Ophthalmic Lens Material and Design

OPTOM FASLU MUHAMMEDOPTOM FASLU MUHAMMED

LENS MATERIALS

Ophthalmic lenses are made from

Glass

Plastic

Glass lenses are often referred to as mineral lenses, whereas if a lens is made from plastic, it is said to be from organic material.

GLASS MATERIALCrown Glass Glass works well for ophthalmic

materials because it resists scratching and is not easily affected by environmental factors.

Disadvantages Glass are weight Impact resistance

Crown Glass The most commonly used clear glass

lens material is made from a type of crown glass .

Having an index of refraction of 1.523.

This material is low in chromatic aberration.

High-Index Glass Will reduce lens thickness for higher powered

prescriptions.

Index 1.60 lenses are readily available

There are fused flat-top bifocals available in indexes up to 1.70

There is a 1.90 index glass available.

High-index glass lens materials generally have Abbé values close to that of polycarbonate

The lower the Abbé value, the higher the chromatic aberration.

RESIN MATERIAL Resin lenses are made up of small molecular

units called monomers which link together to form long chain known as polymers and the process of linking the monomers together is known as polymerization.

There are two processes to make resin lenses

1.Thermosetting

2. Thermoplastic

Plastic Lenses CR-39 For years the most commonly used plastic

lens material was CR-39.

CR-39 was developed by PPG Industries.

“CR” stands for Columbia Resin, and the number 39 denotes the type of Columbia Resin used.

Plastic lenses are roughly half the weight of crown glass lenses.

For low velocity, large mass objects, such as

permanently stick to glass lenses, it does not

adhere to plastic lens material.

High-Index Plastics

CR-39 plastic lenses have an index of refraction of approximately 1.498.

This is the lowest refractive index material used for spectacle lenses.

Polycarbonate Polycarbonate lens material is soft and

requires an anti scratch coating.

When safety is the primary concern, polycarbonate has traditionally been the number one choice.

Safety is the primary concern for children, monocular individuals, people with good acuity in one eye only, and those who are purchasing safety or sports eyewear.

Trivex Lenses

Trivex lens material is a very impact-resistant lens material.

It was developed by PPG Industries.

PPG Industries promotional materials

attribute the “tri” in Trivex to a tri performance lens material; meaning it offers a triple combination of superior optics, impact resistance, and ultra light weight.

It maintains a good resistance to damage from chemicals.

Trivex rivals polycarbonate in impact resistance.

It is the lens of choice for drill-mounted lenses because it does not crack or split at the drilled hole.

The lens is very light weight.

The Abbé value of 43 to 45 is less than CR-39, but higher than its rival, polycarbonate.

NXT Material 2 The resulting lens is a light-weight material

that is extremely strong and also compatible with photochromic pigments and with polarization.

It has already been used in sun and sport eyewear, helmet visors for motorcycles, airline cockpit door view ports, ballistic police shields, and vehicle door armor.

NXT has an index of refraction of 1.53, a density of a 1.11, an Abbé value of 45, and is highly flexible.

Laminated Lenses

Lenses that are made from two or more layers of material are called laminated lenses.

PROPERTIES OF OPHTHALMIC LENSES

When a beam of light falls on a lens surface, some of the light passes through the lens, some of the light is reflected from the lens surface and some of the light is absorbed by the lens material.

PROPERTIES OF OPHTHALMIC LENSES

•The measure of the proportion of light reflected from the surface is called reflectance.

• The measure of proportion of light absorbed is the absorption and the measure of the proportion of light transmitted is the transmittance.

If the intensity of the beam of light is represented by the number 1, reflectance by R, absorption by A and transmittance by T, the intensity may be expressed as follows:

R + A + T = 1

PROPERTIES OF OPHTHALMIC LENSES1. Optical properties

2. Mechanical properties

3. Electrical properties

4. Chemical properties

5. Thermal properties

Optical Properties Abbe Value Abbe value is the number that is

the measure of the degree to which light is dispersed when entering a lens material.

Dispersion stands for the amount that the material spreads out the different wavelengths of light passing through it.

The lower the abbe number, the greater the dispersion of light, causing the chromatic

aberration in the periphery of the lens. The higher the Abbe number, the better the peripheral optics.

Standard plastic lenses have an abbe value of 58.

Abbe value is the property of the lens material and can not be affected by any surface technique.

Reflectance Reflectance is the phenomenon of light

reflection occurs at each of the lens surfaces. The result is the loss of lens transparency and undesirable reflections on the lens surfaces

The reflectance of the lens surface is calculated from the refractive index of the material.

Reflectance = 100 (n – 1)² / (n + 1)² %

Therefore, a material of refractive index 1.5 has a reflectance of

100(1.5 – 1) ² / (1.5 + 1)² Or, 100 (0.5)² / (2.5)² Or, 100 × (0.5 / 2.5)² Or, 3.9 % per surface.

The higher the refractive index, the greater the proportion of light reflected from the surfaces.

Absorption

Absorption of an ophthalmic lens generally refers to its internal absorption, i.e. to the percentage of light absorbed between the front and the rear lens surfaces.

Refractive Index

The refractive index of a transparent medium is the ratio between the velocities of the light in air to the velocity of light in the given medium and is denoted by ‘n’.

n = Velocity of light in air / Velocity of light in the medium

Refractive index of material describes the ability to bend light.

The higher the index of the material, the more it is able to bend light

Mechanical Properties Specific Gravity Specific gravity is the measurement of

physical density or weight of the material in grams per cubic centimeter.

Impact Resistance

Impact Resistance In 1971, the Food and Drug Administration

(FDA) adopted a procedure to ensure the level of protection provided to the consumer for “street wear.

The standard involves dropping a 5/8” steel ball from a height of 50” onto the lens and

the lenses that survive this test are deemed impact resistant.

The Drop Ball Test The Drop Ball Test To be judged

acceptable, a lens is first placed front side up on a neoprene gasket.

It must be capable of withstanding the impact of a five-eighth-inch steel ball weighing 0.56 oz, dropped from a height of 50 inches

When Should the Drop Ball Test be Performed?

Glass lenses must be tested after the lens has been edged and hardened and before it is placed in the frame.

Plastic lenses may be tested in the “uncut-finished” stage before they have been edged.

Scratch Resistance

OPHTHALMIC LENS DESIGN

The basic lens design is determined by the “base curve selection.

The base curve of the lens is the surface that serves as the basis or starting point from where the remaining curves will be calculated.

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