ADD: the additional plus power needed for close vision. It supplements the distance prescription.
Aniseikonia: the condition which results from a significant difference in refractive error, one eye to the other. Among other effects, it describes an image size difference which may lead to double vision or a loss of binocular vision.
Aspheric Lenses: From Latin, the letter “A” prefix means “not”, in this case aspheric means not spherical. Generally, refers to the front curvature of the lens which changes curvature from the center to the edge to correct for optical errors.
Astigmatism: a common component of refractive error, corrected by using a cylinder value at a specific axis in the prescription. Caused by irregular curvatures in the eye.
Atoric Lenses: Similar to aspheric, above, but relating to the cylinder and spherical components of the back surface of the lens. It defines a complex geometry that provides clearer vision over a larger field of view. A feature of freeform technology.
Base Curve: in conventional lens manufacturing it represents the flattest concave curve on the inside of the lens. See also Cross Curve. For all other contexts it represents the front curvature of a lens.
Bicentric grinding: a process to grind compensating prism in a vertical direction on one lens to balance the prism induced at a specific location on the other lens. Also known as slab off prism.
Bifocal: a lens type that corrects vision at 2 focal distances, eg distance and near.
Coatings: coatings describe an exterior surface treatment on one or both surfaces of a lens that can be applied in many ways. Examples include anti-scratch coatings, anti-reflective coatings, mirror coatings, and so on.
Cataracts: describes the increasingly opaque (cloudy) condition of the crystalline lens inside the eye. When the lens becomes too opaque it will be surgically removed and replaced with an interocular implant. The condition can be caused by a congenital defect, trauma or natural aging and is corrected through surgery.
Diopter (D): the unit of measure commonly used in to describe the sphere and cylinder power, abbreviated as D. Technically it is the reciprocal of the focal length of the lens expressed in meters. A related term prism diopter is used to describe units of prism.
Freeform Technology: It uses the ray tracing principle to define multiple points on the lens surface where the solutions for the Rx and optical errors are determined. Unlike conventional surface calculations made in base curves and cross curves, freeform topography is a seamless surface incorporating thousands of discrete points each with its own characteristics. The Persona PRO family of designs uses freeform technology.
Hyperopia: Difficulty seeing near objects clearly. Corrected with plus-power lenses.
Laminated Lens: a lens fabricated through the joining of 2 or more layers together in a permanent fashion. Typically, an optical cement is used between the layers to form a lasting bond.
Lenticular Lens: Commonly described as a fried egg appearance; used in high plus power lenses or post cataract lenses where the central zone contains the prescribed power, and the outside portion called the carrier supports the remaining portion of the lens.
Myopia: Difficulty seeing distant objects clearly. Corrected with minus-power lenses.
PD (Interpupillary Distance): The distance between the centers of the pupils when the gaze is fixed at a distance or near object; distance PD or near PD respectively. May be expressed as a binocular value (between 2 eyes) or monocular (mono PD) pupil center of one eye to the center of the bridge.
PERSONA PRO: Riverside’s in-house brand. Launched in 2006 as PERSONA using freeform technology, and upgrades it represents state of the art lens designs and features.
Photochromic: the property of a lens to change colour and reduce light transmission when it is exposed to UV & visible light.
Polarized: polarized lenses are generally used for sunwear due to their dark colour and light filtering properties. The principal benefit of polarized lenses is that they filter out the glare from reflected light, for instance from road surfaces or water.
Presbyopia: a naturally occurring condition describing the diminishing capacity of the eye to focus on objects at near. Technically it is the loss of accommodation in the crystalline lens within the eye to change its shape and increase power. It is a progressive condition that begins around age 40 -45 and stabilizes around age 60-65. Corrected with reading lenses or multifocal / PAL lenses.
Progressive Lenses (PAL): progressive addition lenses, a form of multifocal lens with no visible or distinct lines of reference and where the power changes from distance to reading in a seamless fashion.
Prescription: the prescribed lens power to correct a patient’s vision or, refractive error, as determined by a qualified and licensed ECP.
Single Vision (SV): Lenses for one distance only—near or far.
Sphere (SPH): a lens that has the same power in both meridians, or a surface with the same curvature throughout.
Trifocal: a form of lined multifocals with 3 focal points, distance, intermediate and near.
Tints: this term describes either the process or the colour to treat lenses. Tinted lenses are used for sunwear, for a cosmetic appearance or to mitigate vision issues.
Vertex Distance: the measurement of distance between the front surface of the eye (cornea) to the back surface of the corrective lens. Can be stated as fitted vertex distance between the cornea and back surface of the lens mounted in the frame or refracted vertex distance between the cornea and phoropter. It can be used in a calculation to adjust lens power.
Visual acuity (VA): Refers to the visual sensitivity of the eye to recognize images. It is often abbreviated as VA. It is a measure of how well the subject eye sees a target image compared to the average population. Common terms used are 20/20, 6/6 or 10/10.
Wrap Angle: describes prism that is prescribed with the bases stated in the same gaze direction. It is frequently used to correct visual/spatial displacement for people with traumatic brain injury or concussion.
