The secret behind the perfect spectacle lens design
A lot of experience, mathematics and exact knowledge of the spectacle wearer’s needs are required to provide a tailor-made solution
What do you think? Which product incorporates more optical knowhow – a spectacle lens or a modern high-performance camera lens containing five individual optical elements? Camera lenses would be the immediate reply that a lot of people would give. What advanced optical properties can a thin, run-of-the-mill plastic lens possibly offer? The hidden secret of a spectacle lens is in its design. A camera lens features symmetrical optics and is produced with the same shape over and over again. This is a rare exception in the field of spectacle lenses, where asymmetry is the rule. The lens shape is considerably more complex and is unique for every single lens. The performance of spectacle lenses is the result of many different optical surfaces of immense complexity. Why is this? This was the question BETTER VISION asked Gerhard Kelch, qualified mathematician and Head of Optical Design at ZEISS Vision Care.
BETTER VISION: Mr. Kelch, ophthalmic experts always strive to develop better and better lens designs. Why is this subject of such key importance? How does this all benefit me as a spectacle wearer?
Gerhard Kelch: The primary job of a spectacle lens is to correct defective vision – short-sightedness or long-sightedness, for example. It can also remedy other visual impairments such as astigmatism or a condition that the experts call presbyopia, or the increasing difficulty to focus at close range as we get older. The minimum requirement to be met by the lens is that the eye can see clearly again when looking "straight-ahead." This does not necessarily mean that we can see as well as people with perfect vision. And this is a challenge that we have been addressing at ZEISS for more than 100 years. Our goal is to provide spectacle wearers with clear, natural vision in all directions, not just straight-ahead.
Our eyes move behind our lenses. And the movement of our eyes varies from one person to another, and every frame sits differently on everyone's face – with a different tilt angle, for example. And this is where the design of modern spectacle lenses come in.
A modern lens takes all of this into account thanks to individualised designs. A prescription lens has a different optical power toward the top or side of the lens than when we are looking straight-ahead. The design must therefore be optimised in such a way that the best possible correction is achieved at the top and the periphery of the lens. This must all be adapted to the wearer's prescription and to the fit and shape of the frame.
Our eyes move behind our lenses. And the movement of our eyes varies from one person to another, and every frame sits differently on everyone's face.
BETTER VISION: Is the way the lens is produced the secret behind it all?
Gerhard Kelch: Well, we at ZEISS invest a lot of money in the ongoing improvement of our lens designs and in adapting them to the changing lifestyles of spectacle wearers. Our many years of experience in the field of optics – and not only in the area of spectacle lenses – provides us with an ideal foundation for our work. Our products cover the entire process from the initial eye examination using our measuring systems to the production and coating of our lenses. The vast majority of the systems we use are products, instruments, techniques and software tools we have developed ourselves. We match all stages of the process precisely and exactly to each other. ZEISS leaves absolutely nothing to chance. Only tried and tested products and technologies reach the market.
If we are developing a new lens design – our latest product ZEISS Digital Lenses, for example, we start by analysing exactly what the new design should achieve. We conduct wearer trials at a very early stage. After all, what good is even the most sophisticated design if it's totally worthless in everyday use?
BETTER VISION: How do you choose the wearers for these trials?
Gerhard Kelch: It all depends on the product, of course. For progressive lenses, we have to choose wearers who are over 40 and already experiencing their first difficulties focusing at close range, or who already wear progressive lenses. Otherwise, we select a typical cross-section of people: e.g. with low and high prescriptions, of different ages and gender, and with different shapes of face … As we operate all over the world, there are various features specific to each region that have to be taken into account, e.g. shape of face, position of eyes, and different tastes when selecting spectacle frames. All of this must be incorporated in our initial design considerations. A tested, standardised procedure using questionnaires then helps us to optimise the designs and to decide between different versions of the product. The more we know about the wearers, the better the lens will be in the end! Needless to say, we also need the best optical design software which we develop ourselves and constantly enhance with new data.
The more we know about the wearers, the better the lens will be in the end!
BETTER VISION: But there are also lenses that are optimised for each and every wearer. How does this work?
Gerhard Kelch: In the past, it was only possible to design and produce lenses using spherical or toroidal surfaces. Today, we can design aspherical and atoroidal surfaces and incorporate them in lenses in many different ways. Right up to freeform technology that allows any type of individualisation. This is interesting not only for progressive, but also for single vision lenses. One example: we can now produce modern sports eyewear with wrap frames in such a way that the wearer no longer experiences peripheral blurring. Powerful computer technology now allows us to design individualised lenses with extreme precision on the basis of the data we receive from your optician – and all in a matter of seconds. In the 1980s, a whole night was needed to design a freeform lens surface.
Nowadays, we calculate 12,000 individualised progressive surfaces each and every day in our Aalen factory in Germany.
BETTER VISION: This must involve huge amounts of data.
Gerhard Kelch: Correct. The measuring data we receive for the individual spectacle wearer – data privacy is guaranteed at all times, of course, – is essential for the production of each individual lens. Imagine a lens gets lost during transportation or is broken during subsequent wear, we have the wearer's data ready to hand and can produce the replacement lens immediately. The data additionally helps us to predict new trends – e.g. in frame fashion – and incorporate these in our lens designs and wearer trials. The past three years have shown that there is now a trend towards larger frames, for example. This fashion trend also affects our designs: a lens sits very differently in front of the eyes in larger frames. The optics must be adapted accordingly.
BETTER VISION: One last question: What would you as a mathematician recommend to every spectacle wearer?
Gerhard Kelch: I firmly believe that clear, comfortable vision is so essential in our everyday lives that more importance should be attached to it. Today, outstanding technologies permit us to create solutions that offer the ultimate in visual comfort. So, I would recommend everyone to have their eyes regularly tested by an eyecare professional. Poor vision may be the cause of headaches, for instance. The right lenses may be the remedy.