Special type of lenses

OPTOM FASLU MUHAMMED

Special Type of Lenses

Special Type of Lenses

Lenticular lenses

Aniseikonic lenses

Aspheric Lenses

Fresnel lenses & prisms

Lenticular lenses

These lenses have been created particularly for use in high powers.

Here a central portion of the lens is grounded to have the power and this is called the aperture. peripheral part of the lens act as a carrier. The aperture is usually 30-40 mm in diameter .

Lenticular lenses

Thickness of central part poses a difficulty while fixing the lens to the frame. This Difficulty is offset by grinding the peripheral part ,which reduces the thickness by 1.2-2.0 mm and improves the cosmetic appearance as well.

Various types of Lenticular lenses

Solid lenticulars : carrier has got convex slope.

Plano lenticulars: the carrier is plane and the aperture is either convex or concave .A concave plano lenticular is called Myodisk.

Cemented lenticulars :the aperture part carries the sphere and is glued on a carrier on which the cylinder is incorporated.

Profile lenticulars: aperture edge follows the same shape as whole lens shape so that the aperture is made as inconspicuous as possible.

Fused lenticulars:the aperture is ground on the back surface of a plus lens and is filled with glass of higher refractive index and heated at 600 degree C.The front surface is ground to reduce the same amount of power.

Lenticular lenses

Anisometropia

Anisometropia is when there is a difference in refractive power between the left and right eyes.

A significant amount of Anisometropia ends up creating problems.

When Anisometropia is corrected with spectacle

lenses, problems are not always over.

ANISEIKONIA

Aniseikonia is a relative difference in the size and/or the shape of the images seen by the right and the left eyes.

Types of AniseikoniaPhysiologic AniseikoniaSymmetrical AniseikoniaAnatomic Versus Optical Aniseikonia

Knapp’s Law and Axial Ametropia

According to Knapp’s law, “When a correcting lens is so placed before the eye that its second principal plane coincides with the anterior focal point of an axially ametropic eye, the size of the retinal image will be the same as though the eye were emmetropic.”

(It should be noted that for Knapp’s law to be fulfilled, the ametropia must be purely axial, and there must be no anatomic Aniseikonia present.)

CORRECTING ANISEIKONIA WITHSPECTACLE LENSES

If an exact amount of aniseikonia is found, modifications to the spectacle lenses that

change relative spectacle magnification will be of benefit whether the anisometropia is axial or refractive.

This is because there are specific modifications that can be made to spectacle lenses that will change their magnification.

There are several ways to approach the problem of Aniseikonia:-

1.Use a frame with a short vertex distance

2. Use a frame with a small eye size. This secondarily

reduces vertex distance.

3.Use an aspheric lens design. This usually flattens the base curves.

4.Use a high-index lens material. This will thin pluslens center thickness

ASPHERIC LENS

What Is an Aspheric Lens?

The term aspheric means “not spherical.”

The degree of curvature of a spherical lens is continuously uniform with a consistent radius of curvature throughout its entire surface, like that of a ball or sphere.

An Aspheric lens surface changes shape. It does not have the same radius of curvature over the entire surface.

Purposes for Using an Aspheric Design

1.The first reason is to be able to optically correct lens aberrations.

2.To allow the lens to be made flatter, thereby reducing magnification and making it more attractive.

3. To produce a thinner, lighter weight lens.4.To ensure a good, tight fit in the frame.5. To make a lens with progressive optics.

Asphericity for Optical Purposes

Once lens powers go beyond the +7.00 D to−23.00 D range, however, it is necessary to use an aspheric

design.

Asphericity for Flattening Purposes

For lenses with spherical base curves, higher plus power always results in steeper base curves Unfortunately, for high plus lenses the steeper the base curve, the worse the lenses look.

Choosing a flatter base curve will make the lens look less bulbous and also reduce magnification. Cosmetically the lens looks much better. It even looks considerably thinner.

To Ensure a Good, Tight Fit in the Frame

Asphericity for Producing Progressive Power Changes

Asphericity for Thinning Purposes(Geometric Asphericity)

FRESNEL LENS

A Fresnel lens is similar to a series of concentric prisms, each with a slightly higher prismatic effect.

When Are Fresnel Lenses Used?

Nonspectacle Uses Fresnel lenses are not just used for

spectaclesLarge minus Fresnel lenses are sometimes

applied to a window to create a wider field of view, or are used for the warning beams of seaside lighthouses.

Short-Term WearCreating Adds

High density lens

The eye-care industry differentiates lens materials by their "refractive index." Simply put, this is the material's ability to displace light .

Density and refractive index are not the same, but they go hand-in-hand. As material density increases, so does its refractive index.

There are higher index glass lens materials available that will reduce lens thickness for higher powered prescriptions.

Plastic and Glass Lenses

Plastic is the most common material used in spectacle lenses. Standard plastic lenses have an IR of 1.49 or 1.50.

Glass lenses are not as popular as plastic because glass is about twice as heavy. Standard glass lenses have an IR of 1.52.

A lens with an IR higher than 1.52 is considered to be a Hi-index lens. The most common range for hi-index lenses is between 1.54 and 1.74.

Index of Refraction

The index of refraction (IR) is the difference in the speed of light as it passes through air and into lens material, and is expressed as a number.

Regular plastic lenses have an IR of 1.50. High index lenses bend light more efficiently because the lens material is compressed. Thinner lenses have a higher IR number.

Polycarbonate

Polycarbonate lenses are lighter than plastic and recommended for children, safety and sports because the material is impact-resistant.

Polycarbonate is a hi-index material with an IR of 1.586. The lenses are lighter but, in certain types of prescriptions, the optics are not as crisp or clear as with plastic or glass lenses

High-index glass lens materials generally have Abbé

values close to that of polycarbonate.

Unfortunately, high-index glass lenses are composed

of materials with a higher specific gravity, making them heavier.

Varifocal lens

Varifocal lens is also called as progressive addition lenses (PAL), progressive power lenses, graduated prescription lenses, and or multifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia.

They are characterized by a gradient of increasing lens power, added to the wearer's correction for the other refractive errors.