**Lens Edge Thickness Formula**. = s 2 + c.t. The thickness of a minus lens can be minimized by keeping the center thickness of the lens to a minimum.

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A +2.25 sphere lens has the same power at all axes on the lens. From the given figure. we notice that abo and a’b’o are similar. There are things like spherical and aspherical lens where aspherical lens is thinner than spherical for the same index.

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From the given figure. we notice that abo and a’b’o are similar. And n its index of refraction.

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A +2.25 sphere lens has the same power at all axes on the lens. In the case of hard. corneal lenses. several authors have expressed the opinion that the optimal value for edge thickness is between 0.12 and 0.18mm (see table i).

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Point of the lens) and additional center thickness for minus lenses (also the thinnest point of the lens). An object ab is held perpendicular to the principal axis at a distance beyond the focal length of the lens.

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The minimum thickness. which is usually the center thickness of minus lenses and the edge thickness of plus lenses. will generally be around 1.5 to 2.0 mm. So the power in the two meridians is [email protected] and [email protected]

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A real. inverted magnified image a’b’ is formed as shown in the figure. It is important to note that for small radii of curvature. for example. lenses where the diameter is ≥ 0.85 x radius of curvature. no bevel is needed due to the large angle between the surface and edge of the lens.

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Please note. this formula is only valid for lenses whose thickness is small when compared to the calculated focal length. The curvature of the lens. its thickness and weight will depend on the amount of long or short sight it is designed to correct.

#### When We Do This. We Minimize The Saggital Value (Sag) Of The Lens.

For plus lens the result is for centre thickness. The lens is held on a good chuck and the optical axis is aligned to the axis of rotation. Once corneal astigmatism gets to about 2.00d a further 0.02mm is needed. so if you are fitting an astigmatic cornea with a spherical lens you might need to order something a bit thicker.

#### For A Given Minimum Thickness. Which Usually Occurs At The Edge Of Plus Lenses And The Centre Of Minus Lenses. The Maximum Thickness At The Centre Of A Plus Lens Or The Edge Of A Minus Lens Can Be Estimated Using The Following Formula With The Accompanying “K” Values From Table 1:

Edge s center 1 s2 figure 3. A specification based on these two values might read: The thickness of a minus lens can be minimized by keeping the center thickness of the lens to a minimum.

#### Calculation Of Axial Edge Thickness Standard Textbooks On Contact Lens Practice Such As Those By Bier And Lowther (1977). Stone And Phillips (1980) And Mandell (1981) All Describe.

A +2.25 sphere lens has the same power at all axes on the lens. The edge thickness is given by: The curvature of the lens. its thickness and weight will depend on the amount of long or short sight it is designed to correct.

#### = Edge Thickness (In Mm) E.t.

For minus grind lens use ct = 1.90. = s 1 + e.t. Convex lens the thickness at the centre of a knife edged lens concave lens the thickness at the edges. for zero thickness at the centre expressing the radii as a ofunction of the powers of each surface gives us t s when d is the diameter of the lens. in metres;

#### It Is Important To Note That For Small Radii Of Curvature. For Example. Lenses Where The Diameter Is ≥ 0.85 X Radius Of Curvature. No Bevel Is Needed Due To The Large Angle Between The Surface And Edge Of The Lens.

Please note. this formula is only valid for lenses whose thickness is small when compared to the calculated focal length. Lens indexes exist are of 1.50. 1.53. 1.59. 1.70 and more. Maintained by john loomis . last updated 1 oct 1997.