Safety standard for opthalmic lenses 2

1. SAFETY STANDARD FOR
OPHTHALMIC LENSES
OPTOM FASLU MUHAMMED

2. British standards of spectacle
 REGULATIONS AND STANDARDS RELATING
TO EYE PROTECTION:-
 The British and European Standards for eye
protectors BS EN 166:2000 Personal Eye
Protection Specification is at the heart of the
strategy for eye protectors, but other standards
like USA …. ANSI Z 87.1, Canada …. CSA Z
94 etc are also linked to it.

5.  A typical safety spectacle has the following lens
markings:
5- 3.1 “AOS” 1 F
 5 is the code number. This specifies the wavelength of
the radiation that the filter offers protection against.
 The second 3.1 is the shade number. The shade
number signifies the tint density. The shade number
ranges from 1.1 to 4.1.
 The optical class is signified by the number.

6.  F - Impact resistance.
 The letter ‘T’, for example, FT after impact
resistance shows that the eye protectors will be
effective at higher temperature also.

12.  This is Z80.1-2005 American National
Standard for Ophthalmic—Prescription
Ophthalmic Lenses (ANSI)Recommendations.

13. Base curve
 The beginning curve on which the lens power is
based is called Base curve.
 The lower numerical surface power of a curved
lens.
 Mostly the base curve is added on the anterior
surface.

14.  To get plus power lenses negative base curves
are used and vice versa ,e.g.:- to get a +2.50 DS
a lens blank with BC-6.00D is taken and then a
curve of +8.00D is ground on the other surface

15.  For spherical lenses: In the case of spherical
lenses, the front sphere curve is the base curve.
 For plus cylinder form spherocylinder lenses: If
the lens is in plus cylinder form, there are two
curves on the front.
 The base curve is the weaker, or flatter, of the
two curves. The other curve becomes the cross
curve .The back surface is quite naturally
referred to as the sphere curve since it is
spherical.

16.  For minus cylinder form spherocylinder
lenses: If the lens is in minus cylinder form, the
front spherical curve is the base curve. The
weaker back surface curve is known as the toric
base curve; the stronger back-surface curve is
known as the cross curve .
 A meniscus lens of BC 6D is Deep meniscus
 A lens with BC of 1.25 D Periscopic lens, not
much used nowadays.

17. APPROPRIATE BASE CURVES
 If an incorrect base curve is selected, the
quality of vision is acceptable while looking
straight ahead. But vision will be degraded
when turning the eyes to view an object off to
the side due to aberration.

18. Base Curve Formulas
Vogel’s formula
It states that, for plus lenses, the base curve
of the lens equals the spherical equivalent of the lens
power plus 6 diopters. Written as a formula this is:
Base curve (plus lenses)= spherical
équivalent + 6.00 D

19.  For minus lenses, Vogel’s formula for base curve
begins with the spherical equivalent of the lens,
divides the spherical equivalent by 2,then adds 6
diopters.
 Written as a formula this is:
BC (minus lenses ) = spherical equivalent + 6.00D
2

20.  Remember that this formula is to help in determining
approximately what base curve might be expected for a
given lens power. Actual base curves for lenses will
vary.

21. Q:Using Vogel’s formula, find an approximate
base curve for a lens having a power of +2.00 D
sphere.
Solution
 For spheres there is no need to calculate a
spherical equivalent. So for this lens, the base
curve is:
 BC = +2.00 D +6.00 D = 8.00 D

22. Q:Suppose a lens has a prescription of +5.50 −1.00 ×70.
Using Vogel’s formula, what is the base curve?
Solution
 Since this lens has cylinder, we begin by finding the
spherical equivalent of the lens.
S.E =+5.50 +(-1.00)/2 =+5.50D
The approximate base curve is:
BC = +5.00D+6.00D
=11.00D

23. Design of high Powered lenses
 HIGH PLUS LENS DESIGNS
1.Regular Spheric Lenses
It is possible to use a regular, spherically based
lens for a high plus wearer, even though the optics
are not as good.
 Sometimes these lenses are called “full-field
lenses”

24. 2.High-Index Aspheric
 Whenever possible, it is best to use a high-index
aspheric lens for high plus lens wearers.
 High index aspherics may not be available in some
of the very highest plus powers.
3.Lenticular lens is one that has a central
area with the prescribed lens power surrounded by
an outside area of little or no power

25. The Development of High Plus Multidrop Lenses
 The Welsh 4-Drop lens was developed in an
effort to overcome the cosmetic negatives of the
lenticular design while maintaining a thin lens.
 The Welsh 4-Drop had a back surface curve that
was almost flat.
 The front surface of the lens had a 24-mm
spherically based central area.
 Outside of that central area, the lens surface
became aspheric and dropped in power, 1 diopter
at a time, for a total of 4 diopters

26.  For example, if the lens had a central base curve of
+14.00, there were four outer concentric areas with
powers of +13.00 D, +12.00 D, +11.00 D and +10.00
D. Each area blended into the other so that the
changes in power were not visible.

27. HIGH MINUS LENS DESIGNS
 Perhaps the greatest lens problem facing the high
minus wearer is thick edges.
1.Lenticular Minus Designs
Central area of the lens contains the
prescribed refractive power of the lens. The
peripheral (carrier) area serves only to extend
the physical size of the lens without increasing
its thickness.
 Lenticular minus lenses can be found in several
forms; one of which is the myodisc.

28. The Myodisc
 According to the traditional definition, the
myodisc design has a front surface that is either
flat or almost flat.
 The front usually contains the cylinder
component of the prescription.
 A myodisc also has a plano back carrier area.
 Myodisc was a trade name.
 In a myodisc type of lens, the carrier is near
Plano.

29. Minus Lenticular
 A high minus lens with a lenticular design can
be made so that the carrier is not Plano.
 If the back side of the carrier is made positive.
 The minus bowl is ground into the “front” of
the semifinished lens. This will become the
back of the minus lenticular lens.

30. Thank u......