Deal correction becomes reality

NULUX EP

An aspheric lens can help compensate for shortfalls in optical accuracy in existing lens designs. This compensation only accurately deals with the horizontal and vertical axes, however, while people actually look in all other directions in-between. Recognising that natural vision is very much a dynamic process, Hoya has developed Nulux EP, a double aspheric lens design. The exceptional new calculation technique is based on measured Visual Acuity, integrating eyeball movements according Listing's Law theory as new parameters. Combined with flat front and back curves, created by Hoya's 'FreeForm' cutting technique and a radically improved polishing technology, Nulux EP ensures that uninhibited sharp vision and a maximum clear visual field, achieved from 'edge to edge' and in all directions of sight.

Nulux EP: The perfection of natural vision in all directions

With the introduction of Nulux EP Hoya has made sharp vision in all directions of sight a reality. A new evaluation parameter and new calculation methods for this unique bi-aspheric, atoric lens design that has set a new standard in unrestricted natural vision. Looking beyond principal meridians

Until now, the most important factor in

calculating lens designs was on issues

such as the material's refractive index,

the curvature and the thickness, taking the prescribed correction value as the basis.

To achieve the best optical correction meant relying on the evaluation results of two primary evaluation parameters related to Visual Acuity.

Power Error and Astigmatism.

 

(A) Power Error                      (B) Astigmatism

Optical accuracy in all sight directions

These two calculation factors were, however, insufficient for cylindrical

correction lenses when it came to achieving an equal and exact calculation of the total field of vision in all directions of sight in areas in which the eyeball moves outside the two main axes (principal meridians), such as when looking in oblique direction.

The physiological properties of the eye, the measured visual sharpness

(Visual Acuity) and the movements made by the eye in all directions were

left out of the equation. Put simply, the available techniques and insights made

it impossible to take them into account.

Meanwhile, calculation techniques have started taking the human eye as the starting point and Calculated Visual Acuity plays a central role herein. The key issue is to optimise a person's visual sharpness, not only in theory, but achieved by the prescribed lens and taking into account directions beyond the rotation in the two primary meridians. If for example a person has a visual sharpness of V 1.0 or 6/6, this

is the level at visual sharpness (Visual Acuity) at which we want to achieve an unrestricted field of vision in all directions, regardless of the correction value of the lens.

Visual Acuity means the eye's ability to discriminate or resolve spatially organised details in images. A number of factors affect Visual Acuity, such as health, age, refractive

error, illumination, contrast and the location of the retina being stimulated. Astigmatic error and power error aberrations, produced by the limitations (aberrations) of spectacle lenses, can also influence Visual Acuity.

The aim is to find the highest clear vision for myopia, hypermetropia and astigmatism, where the focused image is on the retina (measured Visual Acuity). It requires spectacle lens corrections retaining the same vision at all

directions (Calculated Visual Acuity).
 

The relationship between lens power and measured Visual Acuity

The relationship between lens power and measured Visual Acuity (visual sharpness) was the subject of a research project by Dr. Henry B. Peter. He also investigated the

relationship between Visual Acuity on the one hand and age and the influence of spherical aberration and astigmatism on the other. The results of this work provided Hoya with the input it required to start designing lenses with minimal, peripheral

aberrations, and to optimise Visual Acuity in all directions of sight. Put another way, Hoya was able to embark on a quest to enhance visual performance and quality, while also creating the largest possible unrestricted field of vision.

Three evaluation factors for optimising lens design properties

As a first step, Hoya incorporated the relationship between spherical and cylindrical prescription powers with the associated aberrations and the measured Visual Acuity.

This applies also to the effects of the optical design on the Visual Acuity due to the way in which rays of light are bent at different transmitted ray incidence heights.

Finally, the Hoya designers used data related to eye movements, no longer basing their efforts on vision through the optical centre of the lens alone but in all directions.

Until now it was possible to overcome optical aberrations using aspheric lens designs, applying flatter curves with plus powers and less with minus powers. The disadvantage of this solution was that the eventual calculations were only based upon two coordinates: the principal meridians. The human eye of course looks in all

directions, outside the optical centre of the lens.

The measured Visual Acuity as a basis parameter for optical accuracy in all sight directions.

(A) primary direction

(B) secondary direction

(C) tertiary direction

The goal: optimal aspherisation in all directions of sight

Cylindrical correction lens are a particular problem. Major differences occur with astigmatic lenses between vision through the optical centre and vision through the peripheral areas of the lens. A solution therefore had to be found to avoid peripheral residual astigmatism. Achieving a balanced distribution of the different aberrations in the two primary meridians is already a complex matter.


                

Image clear visual field spheric design           Image improved clear visual field                                                              standard aspheric design


But the formation of astigmatism by slanting beams of light increases this complexity still further when looking outside the cylindrical axes. Oblique astigmatic error and mean oblique error occurs when looking at an object outside the optical centre in

an oblique direction through a cylindrical lens. The existing calculation techniques could only offer corrections in two directions: principal meridians respectively. But the goal was to achieve Visual Acuity (visual sharpness and unrestricted visual field) by means of optimal aspherisation in all sight directions. In fact, therefore, we needed new coordinates in order to have a balanced distribution and to minimise aberrations, even when looking obliquely.

The human eye's ability to evaluate the performance of a lens in all sight directions forms the basis to define the requested optical performance and calculation

parameters for designing Nulux EP.

Listing's Law

This next step was achieved by taking into account the movements of the human eye according to Listing's Law when calculating the aspherisation of the lens design. Listing studied the movement made by the eye in all directions - not just horizontally and vertically but also on the tertiary axes (looking obliquely).


When the line of sight moves from the primary position to another position, it is as if the eye rotates about a fixed axis, which is perpendicular to the line of sight in the two positions. As Listing's plane is fixed in the X-Z plane (horizontal and vertical respectively), it is perpendicular to the Y plane (visual axes) and carries on through the rotation axes of the eye. The X-Z

reference planes determine the erect head position. The head is erect when two planes are vertical.


The theory of Listing's Law give us more coordinates to adjust the calculation for all directions of sight. The evaluation function is used in optimising the calculations used in designing a bi-aspherical lens, in addition to the visual acuity evaluation function, derived from a visual acuity measured value of V. The Calculated Visual Acuity should be based on the coordinates of eyeball movements that rotates according to Listing's

Law.


Integrating Listing's Law when designing the next-generation Null EP bi-asphefic lens. Listing's plane is a front plane passing through the centre of rotation of the eye.

Rotational movements of the eye in Primary,Secondary

and Tertiary positions of gaze, rotation about a fixed point, the centre of rotation, according to Listing's Law.

Traditional Best Form lens terminology and Oblique Astigmatic Error (OAE) and Mean Oblique Error (MOE)

The next drawing illustrates the traditional Best Form lens terminology for distance vision in which oblique transmitted rays are shown and in which the tangential and sagittal focal lines give rise to Oblique Astigmatic Error and Mean Oblique Error from the deviation (D) and Far Point Sphere (FPS).

Two important references are shown in this figure, the Vertex Sphere (VS) and the Far Point Sphere. These are formed by rotating point S around the eye's centre of rotation R and the far point focus MR respectively. The object is a distant point on the main (meridian) ray (Mr), so the tangential and sagittal line foci are labelled F'T and F'S respectively. The Disc of least confusion is marked D. Note that the focal lengths F't en F's are measured from the Vertex Sphere.

The Vertex Sphere is constructed by describing an arc of radius s with its centre at the eye's centre of rotation R. The Vertex Sphere has a radius s and vergences are measured at the point Q so that they can be compared with back vertex power (in ophthalmic practice a lens power is specified by its back vertex power).


Illustration Oblique Astigmatic error and Mean Oblique error from oblique transmitted rays

and oblique vertex sphere focal length F't and F's and Disk of least confusion

The Far Point Sphere is constructed by rotating the far point around the eye's centre of rotation. The Far Point Sphere indicates the position of the eye's far point in an oblique gaze. Ideally, when looking at a distant 'off-

axes' object, we would like the lens to focus on the pencil of rays on the far point sphere so that, after refraction by the eye, a focus point MR) at the centre of the fovea (centre of the

retina) is achieved. Actually, those ideal lens design will be hard to realize in spectacle lenses.

New Best Form lens by Hoya

 

By integrating a flat, aspheric convex curve with an aspheric and atoric concave curve, Nulux EP is a bi-aspheric lens that is designed according to the principles of

Calculated Visual Acuity and Listing's Law.

We have introduced the Calculated Visual Acuity as a new evaluation function, optimising calculation in order to obtain maximum visual acuity on each of the evaluation points of the surface. In addition to the spherical and astigmatic surfaces, an atoric surface expresses optimisation in all directions of sight, including outside

the spectacle principal meridians.
 

OAE= FT- FS. MOP (Mean Oblique Power) = 1/2 (F'T+F'S)

MOE (the amount by which MOP differs from Back Vertex Power F'V) -- MOP- F'V.
 

Illustration of ideal best form lens. OAE=O, MOE=O.

Incorrect result without (A) and correct result with (B) concideration to human eye movement for spectacle lenses, based on Listing's Law.

Integrating Listing's Law theory calculation provides for correct aspherisation in all directions of sight. Mapping B and illustration 3 show the results - a stable and clear vision area, which is the same in all directions of sight and according to our new definition for improving uncompromised visual field based on calculated Visual Acuity and Listing's Law.

In summary, the material used is no longer the starting point when designing a lens. Now Hoya takes the human eye- and in particular the visual Acuity and movements made by the eyeball - as the key reference points in creating a lens that offers an unprecedentedly sharp image field. Nulux EP is the perfection of dynamic vision, an aspheric spectacle lens which takes the measured visual sharpness as the crucial starting point for optimal vision.

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