OPTOM FASLU MUHAMMED

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.

Crown 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

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.

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 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

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 notadhere to plastic lens material

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 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 lens material is a very impact-resistant lens material. It was developed by PPG Industries.

PPG Industries promotional material s 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

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.

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

PROPERTIES OF OPHTHALMIC LENSES

•The measure of theproportion 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

Abbe Value Abbe value is the number that is the

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

Standard plastic lenses have an abbe value of 58. Most high index materials have a much lower Abbe value.

Reflectance is the phenomenon of light reflection occurs at each of 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%

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

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.

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

Specific Gravity Specific gravity is the measurement of

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

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 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.