Lens:
A lens is an optical device which transmits and refracts light, converging and
diverging the beams of light. There are three forms of lenses namely:
A lens is an optical device which transmits and refracts light, converging and
diverging the beams of light. There are three forms of lenses namely:
1.
Single
vision lenses
Single
vision lenses
2.
Bifocal
lenses
Bifocal
lenses
3.
Multifocal
lenses
Multifocal
lenses
1. Single vision lenses: single vision lenses are
lenses that has only one distinct power, the single vision lenses are divided
into spherical lens which is plus (+) and minus (-) lens
lenses that has only one distinct power, the single vision lenses are divided
into spherical lens which is plus (+) and minus (-) lens
2. Multifocal lenses: These are lenses that have three
or more different powers of the lens. It is also known as progressive lenses,
such lenses are trifocal and varifocal lenses or varilux
or more different powers of the lens. It is also known as progressive lenses,
such lenses are trifocal and varifocal lenses or varilux
3. Bifocal lenses: These are lenses that have two
distinct power which has demarcation of upper and lower portion of the lens.
The upper portion is for distance vision while, the lower portion is for near
vision. Bifocal lens enable clear vision at two different distances usually for
distance and near vision mostly reading purpose. Bifocal lenses eliminate the
need to swap between different pair of glasses for different virtual task.
distinct power which has demarcation of upper and lower portion of the lens.
The upper portion is for distance vision while, the lower portion is for near
vision. Bifocal lens enable clear vision at two different distances usually for
distance and near vision mostly reading purpose. Bifocal lenses eliminate the
need to swap between different pair of glasses for different virtual task.
Usefulness: bifocal lenses are prescribed for
adults over aged 40 to compensate for presbyopia in some cases bifocal are also
usually prescribed for young adults, who have eye straining or focusing problem
that cause eye strain when reading. In this cases the lower portion of the
bifocal lens reduces the amount of focusing effort required to see near object
clearly.
adults over aged 40 to compensate for presbyopia in some cases bifocal are also
usually prescribed for young adults, who have eye straining or focusing problem
that cause eye strain when reading. In this cases the lower portion of the
bifocal lens reduces the amount of focusing effort required to see near object
clearly.
Types
of bifocal lenses: There
are different types of bifocal lenses namely;
of bifocal lenses: There
are different types of bifocal lenses namely;
1.
Fused/Round
top bifocal lens
Fused/Round
top bifocal lens
2.
D-Top/Flat
top bifocal lens
D-Top/Flat
top bifocal lens
3.
Executive/Straight
bifocal lens
Executive/Straight
bifocal lens
4.
Invisible/Blended
bifocal lens
Invisible/Blended
bifocal lens
5.
Solid/Curved
bifocal lens
Solid/Curved
bifocal lens
Fused/Round top bifocal lens: This is a common
types of bifocal lens in which the lower segment is round and pronounced in the
lens.
types of bifocal lens in which the lower segment is round and pronounced in the
lens.
D-Top/Flat top bifocal lens: This is a types of
bifocal lens in which the lower portion indicates as D shape in the lens. It is
well pronounced.
bifocal lens in which the lower portion indicates as D shape in the lens. It is
well pronounced.
Executive/Straight bifocal lens: This type of
bifocal in which a straight lines demarcation appears in the lens, it is also
pronounced just as the name implies.
bifocal in which a straight lines demarcation appears in the lens, it is also
pronounced just as the name implies.
Invisible/Blended bifocal lens: This type of
bifocal lens in which the lower segment is round and unpronounced as the name
implies. It is on like fused bifocal lens.
bifocal lens in which the lower segment is round and unpronounced as the name
implies. It is on like fused bifocal lens.
Solid/Curved bifocal lens: This type of bifocal
lens in which the lower portion is being curved.
lens in which the lower portion is being curved.
Form of bifocal lens prescription
a.
Plano
addition
Plano
addition
b.
Power
addition
Power
addition
Plano addition: this form of bifocal prescription
whereby the upper portion (distance) is as normal eye without any corrective
purpose.
whereby the upper portion (distance) is as normal eye without any corrective
purpose.
Power addition: this form of bifocal prescription
whereby the upper segment (distance) has corrective power of the lens.
whereby the upper segment (distance) has corrective power of the lens.