SAFETY STANDARD FOR

OPHTHALMIC LENSES

OPTOM FASLU MUHAMMED

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.

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.

F - Impact resistance.

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

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

Base curveThe 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.

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

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.

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.

APPROPRIATE BASE CURVESIf 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.

Base Curve FormulasVogel’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

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

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.

Q:Using Vogel’s formula, find an approximate base curve for a lens having a power of +2.00 D sphere.

SolutionFor 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

Q:Suppose a lens has a prescription of +5.50 −1.00 ×70. Using Vogel’s formula, what is the base curve?

SolutionSince 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

Design of high Powered lensesHIGH PLUS LENS DESIGNS1.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”

2.High-Index AsphericWhenever 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

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

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.

HIGH MINUS LENS DESIGNS

Perhaps the greatest lens problem facing the high minus wearer is thick edges.

1.Lenticular Minus DesignsCentral 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.

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.

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.

Thank u......