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Optical Terms
A Measurement (A) — The horizontal (side-to-side) measurement used in the boxing system. Addition (Add) — Plus power at near. Usually the segment power for reading, but sometimes a high-powered add for special use. The difference in power between the reading, and distance parts of a prescription.
Amblyopia — Reduced acuity that is not correctable with glasses. There will be no obvious cause. Also called ”lazy eye”. Aphakia — Is the absence of the lenses natural crystalline lens. This can be the result of surgery, or trauma.
Aphakic — An eye that has had the crystalline lens removed, sometimes by accident, but usually by cataract surgery. Aspheric — A non-spherical base curve designed for flatter curves, thinner lenses and improved peripheral vision.
Astigmatism — When astigmatism is present there will be power in the cylinder and an axis. An Astigmatic eye has variations of the error of refraction in different meridians. Axis (x or X) — An imaginary line running through the optical center used to designate cylinder orientation in a lens. Full cylinder power is always 90 degrees from this line.
B Measurement (B) — The total vertical (top-to bottom) measurement used in the boxing syste

The widest point of a prism, towards which light is bent.
2. The direction of prism power.
3. See; Base Curve

Base Curve (BC or Base) — The surface, or main curve used as the reference point for figuring the power in all other curves. Bifocal (Bi or Bif) — A bifocal lens has two separate focal lengths. It is in effect, two different lenses, the distance lens, and the near segment.
Binocular — Pertains to vision in both eyes. Bridge1. The part of the frame that connects the two lenses.
2.  The part of the nose that a pair of glasses sits on.
Bridge Size — The size of the frame that bridges the two lenses. Sometimes, but not always the same as the DBL, or distance between lenses. Browbar — On frames with double bridges, the top bar.
C Measurement (C) (Circ) — The vertical (top-to bottom) measurement at geometric center used in the boxing system. Canthus — Points where the upper and lower lids meet. The inner and outer canthus.
Cataract — A condition of the eye in which the crystalline lens becomes opaque, or yellows a causing partial or total vision loss. The three types of cataracts are:
1. Senile
2. Congenital
3. Traumatic.
Center, Geometric (GC) — The point on a lens that is an equal distance horizontally from the A measurement reference points.
Center, Mechanical (MC) — The point of a lens around which it is cut and edged. Center, Optical (OC) — The visual center of the lens, through which light rays pass unbent. The point of no prism.
Centimeter (CM) — There are 10mm to a cm and 100cm to a meter. Chassis — The metal or semi-rimless part of a font on a combination frame.
Circumference ( C ) (Circ) — Also called the C Size The lens circumference measured in millimeters. Usually obtained with a circumference gauge. Concave — A surface that curves inward.
Convergence — The act of the two eyes coming to focus on the same object. Cornea — The transparent front surface of the eye that acts as the first part in transmitting light through the eye. Layers are Epithelium, Bowman’s Membrane, Stroma, Discemets Membrane and Endothelium.
Corneal Reflection Pupillometer — Used to measure the Interpupillary distance of a patient. Uses light reflected off the cornea. CR-39 — One of the trade names for plastic lens material.
Crown Glass — Type of glass most commonly used today in spectacle lenses. Index of 1.523. Crystalline Lens The lens of the eye. It is biconvex in shape and focuses light on the retina. Diopter power ranges from +10.00 to +58.00.
Cylinder (Cyl.)1. Power of the astigmatism in a lens.
2. A lens surface having more than one curve.
Decentration — Distance of displacement between the optical center of a lens, and it’s mechanical center. You will decenter a lens to put the O.C. directly in front of the pupil, or to create, or remove prism.
Diopter (D) — Unit of the measure of the refractive power of a lens. A one-diopter lens has a focal length of 1 meter; a .25 diopter lens has a focal length of 4 meters. Diopters are also the unit of measurement for lens curves, and prism power. Distance Between Lenses (DBL) — The shortest distance between 2 lenses at the nasal. This is sometimes, but not always the same as the bridge size.
Double Gradient Tint (Dbl Grad) — A tint that starts at both the top and bottom of the lens and gets lighter as it nears center. May, or may not be the same color from both directions. Effective Diameter (ED) — The smallest possible diameter of a lens, as figured by the radius from Geometric Center.
Exotropia — Describes an eye with a definite and obvious turn outward. At least one eye clearly pointing temporally. Walleyed. Eyesize — The widest horizontal measurement in the eyewire.
Eyewire — The part of a frame that encircles a lens. Farsighted (Hyperopia) — Requires a Plus lens for correction.
Flint Glass — Flint glass contains lead, and has a higher index of refraction than the more commonly used crown glass. 1.80 index. Focal Length — The distance between the back surface of a focusing lens, and the focal point.
Focal Point — The point at which rays from a light pencil converge, or from which they seem to diverge. Frame The unit that holds a pair of spectacle lenses, consisting of a front, (not always continuous), and a pair of temples.
Frame PD (Geometrical Center, Distance) — Frame PD minus total distance PD will give you total decentration for a pair of lenses. This can be divided in half to give you decentration for each eye, or you can determine separately for each eye. The Frame PD can be estimated by adding the eyesize and bridge together. Front — That part of the complete frame that holds the lenses, and to which the temples attach.
Geometric Center (GC) — The center point on the datum line of the boxing system. Equal distances from all opposite edges of the lens box. Glass — Material used in eyeglasses and other items. In eyeglasses, Crown Glass is most commonly used today.
Glaucoma — A disease of the eye that causes increased pressure of the eyes fluids. Early treatment can arrest the disease; late diagnoses can result in blindness. Gradient Tint (Grad.) — A tint that starts at the top of the lens and gets lighter as it gets lower in the lens.
Hyperopia (Farsighted) — Light rays come to a focus behind the retina. A convex/plus lens is used to correct for hyperopia. Image Jump — Used to describe the rapid change in image size as the eye moves from one part of a multifocal to the other.
Index of Refraction (Index) — Is the ratio of speed of light in air, as compared to the speed of light in a second substance. Indicates the ability of a substance to bend light rays. Infrared — Invisible light rays with longer wavelengths than visible light.
Inset — The amount in millimeters that the O.C. of a lens is moved IN nasally from G.C. The bifocal section of a lens is inset from distance PD nasally to allow for the convergence of the eyes at the near PD Intermediate — The trifocal lens portion, or power. This is usually 50% of the bifocal power.
Interpupillary Distance (PD) — See Pupillary Distance. LASIK — Flap and Zap laser surgery. Most recent and fewest problems.
Lens — A light refracting medium with curves designed for certain powers. At least one surface must be curved. Lens Blank — Generally, a Semi-Finished lens. One having a finished front base curve that will require surfacing for the lens to have power.
Lens Measure — Used to measure the curves of a lens. Also called a Lens Clock or the Geneva Lens Measure. Resembles a pocket watch with 3 prongs. Center prong moves in and out when pressed against lens allowing user to measure curvature of lens. Usually set for glass. Lens Washer — A plastic material that is used as filler between the lens and the frame used when then lens is too small.
Lens, Compound — Lens having more than one correction. A lens having cylinder. Lens, Finished — A lens with both surfaces completed. This type lens is ready to be cut and edged into a frame.
Lens, Meniscus — A lens that has a curved/crescent shape. Lens, Plano — A lens having no power. The front and back surface curves are spherical and identical.
Lens, Plano — Cylindrical A lens with cylinder power, but no sphere power. Example: Plano -.75 x 65. Lens, Semi-Finished — One having a finished front base curve that will require surfacing for the lens to have power.
Lens, Spherical — A lens having two spherical surfaces with the same, or different curvature, but no cylinder. Lensometer — Instrument used to read lens powers, Spotting of prism, and layout of a lens when blocking. Lensometer is a trade name, also known as a Vertometer.
Limbus — A dark, circular area where the cornea and the sclera join. Macular Degeneration — A number of causes will atrophy/degenerate the macula. Loss of vision can range from blind spots to near complete.
Major Reference Point (MRP) — The point on a lens where prism matches what was ordered. Mechanical Center (MC) — The point on a lens used as a reference center when edging.
Meniscus — A crescent shaped lens. Meter (M) — Unit of measurement in metric system.
Millimeter (mm) — 1000 mm to a meter. Minify — Minus lenses have the opposite effect of plus lenses. Where plus lenses magnify objects viewed, minus lenses minify, or make the object look smaller.
Minus ( - ) — The minus sign - shows a minus lenses power, or curve. Minus Cylinder (Grinding) — Lens ground with the base curve on the front surface and two back surface curves in different meridians.
Minus Cylinder (Written Form) — Of the two written forms of prescription, this one shows the cylinder power written as a minus number. Monocular — Referring to “one eye”.
Mydriatic — Type of drugs used to dilate patient’s pupils. Myopia (Nearsighted) — Light rays come to a focus in front of the retina. A concave-minus lens is used to correct for hyperopia.
Nasal (N) — Refers to the nose side (inside) portion of a frame, or lens. Near Rx — The total reading power of a lens. Achieved by adding the bifocal power to the distance.
Nearsighted — See: Myopia. Neutralization1. Offsetting the refractive power of a lens using countering lenses of equal, but opposite power, such as from a trial lens set.
2. Reading the prescription of a lens, or pair of lenses using the lensometer.
Occlude — To block light coming to the eye. Oculus Dexter (OD) — The right eye, or pertaining to.
Oculus Sinister (OS) The left eye, or pertaining to. Oculus Uterque (OU) — Both eyes, or pertaining to both eyes.
Optical Center (OC) — The point of no prism in a lens. Light rays will pass through unrefracted. Any point away from optical center will have some prism. This will always be the thinnest point on a minus lens and the thickest on a plus. Pantoscopic Angle — Tipping in towards the face of the bottom of the frame.
Phoropter — Device containing a series of lenses and placed in front of the patient’s eyes to determine prescription. Photochromic — A lens that changes its light transmission when exposed to light.
Plano (Pl) — A lens surface, or a lens having no refracting power. Plastic1. Plastic lenses are also referred to as CR-39 and have an index range of 1.49 to 1.51.
2. Plastic frame material was originally referred to as Zylonite, or zyl. Newer plastic frame can be made of a variety of materials all lumped under the label plastic.
Plus (+) — The plus sign + shows a plus lens power, or curve. Plus Cylinder (Grinding) Lens ground with the base curve on the back surface and two front surface curves in different meridians. This is an older style, seldom seen today.
Plus Cylinder (Written Form) — Of the two written forms of prescription, this one shows the cylinder power written as a plus number. Polycarbonate (Poly) — The safest material available. Nothing is unbreakable.
Presbyopia (Means – “Old Eyes”) — The loss of accommodation at near due to aging. Prescription (Rx) — The written form showing the power needed to correct visual acuity that the patient receives from the doctor.
Prism — A wedge shaped piece of a refracting material consisting of the apex, base, and the apical angle whose purpose is to bend light. The sign for prism is a triangle. Prism Diopter — The unit of prism power (or degree) measurement.
Photo Refractive Keratechtomy (PRK) — Initial laser surgery for refractive correction. Painful and slow to heal compared to LASIK. Progressive Addition Lens (PAL) — Generic term for progressives.
Progressive Power Lens (PPL) — Generic term for progressives. Pupil — The aperture of the eye through which we focus.
Pupillary Distance (PD) — The distance between the centers of a patients pupils when focused at a specific distance, either at infinity (distance), or up close, (near, or intermediate). Pupillometer — See: Corneal Reflection Pupillometer
Radial Keratotomy (RK) A surgery using a series of incisions designed to correct myopia. Refraction1. The deviation, or change in direction of a light ray as it passes through an optical medium at any angle other than 90 degrees.
2. That part of an eye exam that determines patient’s prescription.
Retina — The retina is composed of a layer of cells (rods and cones) that are sensitive to different aspects of light. The retina is the inside portion of the back of the eye and connects to the optic nerve. Retroscopic Angle — Tipping out away from the face of the bottom of the frame.
Rods — Nerve cells that specialize in detecting low light. Sclera — The white of the eye.
Segment (Seg) — That part of a lens having more power than the main part of the lens. The multifocal part of the lens. Semi-Finished — A lens blank that has only had one side ground and polished.
Slab-Off — A lens with bicentric grinding used to compensate for vertical imbalance. Snellen Chart — Most common chart used today for visual acuity.
Sphere (Sph) or (S) — Usually refers to the sphere power of a lens. Also, a lens having no cylindrical value. Surfacing — The procedure used to obtain the curves required for a prescription. Grinding the surface of a lens.
Temple — The part of frame connecting to the front and wrapping behind the ears. Also called bows, sidepieces, and those things that go behind the ears. Temple Length — The length of the temple. May be measured from several different points.
Temporal — Refers to the temple side (outside) portion of a frame, or lens. Transitions — Most common Plastic photochromic lens type.
Transposition — Changing the "written" form of a prescription. Trifocal (Tri) — A lens having three distinctly different focal powers. Usually distance (top), Near(bottom) and intermediate (middle).
Ultraviolet Light (UV) — Ultraviolet light is a somewhat shorter wavelength than visible light. Has been linked to sunshine cataracts. Vertex Distance — Is measured from the back surface of the lens to the front surface of the eye.
Visible Light — That part of the spectrum a normal eye can see. With Motion — Minus lenses have "with motion", when a minus lens is held over an object and moved back and forth, the object will seem to travel "with" the motion of the lens.