Introduction
Bifocal lenses are lenses that feature two distinctive power which is the distance and near segment. Varifocal lenses are known as progressive additional lenses (P.A.L). varifocal lenses are corrective lenses used in eye glasses to correct presbyope and other disorder of recommendation. They are characterized by a gradient of increasing lens power added to the wearer correction for the level of presbyopia of the patient to a lesser extent existing prescription.
Until the early 1980s, almost all multifocal were seqmented bifocal and trifocals lenses since that time, there has been steady growth in the usage of progressive additional lenses (P.A.L) and approximately 50% multifocal currently dispensed in the United States are of progressive design.There are significant optical difference between and within the segmented and progressive multifocal lenses that affect the vision that is providing to the patient in term of comparison varifocal, distance, intermediate an near vision, but if the bifocal is not properly measured in terms of segment height, it can create image jump. Varifocal lens requires utmost care and skills from the optician while bifocal can be easily fitted.
Types of Bifocal Lenses
- Fuse Bifocal
- D- Top Bifocal
- Executive Bifocal
- Invisible Bifocal
- Solid Bifocal
Varifocal lenses also known as progressive lenses, are used for prescription upper portion for distance while down portion for reading. Varifocal lenses have a gradual change in strength from the top to the bottom with multiple focal point allowing the wearer to see all distance and focus point just through one lens. This means wearer does not need to buy and care for two pairs of glasses or need to switch between glasses for different vision needs. Unlike bifocal lenses, they don’t have dividing line within the lens which makes them more cosmetically appealing and also disguise Presbyopia, the need for reading glasses which can often be associated with age as it typically occur from the age of 40 and above.
Varifocal lenses work by having a gradual change in strength from the top of the lens to the bottom and multiple focal point in between. The upper part of the lens contain the distance power, the middle of the lens has the intermediate rangers and the lower portion the reading part.
Below are basic region of varifocal lens:
Distance zone: The upper portion of the varifocal lens provides the distance viewing area.
Intermediate zone: The distance and reading areas are connected via a corridor of intermediate power and provides view for mid-range vision. Near or reading zone: This portion of the lens provides reading power.
Focal points of varilux lens.
How to use varilux glass
Varifocal glasses are easy to use, and can be adapted to quickly. They are positioned in a way that the user can look through the top of the lens and be viewing the near focus to read, the gaze is simply lowered so that the eye can look through the lower portion of the lens, the reading part.
Varifocal lenses also have additional benefit in that they have intermediate power range which is useful in between distance such as looking at shelves in the super market or dash board of a car and you simply lower or raise your head slightly until the view come into focus.
Types of varilux.
Regular varilux: They give a wide reading area and they require a certain size of frame to allow through vertical height to give a smooth transition from distance vision down to reading.
Concise Varilux:This type of lens is designed to fit into smaller frame because of their size. It takes a skilled optician to fit them properly.
Conceptual framework
Many theories abound through the invention of bifocal and also varilux lens, Benjamin (1784) is generally credited with the invention of bifocal lenses. Bruneni (1994). Optical laboratory association wrote that there has been a steady growth in usage of progressive addition lenses (PAL) and approximately 50% of PALs currently dispensed in the united state are of progressive design, he also wrote that result shows reveals the inability of others to read or write Were if not for recent invention of glasses.
According to Bomile (2013), presbyopia patients although patients that are getting older. When working with presbyopic patients it’s even more important to consider the psychological and emotional implication of the recommendations, because presbyopia is a very big deal. Not only does it greatly increase their complexity of choices in vision care. The aging population increasingly demands excellent near vision for viewing computers and other digital devices and at the same time, they have higher expectation for distance, intermediate and near vision as well as all spectacle dispensed for presbyopic patient, the key part of the process of accepting presbyopia is the patient understanding that a single pair of glass simply will not meet all of their needs, for patients with a distance prescription, the selection of multiple single vision prescription lined bifocal or progressive addition lenses (PALs) for everyday use is based on a combination of life style and desire.
According to Stark and Obrecht (1987), new digitally surface (PALs) most patient rapidly adapt to them, but patient whose primary concern is to have the very widest width of reading area may be better to lined bifocal. Traditional (PALS) are not design to optimize intermediate vision, so office workers should be educated on the benefits of near variable focuses or computer.
Many patients choose to use bifocal lenses because of easy adaptation and easily affordable for the patient in terms of price. John Isaac Hawkins, the inventor of trifocal lenses coined the term bifocal in 1824 and credited Dr. Franklin, in 1955, Irving Rips of young optics created the first seamless or “invisible” bifocal, a precursor to all progressive lenses, the first commercially viable blended lens in 1955 called the younger seamless bifocal. PALs have both cosmetic and vision advantages compared to segmented multifocal. The cosmetics benefit results from the seamless design and are apparent. The vision advantage results from elimination of the bisecting region of unable vision associated with the top of the bifocal segment, resulting in contiguous visual space from distance through intermediate to near.
The vision advantage with PALs compared to bifocal is supported by a study that showed patient preference for PALs compared to a blended bifocal by Fowler(1989) both of which are seamless. The strong preference for PALs compared to bifocal is further supported by another study by Sheedy (2004) in which 265 habitual bifocal wearer were fitted with PALs,72% of these patients preferred the PALs because progressive lenses have no image jump and no area of intermediate blur, many wearers describe their vision with progressive lenses as more natural than with bifocal. The widths and areas of the three viewing zones (distance, intermediate and near) and the magnitude unwanted astigmatism have been reported by Euerbrock (1942). He said that even the largest intermediate and near error free zones are smaller than those required to view a typical computer screen or standard paper respectively and also smaller than the normal amount of ocular rotation used to view non central target. Schaitmann (1873) produced and patented the first one piece bifocal by grinding the top of a leading lens to produce the distance power in the upper portion of the lens. An unfortunate consequence, however was poor optics resulting from a large amount of base- down prism in the top portion of the lens.
Hanna (1884) developed and patented the cemented bifocal, which consist of a thin round reading lens cemented to the back surface of distance lens with cardian balsam. This lens had better optics than previous designs but still had the difficulties associated with two piece lens. Hanna (1886) also introduced the perfection grooved bifocal, which was an improved two piece lens similar to the Franklin bifocal, it had a half moon curved reading portion and also a bevel or groove interface between the two lenses that improved stability.
The modern multifocal era began with introduction to kryptok, first fuses bifocal. It was invented in 1908 by John L. Borsh a Philadelphian ophthalmologist, a 2mm round bifocal segment was made of flint lens fabricated for crown glass (index 1.654) fuse with heat (more than 10000F) into a distance lens fabrication of crown glass (index 1.523).
In 1910 Connor an Indianapolis Optician constructed a one-piece bifocal cuttex, this design involved grinding a round bifocal segment onto the distance portion of a single piece of glass. The result was a lens having a construction similar to that of cemented bifocal but without any of the previously mentioned disadvantages.
Courmettes in 1915, a French citizen and resident of New York city constructed the flat-top bifocal. This was one of the segment styles introduced and manufactured by Univis lens Company of Dagion, Ohio in 1926. In 1950 the one piece Franklin- style bifocal was introduced by America optical company due the trade name “executive” this lens on the same principal as the first bifocal invented by Benjamin Franklin with the optical centers of the distance and near portion placed adjacent to each other. Hawkins of London in 1826 introduced trifocal, the first trifocal.In 2002, the Lord et al., study unveiled a variable of potentially enormous effect. Use bifocal and multifocal eye evidence suggest that bifocal lenses in pair distance vision in the lower visual flied (Joerger, 2003) supports the finding that the new-use of bifocals in able bodied individuals can play a contributing factor in guilt with potential to increase the risk of falling even in a relatively young set of subject. In 2008, Essilor designed the various experiences, a virtual simulation concept using the various ophthalmic solutions offered to presbyopes.
References
Aves, O. (1908) improvement in and relating to multifocal lenses and the like and method of finding source. G. O 15 – 735.
Benete, A. (1973) Variable and Progressive power lenses manufacturing Optic Mar, 137 – 141.
Benjamin Franklin (1784). Invector of bifocal.
Boorish, I.M, &Hitzeunan, S.A. (1983). Comparison of the disaceptance of progressive additions, multifocal with blended bifocal.JAM Optom ASSOC 44:415
Bomile, W. (2012) Psychological and emotional implication of the recommendation.
Boroyan, H.J., Cho, M.H.,Fuller, B.C.et al., (1995):line multifocal wearer prefer progression addition lenses AMJ Optom ASSOC 66: 296
Bruneni, J. (1994). Looking Bask :An Illustrated history of the American Optician industry. Torrance CA: optical Laboratory Association,
Maintenaz, B. (1966). Four steps that lead to varilux: Am J Optom frch Am Acad Optom 43: 441
Pedrone, C., Obrecht, G. &Stark, L. (1987).Eye-head coordination with literally ‘modulated ‘gaze field. Am J Optom Physiol Opt 64: 853.
