Wide field-of-view bifocal eyeglasses

Presbyopia is the age-related diminished ability to focus on nearby objects. It affects most people after the age of 45, and in many cases, presbyopia is accompanied by myopia or hyperopia (loss of focus for far vision). In the presence of both defects the image is blurred (in different ways) when looking at far objects and at those nearby. Consequently, monofocal spectacles are not a satisfactory solution. The typical alternatives, still based on eyeglasses, include segmented focal regions (bifocal or trifocal lenses) or progressive addition lenses (PALs).

In bifocal lenses the smaller part, located nearer the nasal region, is designed for near-distance vision (thus providing higher optical power), whereas the larger part is used for far-distance vision. The jump between the distance and near regions of the lens generates prismatic discontinuities and additional undesired visual effects. In PAL designs, the lens surface is smooth, and the optical power changes gradually as the eye moves downward and in the nasal direction, thus providing a continuous power change. However, PALs have the drawback of reducing the field of view for each power position (for far, near, or intermediate object locations), which restricts the natural eye-head movements of the wearer.

To avoid these serious limitations of traditional bifocals and PALs, we have developed a novel type of bifocal, or continuous focal: power-adjustable lenses that ensure a wide field of view for every viewing distance, without the problem of discontinuities. The design is based on Alvarez's principle: when two lenses with cubic-type surfaces are laterally shifted with respect to each other, they induce a spherical power change. We then used optimal spectacle design methods to provide good vision at all relative shifts of the two lenses. We previously used this approach to design affordable adjustable ophthalmic lenses, thereby addressing the problem of functional blindness caused by uncorrected refractive errors, which is common in some developing countries.^1–3 In these previous designs the optical axis was fixed between different focal positions. However, when the spectacles are used to correct vision for different viewing distances, the designer must take into account that the principal gaze direction changes depending on the object location, and consequently the optical axis is displaced. Figure 1 shows this effect. For our design, we obtained a merit function as a weighted sum of the squared spherical and cylindrical errors for different eccentric gaze directions (covering a field-of-view range up to 45°) for two object viewing locations. The merit function was optimized by a cascade approach, where different surface parameters were adjusted at successive steps. Further details of the design methodology can be found elsewhere.^2–4

Figure 1. Eye movement when looking toward a nearby object (point P). (a) Vertical rotation. (b) Horizontal rotation (toward nasal region). C: Eye center of rotation.

To illustrate our design's potential, we may consider an example where we aim to enable wide field-of-view far-distance and near-distance viewing by a person whose left eye needs an addition of +2.0D. The refractive index of both lenses is n=1.586. For the ‘ rest position’ (no lens movement), the optical power at a point aligned with the primary gaze direction is 2.3D. When we move the lenses to a first relative position, achieved by a horizontal shift of 2mm of the front lens in the negative x-direction from the rest position, the optical power at a point aligned with the primary gaze direction is 3D. While the lenses are in a second relative position, achieved by a horizontal shift of 4mm of the front lens in the positive x-direction from the rest position, the optical power is 1D. Figure 2 shows the power (a, c) and cylinder (b, d) error distribution (deviations from nominal values) for far (a, b) and near (c, d) vision viewing conditions. The graphs show that the deviation, for both sphere and cylinder, is less than 0.25D within an elliptical optical window, having major and minor axes of at least 40° by 40° of eye rotation.

Figure 2. Power error as function of eye rotation for far vision (a) and near vision (c). (b) Cylinder error for far vision. (d) Cylinder error for near vision.

In summary, we have developed wide field-of-view eyeglasses, while also achieving the typical field of view available when looking through conventional monofocal eyeglasses. This technology represents a breakthrough over conventional bifocal lenses, where the desired power is usable in only half of the visual field. It may also be possible to use the same methodology to design trifocal wide field-of-view eyeglasses, and in fact continuous focal change, without reducing the field of view. In our future work, we plan to integrate into the design a mechanical frame capable of holding the optical lenses.