vision, daytime vision, and the central portion of the visual field.

Front 30

Conjunctiva

Back 30

The clear membrane that lines the eyelids and covers the exposed surface of the eyeball.

Front 31

Cornea

Back 31

The refractive, transparent, anterior wall of the eye, equal to 43 dipters power. The five layers of the cornea in order are:
Epithelium
Bowman's Membrane
Stroma
Descemet's Membrane
Endothelium
The cornea is the fastest healing part of the body.

Front 32

Crown glass

Back 32

Pure silicate glass of general excellence for the spectacle lens; index of refraction 1.523.

Front 33

Crystalline Lens

Back 33

A transparent, colorless body suspended in the front part of the eyeball, between the aqueous humor and the vitreous humor. Its function is to bring light rays to a focus on the retina.

Front 34

Cycloplegia

Back 34

Paralysis of the ciiary body; cycloplegic drops are used for diagnostic purposes.

Front 35

Decentration

Back 35

Distance between the optical center placement of a lens and its geometric center, usually expressed in millimeters.

Front 36

Diffusion

Back 36

The scattering of light.

Front 37

Diopter

Back 37

Unit of measurement of the refractive power of a lens. A lens whose focal length is one meter has a power of one diopter. The higher the dioptric power, the shorter the focal length.

Front 38

Diplopia

Back 38

The seeing of one object as two, due to uneven tracking of the eyes. Commonly known as double vision.

Front 39

Distortion

Back 39

Defect in a lens which causes a straight line to appear curved.

Front 40

Divergent

Back 40

Two or more light rays proceeding outward from a point.

Front 41

Emmetropia

Back 41

The refractive condition of a normal eye. WHen the eye is at rest, the image of distant objects is brought to a focus on the retina.

Front 42

Esophoria

Back 42

A tendency for one eye to deviate off the visual axis toward the other eye.

Front 43

Esotropia

Back 43

Actual deviation of one eye toward the visual axis of the other eye (crossed eyes).

Front 44

Exophoria

Back 44

A tendency for one eye to deviate off the visual axis away from the other eye.

Front 45

Exotropia

Back 45

Actual deviation of one eye away from the visual axis of the other eye (wall eyed).

Front 46

Far Point

Back 46

Point in space which is sharply focused on the retina with the eye's accommodation relaxed.

Front 47

Flint Glass

Back 47

A glass of high refractive index (1.65), containing lead.

Front 48

Focus

Back 48

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

Front 49

Fovea

Back 49

A small depression in the retina, at the back of the eye. It is the part of the macula adapted for the clearest vision.

Front 50

Fusion

Back 50

The power of coordination by which the images received by the two eyes become a single image.

Front 51

Geometric Center

Back 51

Point where the diagonals of a boxed lens meet.

Front 52

Glaucoma

Back 52

An ocular disease having as its primary characteristic a sustained increase in intraocular pressure that the eye cannot withstand without damage to its structure or impairment of its function. Primary symptom is a loss of peripheral vision.

Front 53

Hyperopia

Back 53

Also known as farsightedness. A refractive error in which, because the eyeball is short or the refractive power of the lens is weak, the point of focus for rays of light from distant objects falls behind the retina. Correction to increase refractive power (a plus lens) is necessary for distance vision as well as near vision.

Front 54

Index of refraction

Back 54

The ratio of the speed of light in air to the speed of light in a given medium.

Speed of light in air / Speed of light in medium = Index of Rrefraction

Front 55

Infinity

Back 55

(Optical infinity) Distance great enough that the rays of light appear to be parallel; 20 feet or beyond, since accommodation in in effect for anything within 20 feet.

Front 56

Inset

Back 56

Amount of decentration from the distance optical center to the optical center of the bifocal segment, usually 1.5mm per eye.