• aberration - An imperfection in a lens or optical system which prevents a point focus.
  • accommodation - The act of increasing the dioptic power of the eye by an increase in the power of the crystalline lens to permit objects at different distances to be clearly seen.
  • acuity - Sharpness of vision.
  • addition - Dioptic power added to distance prescription to supplement accomodation for some finite distance; such as reading. The dioptic power of a bifocal segment.
  • amblyopia ex anopsia - Dimness of vision resulting from non-use; as, the non-fixing eyein strabismus.
  • antireflection coating - A thin film of magnesium flouride deposited upon a glass surface by vaporization of the metal in vacuum.
  • aphakia - The condition of an eye without the crystalline lens in the path of incident light. Usually lens removal in cataract surgery, but also can be cause by luxation of the lens. Loss of lens adds more than +10.00D hyperopia to the eye.
  • astigmatism - The condition in which the focal lengths of opposite meridians of an eye or optical system are not equal, thus producing two separate line images of a point source. Objects may appear to have a doubling or ghosting effect. Occurs when the eye is oval shaped instead of round. May exist alone or in combination with nearsightedness.
  • atrophy - A wasting or diminution of a body structure due to failure of nutrition.
  • axis - An imaginary line with which an object's functions or parts are symmetric.
    • Cylinder a - The meridian of a cylinder lens in which there is least or no curvature. One of the principal meridians of a spherocylindrical lens or surface.
    • Optical a - An imaginary line joining the centers of curvature of a lens or symmetrical optical system.
    • Visual a - The line joining the fixation point through the nodal points with the fovea centralis.


  • base curve - The meridian of least curvature on a cylindrical surface. It may be found on either the convex or concave side of a meniscus lens.
  • bifocal - A spectacle lens with two areas one of which has more dioptric power than the other.
  • binocular - The term used to describe simultaneous use of the two eyes in the act of vision.


  • cataract - Opacification in various degrees of the crystalline lens substance or capsule which can diminish visual acuity from normal to bare light perception. It may be congenital or caused by metabolic derangement or by trauma. Vision is re-established by the surgical removal of the lens and capsule.
  • choroid - The highly vascular middle coat of the eye globe lying between the sclera and retina. Its principal function is blood supply to sustain the retina.
  • chromatic aberration - The dispersion of colors arising from unequal refraction of light of the different wavelengths of the spectrum. This defect of an optical system causes the image to be surrounded by a halo of colors. It is corrected in an achromatic lens system.
  • ciliary body - An extension of the uvea which is comprised of the ciliary muscle and ciliary process. Its function is to assist in the changing of the crystalline lens in the act of accomodation.
  • cone - One of the two types of nerve endings of the retina. In the macular area they are tightly bunched to produce maximum visual acuity. They are sensitive to color, much less sensitive to motion.
  • contact lens - A thin plastic shell shaped like the front of the eyeball which is held in position by the suction of a thin layer of tears. This lens is used to correct refractive errors including irregularly shaped cornea as a cosmetic lens to change iris color or to cover unsightly scars.
  • convergence - The movement of the eyes in which the internal recti turn the visual axes to intersect at some finite point.
  • cornea - The transparent anterior portion of the scleral coat of the eye. The radius of curvature, about 8.00 mm is less than the sclera. The power of the cornea is about 43.00D. (about 75%) of the dioptric power of the eye.
  • crystalline lens - The biconvex lens situated behind the iris of the eye. It is suspended from the ciliary body by a yoke of zonular fibers. By the force of the muscle fibers surrounding the lens its center thickness increases, its diameter decreases and the anterior portion bulges forward. This increases its dioptic power to focus the eye upon objects near to the eye. This act is known as accomodation.


  • depth perception - Perception of the relief of objects in which they appear as solids and not as flat pictures. Third-degree fusion.
  • deviation - Bending or turning in another direction. The deviation of visual axes in strabismus (squint), the deviation of a ray which is refracted at an interface.
  • diopter, dioptic, dioptry - Unit of focal power in opthalmic optics. The reciprocal of the focal length of a lens or optical system expressed in meters. 1 meter focal length = 1/1 = 1.00D., 0.50 meter focal length 1/.5 = 2.00D., 0.20 meter focal length = 1/.2 = 5.00D.
  • diplopia - The condition in which images of a single object fall upon noncorresponding points of the two eyes and cause the perception of double vision.


  • emmetropia - The state of an eye without refractive error in which visual acuity is at maximum for distant vision without accommodative effort.
  • extraocular muscles - The six muscles which cause movement of the eye internal and external recti and superior and inferior recti and superior and inferior oblique.
  • eye - The sense organ which contributes to the sense of vision.


  • farsightedness - A common term for hyperopia.
  • focus - A point through which rays of light converge or from which rays of light appear to diverge when entering or emerging from an optical system.
  • fusion - The function of merging simultaneous retinal images into a single perception. Breadth of f - The term applied to the limit of the amounts that convergence or divergence can be altered with Base In or Base Out prisms while single binocular vision is sustained.


  • glaucoma - An eye disease caused by impaired drainage of aqueous humor which results in increased intraocular pressure. Finally the peripheral nerve fibers of the retina are atrophied, ending with blindness of central vision for total blindness.


  • hyperphoria - Heterophoria in the vertical meridian in which there is a tendency for one eye to deviate upwards relative to the other.
  • hypophoria - Vertical heterophoria in which one eye tends to deviate downwards relative to the other. This can be differentiated from hyperphoria in the other eye only by evidence of paresis or paralysis of elevating muscles.


  • index of refraction - The ratio of the velocity of light in one medium to the velocity of light in the next medium.
    index = V first medium / V second medium

    water = 1.332
    CR39 = 1.498 (hard resin)
    crown glass = 1.523
    polycarbonite = 1.586
    high index plastic = 1.540 - 1.660
    high index glass + 1.600 - 1.800
    diamond = 2.44
  • infinity (optical) - A distance which is great as compared with the aperture or focal length of an optical system. Six meters (20 feet) the distance used for refraction is over 1000 times the width of the pupil and about 400 times the focal length of the eye. At 6 meters the size of the pupil subtends an angle of less than 4 seconds at the first nodal point.


  • keratoconus - Anomalous development of the cornea in which the cornea assumes a conoidal shape. A.k.a., conical cornea.
  • keratometer - An instrument used to measure the curvature of small areas of the cornea by reflected light.


  • LASIK or laser in situ keratomileusis - See our LASIK Info page.
  • lens - A transparent medium bounded by two geometrically describable surfaces one of which shall be curved - that is, spherical, cylindrical, toroidal or aspheric.
  • light - A form of energy, necessary to see. Visible light is that part of the spectrum that produces the sensation of sight in the human eye. The wavelengths of the radiation in this visible range are very small and are usually expressed in either millimicrons or nanometers. The values given for the limits of the range of wavelengths in visible light will depend on the textbook you are reading, but will be around 380 - 760 nanometers or 400 - 750 nanometers. The rays that cause us to see violet have the smaller, shorter wavelength, around 380 nanometers or milli-microns. Each color of the spectrum, blue, green, yellow, and orange, will have a range of wavelengths up to red at around 660 - 750 nanometers.


  • macula - The point of clearest vision at the centre of the retina.
  • magnification - The property of some optical lenses or systems of projecting a real inverted image of larger area than the object.
  • medium - A substance that transmits light.
  • multifocal - A lens having more than one focal power in USA. A lens having more than two focal powers in UK.
  • myopia - A refractive error in which the far point of the unaccommodated eye is at a finite distance before the eye. Shortsightedness or nearsightedness.


  • nearsightedness - A condition that usually starts in childhood and stabilizes in the late teens or early twenties. Because the eye's focussing powers are too strong for the size of the eye, near objects are seen more clearly and those far away appear blurry. Light is focussed in front of the retina. Myopia.
  • neutral filter - A light filter which dampens illumination by reducing the visible spectrum about equally, thereby causing no color changes.


  • opacity - the change in a material from being transparent to being non-transparent.
  • ophthalmic - With reference to the eye and its functions.
  • Ophthalmologist - A medical doctor who has had three or more years of graduate education in eye care. He diagnoses and treats eye diseases, does eye surgery and prescribes lenses. Eye physician, oculist, ophthalmic surgeon.
  • optic - Pertaining to light or the sense of sight.
  • optician - One skilled in the application of the science of optics, including optical lens and/or instrument designing or manufacturing.
  • optometrist - Independent primary health care providers who examine, diagnose, treat and manage diseases and disorders of the visual system, the eye and associated structures as well as diagnose related systemic conditions.


  • papilla - Head of the optic nerve.
  • parallax - The apparent displacement or change of position of an object when viewed from different places; such as the alternate use of the right and left eve.
  • pd - Abbreviation for interpupillary distance which has been confused by frame manufacturers referring to the total of distance between lenses plus the "A" measurement of a "boxed" lens as the PD of a frame.
  • polaroid - A sheet of cellulose acetate containing crystals of an iodine compound which are all oriented in one direction. The plastic is sometimes laminated in glass. Light polarized by this material is used for stereoscopes, 3-D pictures and movies, for glare protecting lenses and a test for strained glass including case-hardened glass.
  • polycarbonate - The toughest lens material available. Flexes in response to impact, even to the point of being bent permanently without breaking. 20 times more impact resistant than glass. Also very lightweight; 3 times lighter than glass. Polycarbonate is injection molded into lens blanks, UV and IR absorbers as well as lens colour is consistent throughout the lens and cannot be scratched off.
  • presbyopia - Literally, the sight of old age. The condition of vision due to diminished accommodative amplitude which removes the near-point farther from the eye than is convenient for reading.
  • prism - An optical element bounded by two flat surfaces inclined toward one another. The edge at which they intersect is called the apex. The angle between the surfaces is termed the aptical angle.
  • prk - Photorefractive Keratectomy - Procedure where an excimer laser is used to remove a thin layer of tissue from the center of the cornea. During the procedure, the laser's light energy is converted into chemical energy which evaporates molecules of corneal tissue in very precise amounts. By altering the curvature of the cornea, PRK helps reduce or eliminate moderate nearsightedness and astigmatism.
  • progressive lens - a lens that has an infinite number of focal lengths in the central corridor from the distance optical center to the near addition power. A.k.a. lineless bifocal, no-line bifocal, seamless bifocal.
  • pupil - An aperture or its image of a lens or an optical instrument. The aperture of an eye created by the opening of the iris of the eye.


  • ray - A straight line representing the direction of a ray or bundle of rays of light. An element of geometrical optics.
  • refraction - The bending of a ray of light at the interface of two optical media of differing indices of refraction. The term used to describe the act of measuring an error of refraction of the eye.
  • refractive index - See index of refraction.
  • resolving power - The capability of an optical system to separate the images of two closely situated points in the object space.
  • retina - The innermost coat of the eyeball. It is composed of nerve endings which convert optical images into nerve impulses.
  • rk - Radial Keratotomy - Procedure where a doctor surgically flattens the cornea by making a series of small, radial incisions in the periphery of the cornea. This flattens the cornea and changes its focusing power so that light rays fall precisely on the retina. Radial Keratotomy is used to treat nearsightedness and some forms of astigmatism.
  • rods - One of the two principal nerve endings of the retina which are highly sensitive to low variations in illumination but relatively insensitive to color differences.


  • sclera - The outer coat of the eyeball, a tough fibrous membrane.
  • scotoma - An impairment to vision caused by diminished or total lack of function of the retina in a limited area. It may be unnoticed (Mariotte's blind spot) or be seen as a black area in the visual field.
  • specular reflection - A reflection from a mirror surface.
  • sphere - A surface on which every point is equidistant from the center; a surface of revolution.
  • spherical aberration - Breach of a point image of a point object by rays refracted from the periphery of a surface to a shorter focal distance than paraxial rays.
  • strabismus - Anomalous fixation in which the non-fixing eve is turned in another direction, thus retinal images fall upon noncorresponding points. Heterotropia, squint, crossed-eyes, wall eyes.


  • trifocal - A lens with three focusing points; as, distance, intermediate and reading.


  • ultraviolet radiation - Invisible radiation below visible violet from 4000 AU to the beginning of x-rays, at about 150 AU. Responsible for the tanning effect on skin, and harmful to certain ocular tissue such as the crystaliine lens. Sub-divided into three regions called A,B, and C.
    • A Band - The longest ultraviolet rays, adjacent to the visible spectrum with 330 - 400 nanometer range. The ultraviolet rays most likely to present an ocular hazard.
    • B Band - Wavelength range of approximately 275 - 330 nanmoeters.
    • C Band - Band with the shortest wavelengths of the ultraviolet rays, between 200 -275 nanometers.
  • uvea - The middle coat of the eye which consists of the choroid and extends to the ciliary body and iris.


  • vitreous humor - The transparent gel filling the space between the crystalline lens and the retina. It is contained in a hyaloid membrane which is attached to the retina in the area of papilla.


  • x-axis - The imaginary line connecting the centers of rotation of the eyes. The line connecting the geometric centers of a pair of spectacle lenses.


  • y-axis - An imaginary line perpendicular to the x-axis and fixation axis through the center of rotation. A line perpendicular to the x-axis of the spectacle lens and optical axis.


  • z-axis - The fixation axis of the eye, i.e. the line from the point of fixation through the center of rotation. The optical axis of a lens.