Single vision lenses, as the name suggests, correct a single refractive error with a single focal length. When a myopic or hyperopic refractive error exists, spectacles are required to correct vision. Minus powered single vision lenses are used to correct myopia and plus powered single vision lenses are used to correct hyperopia. Single vision lenses can also be used as reading glasses for presbyopic patients who have clear distant vision, but require correction for close objects.
As people age, the lens of the eye tends to harden, resulting in a lessening of the ability to focus on near objects. This condition is known as presbyopia. As presbyopia sets in, if an individual has an existing refractive error, two refractive corrections may become necessary; one for near and one for distance. To accommodate two refractive corrections, bifocals can be used. In a bifocal, the bulk of the lens is used to compensate for the existing refractive error, while a plus correction or add power segment is included in the lower portion of lens, where people already tend to look to view near objects (such as when reading a book), to correct for the presbyopic condition.
Common types of bifocal lenses include: round segment, which has a circular add segment of the lower portion of the lens; flat top, which has a semi-circular add segment; and executive, which uses the entire lower portion of the lens for the add segment. Add segments are available in many different sizes to accommodate a patient's needs and frame requirements.
As presbyopia advances, depending on the patient's activities and lifestyle, there may be a need for intermediate distance refractive correction, in addition to the near correction. Trifocals include an intermediate range add segment on top of near range add segment to accommodate the patient's multi-focal requirements. Trifocals are commonly available in flat top and executive styles. Like bifocals, trifocals segments are available in many sizes and heights to accommodate a patient's needs and frame requirements.
Blended mutifocals are bifocals with no visible line between segments, but unlike progressive lenses have no gradual change or useable region between existing refractive correction and the add power. The line between segments is simply blended, as the name suggests, making it less visible.
Unlike bifocals and blended multifocals, progressive lenses offer a continuous, gradual change in power from the upper distance portion of the lens to the lower near portion. Consequently, in addition to providing correction for all distances, progressive lenses have no visible lines and thus are more aesthetic than bifocals or trifocals. Progressives are available in many different designs to accommodate a patientï¿½s viewing requirements, as well as various fit heights to match different frame sizes. Custom progressive lenses are now even available, which can be ground specifically for an individual, taking precisely into consideration the individualï¿½s anatomy, visual requirements, and frame choice.
Occupational lenses are designed to meet specific occupational or recreational needs of a presbyopic patient. For example, the majority of an individual's viewing may be done in the intermediate distance, as may be the case with frequent computer use, requiring the portion of the lens that would typically be used for distance, to contain the intermediate range power correction. Another example is an individual whose occupation requires viewing near objects over their head, requiring an add segment in the upper portion of the lens. Occupational lenses are available in a number of styles and designs. Contact your lab to find out what is available.