How shape.line improves the disturbing influence of the lens edge on vision

Fig.1: The reflections that occur at the edge of the lens with minus lenses can be elegantly integrated into the spectacle aesthetics by using a shape.line edge coating that matches the color of the frame. Comparison of the effect of identically glazed black frames (-2dpt, n=1.5) with and without black shape.line edge coating.

Everyone knows it, but hardly anyone really notices it: if light falls on the edge of the spectacle lens, scattered light is created there, which is guided through the lens via total reflection, illuminates the entire circumference of the lens and then leaves the lens again through the front, rear surface or edge of the lens. Part of it is directed towards the eye where it is consciously or unconsciously perceived by the wearer.

Fig.1 illustrates how the typical myopic rings adversely affect the overall impression of the spectacles and the spectacle wearer. Even with the low powers used here (-2dpt, n=1.5), they are clearly visible when viewed from the side and significantly change the aesthetics. This effect can be eliminated or greatly reduced by using a color-coordinated edge coating to match the frame. Since the beginning of 2024, very high-quality edge coatings have been available in a large number of colors under the shape.line brand, see Fig. 2. 

Fig.2: shape.line - In an automated process, the lenses are individually coated after edging. Both aesthetic and functional, light-absorbing coatings can be produced in a variety of colors.

On the one hand, edge coatings can significantly improve aesthetics. On the other hand, edge coatings absorb the stray light generated at the edge of the lens and block light entering the edge from the outside, as can be clearly seen in Fig. 1. The scattered light at the edge is “swallowed” by the coating - the bright edge disappears.

Exactly how the stray light from the edge of the lens affects vision is a very interesting point that has been little studied to date. However, it is clear that stray light from the edge of the lens affects vision. Many spectacle wearers report this and a twelve-month study carried out in 2022/2023 clearly confirms this.

The edge of the lens “collects” the light from different directions

How the light reaches the edge of the lens via the front and rear surfaces or the edge itself and makes it glow can be visualized simply but impressively with a point light source (e.g. laser beam), Fig. 3. In this case, the green light from the laser enters the -4dpt lens from above via the front surface, is directed to a point on the edge by total reflection and from there is distributed over the entire edge area by scattering and total reflection in the lens. From there it is scattered again and leaves the lens in part in the direction of the wearer's eye, as the illumination of the eye socket in Fig. 3 clearly shows.

Fig.3: Light entering via the front surface leads to illumination of the edge due to total reflection. A portion of the stray light generated there radiates towards the eye and influences foveal and peripheral vision. Spectacles with -4dpt lenses. 

What is interesting here is that the laser light hits the front surface from above at a flat angle, similar to the light from a ceiling light. The point of impact is quite a distance from the edge and yet the light beam can no longer leave the lens and hits the edge of the lens with its entire power. This is a case that always and inevitably occurs in practice, regardless of the lens edge cover provided by the frame and possible anti-reflective coatings.

This phenomenon becomes even clearer when the light does not enter via the optical surfaces but via the edge of the lens itself, Fig. 4. In this case, the light enters the lens directly via the edge of the lens that protrudes slightly beyond the edge of the frame. This is a case that frequently occurs not only with rimless spectacles, but also with full rim frames. Depending on the frame shape, thickness of the frame edge and lens thickness, this phenomenon can often be observed on the temporal/nasal side of minus lenses and on the top of plus lenses. In the case shown, the minus lens (-4dpt) protrudes slightly backwards beyond the fairly wide edge of the frame. Light entering from the side hits this part of the edge and spreads out in the lens, as described above.

Fig.4: If light enters the lens via an exposed part of the edge, it spreads over the entire edge of the lens due to total reflection.

Lens edge coatings from shape.line eliminate stray light and thus lead to optimum visual quality

In summary, it can be said that disturbing stray light is caused both by light entering the edge from the outside and by the illumination of the edge via the front and rear surfaces of the lens. Some of the resulting scattered light reaches the eye of the spectacle wearer and leads to unconsciously perceived visual stress, which needs to be investigated in more detail.

The disturbing stray light can be reliably suppressed if an edge coating blocks the light from entering from the outside and also has highly absorbent properties. As a result, the absorbent edge coating not only reduces the reflections at the point of impact of the light beam, but also at every other point of the edge, Fig. 5. Stray light hardly occurs at all - optimum visual quality and significantly greater visual comfort are achieved. With the GlareVIS demonstrator developed by shape.line, this important benefit can be made clear to consumers quickly and easily.

Fig. 5: The absorbent edge coating of the left lens eliminates the light entering via the front and rear surfaces and blocks the light hitting the exposed edge of the lens from the outside. The right lens with untreated edge shows the usual myopia rings. Spectacles with identical lenses (-2dpt, n=1.5) in the GlareVIS demonstrator.

The edge coating technology presented here (shape.line) is a real innovation and new territory for many opticians. It was presented to the trade public for the first time at OPTI2024 and is now available to opticians in the form of a service. The coating is applied in an automated process in which the material is applied very precisely to the edge. Prior masking or subsequent wiping off of spilled paint is not necessary. The lens itself is processed as usual, following a few instructions, and sent in for coating. It is not necessary for the optician to purchase their own equipment or aids.

At OPTI 2025, opticians can get a personal impression of the effect of shape.line's edge coatings. Visit the sales and development team in hall C4 at booth C4.560.

Unfortunately you can't make it to Munich? Then visit our website www.shape-line-optic.de for more information.

Authors:

Prof. Dr. Jörg Luderich, Technical University Cologne

Christian Pöpperl, Shape Engineering GmbH