1. Lens Types & Terminology
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2. Types of Lenses
The first type of lens is called a single-vision lens. A single-vision lens is used to correct one focal point. A focal point is a point of focus (such as distance, intermediate or reading). Single-vision lenses are used for seeing at one distance, hence the word ‘single’, meaning one. A single vision lens contains one main viewing area with one optical center.
Single Vision / SV
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3. Flat Top Bifocal
A lens containing two viewing areas is called a Bifocal. It corrects two focal points, one for distance and one for reading, hence the ‘Bi.’ Think of a bicycle, it has two wheels. A bifocal with a flat top segment is called a Flat top bifocal. This lens is also referred to as a D-Seg, because of the segment shape.
Bifocal
Distance
Near
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4. Flat Top Trifocal Trifocal
Flat Top Trifocal - A Flat Top Trifocal lens contains three viewing areas; distance, intermediate and reading.
Distance
Intermediate
Near 4
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5. Blended & Round Bifocals
Blended or Kryptok - bifocal lenses with a round bifocal or reading segment. The Kryptok lens, also called a Round Seg, has a visible segment 22 millimeters in width. The blended bifocal has an invisible segment.
Kryptok
Blended 5
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6. Types of Lenses
Progressive lenses are also called no-lines because they have no visible segments.
To identify a Progressive lens, hold the glasses about 18” from your face and look through the lens at the PROGESSIVE LENS IDENTIFYER GRID. If the lens is progressive you should be able to the see an hourglass shape on the grid.
WE DO NOT USE THESE LENSES 6
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7. Types of Lenses
Occupational Bifocal - is a type of lens with two reading segments, one on top and one on the bottom. This lens is also called a Dual Seg or Double D Seg (because the segments are shaped like the letter D).
WE DO NOT USE THESE LENSES 7
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8. Reading Segments
The segment height (also called seg height) is where the add power portion of a lens is placed in the frame that will provide the patient the best use for reading through the bifocal lens.
The segment height (also called seg height) is where the add power portion of a lens is placed in the frame that will provide the patient the best use for reading through the bifocal lens and is where we read the bi-focal power on the lensometer.
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9. Optical Center
Optical Center is the most accurate part of the prescription lens that is normally placed directly in front of the patient’s pupil unless prism is prescribed and is where we read the distance power of the lens.
Optical Center / OC
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10. Prism & Refraction
A prism is a triangular shaped piece of optical medium with a base and an apex. A prism has no focusing power. It deviates the path of light, sending it in a different direction. Prismatic lenses are often used to help strengthen the muscles controlling movement of the eye.
Refraction is the term used to describe the bending of a light ray. Refraction can be different based on the thickness or shape of the lens as well as the lens material that the light is passing through 10
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11. Prescribed Prism
There is prescribed prism. This is when a doctor is trying to correct a patient’s tendency of the eyes to cross or turn, outward, upward or downward. A patient’s eye will be pulled toward the optical center of a lens.
WE DO NOT USE GLASSES WITH PRISM
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12. Diopter
The unit of measurement of the refracting power of a lens is called a diopter. Prescriptions most commonly use quarter diopter steps. One diopter is written as 1.00, three fourths of a diopter as .75, one half of a diopter as .50 and one fourth of a diopter as These measurements are accompanied by a plus or minus depending on the refractive error: hyperopia (+) or myopia (-). 12
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13. Ultraviolet Protection
UV or Ultraviolet Protection is designed to prohibit harmful UV rays from entering the eyes.
All plano sunglasses we use should be passed under a UV Meter and have a reading of 5 or less. If a UV meter is not available please indicate on the bag “UV NOT READ”
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