BEST FORM SPECTACLE LENSES

OPTOM FASLU MUHAMMED

In actual world, the eyes turn behind the lens to view through off-axis visual points and it is then that the form assumes importance.

OFF-AXIS PERFORMANCE OF LENSESThe aberrations which are significant to the

spectacle wearer are:

Transverse chromatic aberration (TCA)

Oblique astigmatism (OA)

Curvature of field

Distortion

Spherical aberration and coma – are both aberrations of wide aperture systems and although a spectacle lens may be quite large.

TRANSVERSE CHROMATIC ABERRATIONTransverse chromatism can only be

eliminated by constructing an Achromatic lens, that is, a pair of lenses bonded together, in which the chromatism of one component neutralizes the chromatism of the second.

Given power by selecting a material with the highest available V-value.

OFF AXIS BLUR –Under condition of low contrast colour fringes may be noticed. instead ,the effect of TCA is to cause a reduction in visual acuity.

To reduce chromatism by bending the lens into a very steeply curved form (the so-called Wollaston bending –but such lenses are expensive to produce and appear very bulbous.

To a good approximation, the magnitude of the TCA at any given point on a lens is found by calculating the prismatic effect, P, at the point and dividing this by the Abbe number,

TCA = P/V.P = Prismatic effectV = Abbe number

DISTORTION

It is possible to reduce distortion by supplying steeply curved Wollaston form lenses.

FIELD DIAGRAMSA most useful guide to the effects of oblique

astigmatism and curvature of field in a given spectacle lens is obtained by studying a field diagram for the lens form.

FIELD DIAGRAMS A field diagram is a plot of the

tangential and sagittal oblique vertex sphere powers against the ocular rotation of the eye viewing through the lens.

The plane containing the optical axis of the surface is referred to as the ‘tangential plane’

The plan at right angles to the tangential plane is referred to as the sagittal plane.

The vertex sphere is an imaginary reference surface concentric with the eye’s centre of rotation, from which the positions of the tangential and sagittal foci are measured.

.

The far point sphere is the imaginary surface, also concentric with the eye’s centre of rotation, upon which we can assume the far point to remain as the eye rotates to view through off-axis zones of the lens.

The distance between the vertex sphere and the far point sphere measured through the eye’s centre of rotation, Z, is constant and equal to the back vertex focal length of the lens.

FIELD DIAGRAM IN +4.00 PLANO CONVEX LENS

In the case of a perfect lens, such as the +4.00 design whose ideal field diagram is illustrated in the tangential and sagittal oblique vertex sphere powers remain +4.00 for all zones of the lens.

Unfortunately, this performance is impossible to obtain in a single lens with just two surfaces, at least for this power.

The performance of a +4.00 design, made in Plano-convex form. When the eye views along the optical axis of the lens, the power of the lens is, indeed, +4.00D.

OFF AXIS PERFOMANCE OF +4.00 DS LENSWhen the eye rotates through 30° from the optical

axis, In sagittal meridian is +4.25D In tangential meridian +5.25 in the.

This is equivalent to a power +4.25D sphere with a +1.00D cylinder.

This is so different from the paraxial power that it cannot be ignored. Clearly, the choice of a Plano-convex design for a lens of power +4.00D is a poor one.

We shall see that, although we cannot make a +4.00D lens for which the power remains the same for all directions of gaze, we can certainly improve.

BEST FORM SPECTACLE LENSES

A best form spectacle lens is one whose surface powers have been specially computed to eliminate, or at least minimize, certain stated defects in its image forming properties.

BEST FORM SPECTACLE LENSES

Shows how the off-axis performance of +4.00D lenses varies for three meniscus forms with front curves +9.75D, +8.12D and +7.62D.

POINT-FOCAL’ LENS In Figure (BC is

+9.75 Ds) (OAE) = 0 Such a form is

described as a ‘Point-focal’ lens form.

At 35°, the power of the lens has dropped to +3.75D.

MINIMUM TANGENTIAL (T) ERROR’ FORM In figure b(BC is +8.12

Ds ) :- If the form of the

lens is flattened from the point-focal bending, the tangential power increases and now the same as the back vertex power of the lens. Such a form is described as a ‘minimum tangential (T) error’ form.

In Figure 6c (BC is +7.62 D ), the bending of the lens has been reduced still further to a +7.62D base curve and it can be seen in the field diagram.

T and S oblique vertex sphere powers have increased to just the point where the focal lines within the eye would lie either side of, and equidistant from, the retina.

PERCIVAL LENS DESIGN’ At 35°, the off-axis

power of the lens is +3.85DS/+0.30DC, the T is +0.15D too great and the S is 0.15D too weak compared with the paraxial power.

The mean oblique power of the lens is +4.00D. This form of lens is known as a ‘Percival lens design’ and is free from mean oblique error.

The same principles are involved in the design of minus spectacle lenses. illustrates field diagrams for -4.00D lenses made in point-focal form (+5.00 base curve), minimum T-error form (+3.87 base curve) and Percival form (+3.25 base curve).

FIELD DIAGRAM OF -4.00D SPH

TSCHERNING’S ELLIPSES

Spectacle lens design was undertaken by the laborious means of trigonometric ray tracing using six or seven-figure logarithm tables.

This problem was addressed during the 19th century by Airy Coddington and von Seidel who, amongst others, developed approximate equations for the aberrations and for lens forms which exhibit minimum aberration.

The equations for spectacle lenses, which are corrected for oblique astigmatism, are quadratic in form and plot in the form of an ellipse. Such equations were published by Dr Marius Tscherning 2 & known as Tscherning’s ellipses.

Tscherning’s ellipses show very nicely that there is a range of powers that can be made free from a particular aberration and that within this range, there are two different forms of lens for each power.

The shallower form - the ‘Ostwalt Bending’ and is the form which is usually employed in practice.

The steeper form is known as the ‘Wollaston bending’ and, in view of the necessary deep curvatures, is more difficult to produce.

Lenses in the range +7.00 to -23.00 can be made free from astigmatism .

The Ostwalt form for a -5.00D lens requires a back surface power in the region of -9.50D.

The Wollaston form a -5.00 D lens needs a back surface power in the region of -22.00D.

Tscherning ellipses drawn for lenses made in three different refractive indices – 1.50, 1.70, and 1.90.

TSCHERNING’S ELLIPSES FOR DISTANCE AND NEAR VISION POINT FOCAL LENSES. CONSTRUCTED FOR N = 1.50, CRD = 27MM, L1 = -3.00 FOR THE NV ELLIPSE

BEST FORM LENSES OF WOLLASTON FORM

It is for steeper form.

Near vision form is the same as the distance vision form over a large part of the range.

In oblique gaze, bundles of light pass through the lens more nearly in the position of minimum deviation .

Wollaston form lenses. The Contour Optics lensfrom Sola Technologies

BEST FORM LENSES FOR NEAR VISION

Similar equations can be derived for lenses intended to be used for near vision1 and the ellipse for near vision at -33.3cm.

Ostwalt lenses by German firm Rupp+Hubrach.

BEST FORM ASTIGMATIC LENSESCurved form for the same reason as spherical

lenses.

Three different forms of toroidal surface are used in ophthalmic lens manufacture

1.The tyre-formation surface-mass production2.The barrel -formation surface – individual surface working3.Capstan –formation surface

Thank U………