MOC PQA - Optician key terms

The distance from the back surface of a spectacle lens to the front of the cornea, usually 12 to 14 mm. It matters because high-powered lenses change effective power when moved closer to or farther from the eye.

The lens power recalculated for a different vertex distance than the one used during refraction. Used when fitting contact lenses or when a trial frame distance differs from the final frame fit.

The actual power a lens exerts at its position of use. Moving a lens along the optical axis changes its effect on the eye, especially for corrections above about plus or minus four diopters.

The power measured from the rear surface of a lens, which is the value used in prescriptions and focimetry. It differs from front vertex power in meniscus lenses, particularly in higher powers.

The power measured from the front surface of a lens. It is used in some laboratory calculations and differs from back vertex power whenever lens thickness and curvature make the two surfaces non-equivalent.

A calculation stating that induced prism in prism diopters equals decentration in centimeters multiplied by the lens power in diopters. Used to predict unwanted prism from lens decentration or to plan intentional prism.

A unit of prism power equal to a displacement of one centimeter at a distance of one meter. It describes how much a prism deviates light and is used to specify prescribed prism and tolerances.

The orientation of a prism relative to the eye, described as base in, out, up, or down. It determines which way the image is displaced and must match the prescription exactly.

Unwanted prism created when a lens optical center sits away from the line of sight. It can cause asthenopia, diplopia, or dizziness, and is checked against standards during final inspection.

The point on a lens where the optical axis crosses the surface, so no prism is induced there. Its position relative to the pupil determines unwanted prism and is verified with a lensmeter.

The point on a lens, usually aligned with the pupil, at which the prescribed power and prism are meant to be delivered. Layout and blocking are based on this point rather than geometric center.

The point on a lens where the prescribed prism is measured, typically marked during layout. Its location must be confirmed before edging so the prism sits correctly in front of the eye.

The distance between the centers of the pupils, measured monocularly or binocularly in millimeters. Accurate values keep optical centers aligned with the eyes and prevent unwanted prism.

An instrument that measures the distance between pupils and often pupil height relative to the frame. Modern devices use corneal reflections to record monocular distances for progressive and high-power lenses.

An optical instrument that measures lens power, axis, prism, and optical center position. It is used to verify prescriptions, locate optical centers, and check finished eyewear against tolerances.

Another name for a lensmeter, an instrument that neutralizes lens power by focusing a target through the lens. It reports sphere, cylinder, axis, and prism values for verification.

An instrument that measures corneal curvature and astigmatism, giving base curve guidance for rigid contact lens fitting. Readings are reported as radii of curvature and corneal power in diopters.

A device that maps corneal elevation and curvature across the whole surface, producing color maps and indices. It is used for contact lens fitting, orthokeratology, and detecting irregular or ectatic corneas.

A biomicroscope with an adjustable light beam used to examine the eyelids, cornea, conjunctiva, anterior chamber, and crystalline lens. It also supports contact lens fitting evaluation and fluorescein assessment.

The curvature of a lens surface, usually the back surface of a contact lens or the front surface of a spectacle lens. It affects fit, magnification, and cosmetic appearance, and is recorded in millimeters or diopters.

The height of a contact lens or corneal surface measured from a chord to the apex. It describes how a lens vaults the cornea and helps predict tightness or looseness of fit.

The pattern of tear pooling and bearing seen under a rigid contact lens when fluorescein is instilled and viewed with cobalt blue light. It guides changes in base curve, diameter, or peripheral curves.

Swelling of the cornea caused by fluid accumulation, often from hypoxia or overwear of contact lenses. Signs include haze, striae, and reduced vision, and it requires lens discontinuation and review.

The Dk/t value describing how much oxygen passes through a contact lens, where Dk is permeability and t is thickness. Higher values reduce hypoxic stress during daily and extended wear.

A soft contact lens material combining silicone for high oxygen permeability with hydrogel for comfort and wettability. It dominates modern daily and extended wear because it reduces hypoxia-related complications.

A rigid contact lens made from oxygen-permeable polymers that holds its shape on the eye. It provides crisp vision, handles astigmatism well, and requires careful fitting based on corneal shape.

A progressive condition in which the cornea thins and steepens into a cone, causing irregular astigmatism and reduced vision. Rigid and specialty contact lenses are common optical management options.

Astigmatism arising from unequal curvature of the cornea across different meridians. It is measured by keratometry or topography and influences both spectacle correction and contact lens selection.

Astigmatism in which the cornea is steeper vertically than horizontally, so a plus cylinder axis sits near 90 degrees. It is common in younger patients and corrected with the appropriate cylinder axis.

Astigmatism in which the cornea is steeper horizontally than vertically, with a plus cylinder axis near 180 degrees. It becomes more common with age and affects lens orientation and tolerance checks.

The age-related loss of ability to focus on near objects as the crystalline lens stiffens. It is corrected with reading adds, progressive lenses, bifocals, or multifocal contact lenses.

The extra plus power in the near portion of a multifocal lens, equal to the difference between near and distance power. It is prescribed for presbyopia and must be verified during final inspection.

A lens with a continuously increasing plus power from distance to near, with no visible line. Design choices balance corridor length, reading width, and peripheral distortion for the wearer.

The vertical distance over which a progressive lens increases from distance to near power. A longer corridor gives a gentler progression and wider zones but requires sufficient fitting height.

A reference mark on a progressive lens, usually a cross or circle, used to align the lens with the pupil during layout. Correct placement ensures the wearer looks through the intended zones.

The downward angle of the frame front relative to the face, commonly 8 to 12 degrees. It affects the effective power and optical center position of the lens and should be considered in high prescriptions.

The angle at which the frame front wraps around the face, measured between the two lens planes. Increased wrap alters prism and power, so compensated lenses may be needed for wrapped sports frames.

The three-point contact of a frame at the bridge and both temples, which keeps the frame stable and level. Adjusting bridge and temple angles restores comfort and correct lens position.

The measurement from the hinge to the end of the temple, usually 135 to 145 mm. Correct length keeps the frame from slipping and prevents pressure behind the ears.

The distance between the two lenses at the bridge, measured in millimeters. It must match the wearer's nasal anatomy for comfort and to keep the optical centers properly positioned.

A standardized method of measuring frame fronts using the smallest rectangle that encloses each lens. It gives lens width, bridge width, and temple length for ordering and comparison.

A thin multilayer coating that reduces surface reflections and increases light transmission through a lens. It improves cosmetic appearance, night vision, and clarity in photographs and screens.

A hard coating applied to plastic lenses to reduce surface abrasion during normal use. It improves durability but does not make the lens unbreakable or eliminate the need for careful cleaning.

A top-layer treatment that repels water and oil, making lenses easier to clean and reducing smudging. It is often combined with anti-reflective layers in a multi-coat package.

A surface treatment that resists oils from skin and fingerprints, keeping lenses clearer between cleanings. It is usually paired with hydrophobic layers on anti-reflective lenses.

A lens that darkens when exposed to ultraviolet light and clears indoors. Performance varies with temperature and UV intensity, and it is not a substitute for polarized sun lenses in all situations.

A lens containing a filter that blocks light vibrating in a particular plane, reducing glare from water, snow, and roads. It is useful for driving and outdoor sports but can interfere with some displays.

The ability of a lens or coating to block UVA and UVB radiation that contributes to ocular damage. Standards define claims and labeling, and protection should not depend on lens tint darkness alone.

The ability of a lens and frame to withstand a standardized drop-ball or high-mass test without breaking. It is a regulatory requirement for many dress and safety eyewear categories.

A United States standard specifying performance requirements for eye and face protection, including impact, optical, and marking criteria. Safety eyewear must be marked to show compliance with the applicable clauses.

The process of explaining risks, benefits, alternatives, and costs so a patient can make a voluntary decision. It should be documented and is central to ethical contact lens and dispensing practice.

The legal boundaries defining which tasks and clinical decisions an optician may perform. Exceeding them, such as diagnosing disease, creates regulatory and liability risk regardless of experience.

The systematic documentation of prescriptions, measurements, fittings, adjustments, and patient communications. Accurate records support continuity of care, quality review, and defense in complaints or audits.

The permitted deviation of lens power, axis, prism, and centration from the prescription defined by standards. Knowing tolerances helps an optician decide whether eyewear is acceptable or must be remade.