OPTOMETRIC OPTICS OPTOM FASLU MUHAMMED

Antistatic CoatingAn antistatic agent is a compound used for

treatment of materials or their surfaces in order to reduce or eliminate buildup of static electricity generally caused by the triboelectric effect.

Its role is to make the surface or the material itself slightly conductive, either by being conductive itself, or by absorbing moisture from the air.

Static electricity readily accumulates on ordinary plastic lenses, attracting substances such as dust and pollen.

This coating features a special film (conduction film) that releases static electricity, making it difficult for substances such as dust and pollen to become attached to the lens surface.

sagittal depth (sag) formulaThe lens measure operates on the principle of the

sagittal depth (sag) formula. The sagittal depth, or “sag,” is the height or depth of

a given segment of a circle .If both the sag of a lens surface and the index of refraction of the lens material are known, the surface power may be calculated.

The lens measure has three “legs,” or points of contact

with the lens surface. The outer two are stationary, and the center contact

point moves in and out. The vertical difference between the positions of the

two outer contact points in reference to the position of the center contact point is the sag for the arc of a circle. This circle can be thought of as having a chord, the length of which is the distance between the outer contact points of the lens measure

LENS THICKNESSSagittal DepthThe formula that is the basis for

determining lens thickness is the sagittal depth, or sag formula,

Sagittal depth is the depth of the lens surface curve and is shown in Figure 13-24. Remember that a chord is a straight line joining two points on a curve.

In Figure 13-24, the two points on the curve are at the edges of the lens, and the length of the chord equals the diameter of the lens

To find the sagittal depth, it is necessary to know the length of the chord and the radius of curvature of the lens surface.

s = r − (r2 − y2 )This last equation is called the accurate

sag formula

The Approximate Sag Formula

Before the advent of hand calculators, simplified formulas yielding an approximate result were used to find the sag of a curved surface.

To simplify calculations, the more complex formula of s = r − (r2 − y2 ) was simplified to the approximate sag formula.

The approximate sag formula only works when the diameter is small and the radius of curvature is long.

In other words, it only approximates well for small lenses with low-powered surfaces. Because it enjoyed such widespread usage, it is still seen and referred to. However, using the accurate sag formula is no longer as difficult and yields accurate results over the whole range of lens sizes and powers.

Thickness of Meniscus Lenses