Bifocal Lens:  A special type of eyeglass lens that incorporates two different optical powers in the same lens, typically to correct both distance and near visual acuity.

Contact Lens:  A small disc made of various silicone and or plastic materials usually containing an  optical correction which is worn over the cornea as an alternative to spectacles.

Convex Eyeglass Lens:   A lens that is thicker in the center than at the edges, which causes light rays from an object to converge.  This causes light rays in a hyperopic (farsighted) eye to focus on the retina which brings a blurry image in focus.  Farsighted eyes usually focus their light rays behind the retina.

Cylindrical Eyeglass Lens:  A lens that produces a different refractive power in each meridian which is used for correcting astigmatism.

Diopter:  The unit of measure used in ophthalmology to designate the refractive power of a lens.

Divergence:  The spreading apart of light rays as they leave an object or when they are refracted through a minus power lens.

Geometric optics:  The area of optics that deals with the transmission of light rays and is concerned with the effect of lenses on light and the production of images.

Legally Blind:  Best corrected visual acuity of 20/200 or less, in the better-seeing eye.

Low Vision Aids: High powered plus lenses which help patients who have poor vision.

Monovision:  An alternate method of correcting presbyopia.  One eye (the dominant eye) is corrected for distance vision and the other eye (non-dominant eye) is corrected for near.  This can be accomplished with contact lenses or refractive surgery, but usually does not work with glasses.

Multifocal Implants:  An implantable intraocular lens that attempts to incorporate more than one optical power to permit focusing at different distances (distance and near).

Optical Power:  The degree to which a lens, mirror, or other optical system converges or diverges light.

Optician:  A professional who makes and adjusts optical aids such as spectacles and contact lenses from a prescription supplied by an ophthalmologist or an optometrist.

Optics:  The branch of physics that deals with the properties of light and vision, for instance its refraction and reflection by lenses, prisms, mirrors and the eye.  Two principal areas of optics that apply to ophthalmology are physical optics and geometric optics.

Optometrist:  Doctor of Optometry. (O.D.) specializing in vision problems, treating vision conditions with spectacles, contact lenses and vision therapy and may also prescribe medications for certain eye diseases.

Orthokeratology:  (AKA:  Ortho-K):  A system of treating myopia and astigmatism with a sequential series of flatter-than-normal contact lenses that gradually flatten the cornea.  In orthokeratology, patients wear the contact lenses while sleeping.

Patch Therapy:  Treatment of amblyopia which involves occluding an ambyopic patient’s better seeing eye to improve vision in the amblyopic eye.  This form of therapy is most effective when it is begun before age 8.

Physical Optics:  The area of optics that describes the nature of light in terms of its wave properties.

Pinhole device:  An opaque disc with one or more holes which blocks all but the centermost light rays from an object.  It is used as a quick screening device to determine whether reduced vision is caused by refractive error.  (Vision through the pinhole will improve if the decreased vision is caused by refractive error.)

Prism:  Wedge-shaped lens that bends light rays toward its base.  Prisms are used to correct eye deviations (eye turns) that cause eyestrain and diplopia.

Progressive Eyeglass Lens (Invisible bifocals):  An eyeglass lens that incorporates corrections for distance, middle and near vision without any visible bifocal line.

Pupillary Distance (P.D.):  The distance between the pupillary centers of opposing eyes.  This measurement is critical to making eyeglasses as the optical center of each lens needs to be centered over the pupil of each eye.

Pupillometer:  An ophthalmic instrument used to measure the size (diameter) of the pupil.

Refract:  To bend.  In ophthalmology, light rays are refracted when they pass through different types of lenses.

Refraction:  A subjective determination of an eye’s refractive error as well as the best prescription for glasses and contact lenses to correct it.  This test involves a series of lenses in graded powers which are presented to the patient to determine which lens, or combination of lenses, provides the sharpest visual acuity.

Refractive Error:  Optical defect in which parallel light rays are not brought to a sharp focus precisely on the retina producing a blurred retinal image.  Types of refractive errors include myopia, hyperopia, astigmatism and presbyopia which can be corrected with spectacles, contact lenses and refractive surgery.

Retinoscope:  A hand held instrument which is used to determine an eye’s refractive error objectively (with no response required from the patient).  Light from the retinoscope is projected into the eye and the movements of the light reflection are neutralized (eliminated) with corrective lenses.

Retinoscopy:  An objective measurement of an eye’s refractive error by using a retinoscope.

Single Vision Eyeglass Lens:  A lens that corrects only one type of refractive error.

Snellen Chart:  A test chart for assessing visual acuity.  The chart contains rows of letters, numbers or symbols in standardized sizes with a designated distance at which each row should be legible to the normal eye.  The standard testing distance is typically 20 feet.

Spectacles (Eyeglasses):   Optical aid which incorporates a combination of the appropriate lenses (concave, convex and toric) to correct various refractive errors.  There are many types of spectacles which include:  Bifocal,  trifocal, single vision, progressive, and transition lenses.

Toric Contact Lens:  A contact lens that incorporates spherical and cylindrical components which is used for correcting astigmatism.

Transition Eyeglass Lens:  Spectacles with sun-activated tints. Lenses are clear indoors and darken outdoors for greater comfort in the sun.

Trifocal Eyeglass Lens:  A spectacle lens that incorporates three distances (near vision, middle vision and distance vision) into one lens with visible lines of demarcation in the lens.

Visual Acuity:  Assessment of the eye’s ability to distinguish object details and shape, using the smallest object that can be seen at a specified distance, typically at 20 feet.