Patricia Wild Optometrists- Ramsey and Farmhill
Multifocal Lenses for spectacles
There are different terms used which describe multifocal spectacle lenses, all of which fall under the terminology 'multifocal spectacle lenses'. They are most commonly known as progressive spectacle lenses, but are also called progressive addition lenses (PAL), progressive power lenses, graduated prescription lenses, and varifocal or multifocal lenses. They are corrective lenses used in eyeglasses and spectacles to correct presbyopia and other disorders of accommodation.
They are characterised by a gradient of increasing lens power, added to the wearer's correction for distance vision. The gradient starts at the wearer's distance prescription, at the top of the lens and reaches a maximum addition power, or the full reading addition, at the bottom of the lens. The length of the progressive power gradient on the lens surface depends on the design of the lens, with a final addition power between 0.75 and 3.50 dioptres. The addition value prescribed depends on the level of presbyopia of the patient. In general the older the patient, the higher the addition.
The Varilux & Carl Zeiss lenses were the first PAL of modern design. It was developed by Bernard Maitenaz, patented in 1953, and introduced by the Société des Lunetiers (which later became part of Essilor) in 1959.
Early progressive lenses were relatively crude designs. People often had trouble adapting to new prescriptions, but that is no longer the case! Most patients adapt quickly to newer, more modern multifocal spectacle lenses. In the older-style multifocal lenses, right and left were identical variable power lenses with distance and reading power centers in the upper and lower part of the lens, respectively. The glazing was made to accommodate that the wearer changes eye position from distance viewing to reading. By tilting the reading power towards the nasal side in perfect symmetry, appropriate reading power was given to the wearer.The symmetric design, however, was difficult to accept for patients, because the eyes in general work asymmetrically.
Modern sophisticated progressive lenses are designed asymmetrically for greater patient acceptance and include special designs to cater to many separate types of wearer application: for example progressive addition lenses may be designed with distance to intermediate or intermediate to near prescriptions specifically for use as an occupational lens, or to offer enlarged near and intermediate view areas.The typical progressive lens is produced from a so-called semi-finished lens. The semi-finished lens is moulded with an asymmetrical power pattern on the front. On the back side a custom surfacing is made to adjust the power for each patient. This method, however, is problematic especially for astigmatic prescriptions. The reason being that the semi-finished front pattern is designed for a spherical prescription.
Freeform designs are tailored to each prescription and do not have this problem. Since the 1980s, manufacturers have been able to minimize unwanted aberrations by improvements in mathematical modelling of surfaces, allowing greater design control, and improved manufacture. technology.
Today the complex surfaces of a progressive lens can be cut and polished on computer-controlled machines, allowing 'freeform surfacing', as opposed to the earlier casting process, thus explaining the difference in price. In short, the price is based on the technology used and the year the lens came to market, with more modern lenses giving better quality and ranges of vision than older design lenses.