2. ο The total power of a lens can be
achieved by combining different types
of curved surfaces (concave or
convex) and this is called the form of
lenses.
ο Depending up on the from lenses
could be of two types:
Flat Lenses
Curved Lenses
3. ο 1. Both its surfaces have got same
type of curvature,e.g. biconcave or
biconvex
ο 2.One surface is flat and the power is
grounded on the other surface, e.g.
Plano-concave and Plano-convex.
4. ο If both surfaces are convex or both
concave, the lens is biconvex or
biconcave.
5.
6. ο If one surface is Plano and the other an
outward curved plus surface (i.e., a convex
surface), the lens is referred to as Plano
convex.
ο If one surface is Plano and the other
curved inward for minus power (i.e., a
concave surface), the lens is Planoconcave.
7.
8. ο A lens is said to be curved when there
is a convex curve on one surface and
a concave curve on the other surface.
ο Curved Lenses are of two types:
Meniscus
Toric
9. ο These are curved lenses where both
the surfaces are spherical β anterior
surface is convex and posterior
surface is concave.
ο Introduction of these lenses has made
mass production of lenses easier.
10.
11. ο These are curved lenses where one
surface is spherical and the other
surface is toroidal in shape.
ο Toric lenses are used where a
cylinder is also present in the
prescription..
12. ο Lenses can be made in a variety of
forms, with many forms possible
for a lens of the same power.
13. ο The nominal power of a lens is the sum
of its front and back surface powers.
ο When expressed as an equation, this
is
F1 + F2 = F TOTAL
14. ο For example, a biconvex lens of +4.00 D
of power could have surface powers,
such as the following:
F1 + F2=F T
(+2.00 D) + (+2.00 D) = +4.00 D
(+3.00 D) + (+1.00 D) = +4.00 D
(+0.50 D) + (+3.50 D) = +4.00 D
15. ο The same +4.00 D lens power might then
have any one of the following forms,
which represent only a fraction of the
possibilities.
F1 + F2=F T
(+7.00 D) + (-3.00 D) = +4.00 D
(+8.00 D) + (-4.00 D) = +4.00 D
(+10.00 D) + (-6.00 D) = +4.00 D
16. ο These forms are limited only in that
one meridian must have a net power
of zero and the other a net power
equal to the cylinder value.
19. Q: Suppose a lens has a toric front
surface. F1 at 90 is +4.00 D , F1 at 180
is +6.00 D .Back surface has a surface
power ofβ4.00D .What is the total power
of the lens?
20.
21. Q: Suppose a lens has a toric front surface.
F1 at 90 is +4.00 D, and F1 at 180 is +6.00
D. If the back surface has a surface
power of β4.00 D, what is the total power
of the lens?
22. ο When the lens obtains its cylinder power
from a difference in power between two
front surface meridians (i.e a toric front
surface lens), the lens is said to be ground
in plus cylinder form.
23. ο A lens has a cylinder component, but the
cylinder power is a result of a difference
in power between two back surface
meridians, the lens is said to be ground in
minus cylinder form.
24. Q: If a lens has dimensions of F1 = +6.00 D,
F2 at 90 = β8.00 D, and F2 at 180 = β6.00
D, what form does the lens have and what
is its total power?
25.
26.
27. ο Lens shape refers to outline of the lens
periphery with the nasal side and the
horizontal indicated.
28. ο Round Lens
- Ancient lens shapes.
-Not much popular
- Still used for some industrial
goggles and other forms of spectacle
in which the fashion element does not
predominate, because it simplifies
glazing.
29. ο 2.OVAL LENS
- Ancient lens shapes
-Elliptical in shape and not much in
use.
30. ο 3.Pantoscopic Round Oval (PRO)
ο Lower half of a circle and upper half of
an ellipse with the same horizontal
diameter.
31. ο It refer to the lens shape which resembles
the monocular field of vision.
ο Round contour and the squarer contour