Mineral glass is difficult to work, especially the rock crystal beryl initially used to make spectacle lenses, because of its hardness. So anyone who wanted to turn a piece of glass into a visually effective lens needed a lot of patience and perseverance. Whether a lens could ultimately deliver a distortion-free image depended entirely on the precision with which it was worked. Thus it is not surprising that, in the late Middle Ages, methods were sought based on wind and water power to produce spectacle lenses with the required finish, if possible without the need for human labor. The advent of the industrial revolution two centuries ago with its motor-driven machines finally made it possible to manufacture spectacle lenses on a routine basis.
By Dr. Hans-Walter Roth
In the beginning grinding of spectacle lenses was done entirely by hand. Thus chroniclers report dozens of unqualified workers spending days giving the blanks supplied by the glassworks their desired shape. On grinding machines, whose antecedent was the potter’s wheel, one side of the lens was ground to a shape corresponding to a spherical surface. The other side was initially flat, as with lenses for reading placed on the page. Only later did lenses become biconvex. Provided the diameter and refractive index of the material were known, the strength of the spectacle lens could be calculated from the difference between the central thickness and the edge thickness. Prior to the introduction of the metric system two centuries ago when the diopter was introduced as a measure of spectacle lens strength, previously only the focal length was specified. It was easy to determine: you just had to hold the lens up to the sun and, by focusing the rays of light for example on a piece of paper, you could find the focal point. The oldest surviving pair of glasses was found in 1953 under the choir stalls in the nunnery at Wienhausen, founded in the 13th century. Like most glasses at that time, the strength of the lenses was +3.4 diopters, showing that they served as a reading aid to compensate for presbyopia.
The beginnings of automation
With the invention of printing, the demand for reading glasses increased dramatically as more and more people learnt to read. The lengthy process of grinding by hand, however, prevented mass production of reading aids. Thus it became imperative to automate the lens-grinding process. As early as the 16th century, there had already been some quirky constructions to grind several lenses in series simultaneously.
Diderot’s encyclopedia, published in France from 1751, includes illustrations of various devices and tools used to make lenses for a variety of optical instruments, including spectacles. The grinding machine has a solid wooden frame similar in principle to that of a potter’s wheel. A large flywheel is driven by a hand-operated crank, with a leather drive belt transmitting the rotation to a small wheel, thus multiplying the speed of rotation by a factor of 1: 5. The precast biconvex glass lens is mounted in a concave support cup on a vertically mounted rotating rod. This in turn is connected via a wooden bevel gear to the small wheel in such a way that one turn of the hand crank makes the lens rotate five times about its own axis.
Above the lens holder there is a metal bar fixed with two wing nuts, to which various grinding and polishing heads can be attached. These are shown in the subsequent figures with different radii of curvature, showing that different lens thicknesses can be ground on the same machine. The concave polishing heads and the holders for the lenses are made of metal – usually brass – prefabricated on a lathe. A thin piece of leather between the lens and the holder prevents the surface of the lens from being scratched while it is being worked and held securely.
The illustrations shown here are from one of the numerous editions of the Diderot encyclopedia. They were purchased individually from one of the many booksellers on the banks of the Seine in Paris. Unfortunately, today there are hardly any complete editions of the famous encyclopedia from the 18th century on the market; savvy dealers preferring rather to separate them and sell the pages individually, in order to make more money.
Diderot Lunetier 1, Lenses from different sources.
Diderot Lunetier 2, Tools.
Diderot Lunetier 3, Tools.
Diderot Lunetier 4, Cutting and polishing machines.
All pictures in this article are courtesy of the Institute for Scientific Contact Optics Ulm.
Bühler Leybold Optics has organized their first live webinar for the ophthalmic market.
The webinar with the title “meeting market requirements from fast coating solutions to premium processes” will be held on Tuesday, May 26, 2020.
The current ophthalmic market demands specific coating solutions to meet sustainability requirements. In this webinar, Xavier Peréz (R&D Project Manager, Process Development and Application) and Amaury Laureau (Sales Manager Ophthalmic) will present several solutions to tackle that demand in different ways.
List of topics:
- Lens production and sustainability
- Coatings processes: OptiFast, Robust, UMBRA
- Energy management
- Digital solutions
In order to give viewers from different time zones the opportunity to participate, two sessions will be offered:
- 10:30 AM Central European Summer Time
- 05:30 PM Central European Summer Time
Registration is open now.
Wanting to stay connected with labs, but at their customers’ convenience, Satisloh has created their new virtual booth, SatislohLink. Responding to canceled trade shows around the world, the company built this new online tool to keep the ophthalmic market informed and supported about new technologies so when they’re ready to ramp back up to capacity, they know what’s available and how it will impact their production levels and improve their efficiency.
“As spring trade shows were being canceled, we realized we needed to think creatively about how to stay connected with our customers and keep everyone safe,” said Andy Huthoefer, Satisloh Head of Marketing and Product Management. “Our virtual booth, SatislohLink, lets us do that. Users can explore our offerings from the comfort of their homes – until we can meet in person again.”
Visitors to SatislohLink can experience virtual product launches, explore product videos, attend in-depth webinars to learn more about the company’s new technologies and processes, sign up for limited-time special pricing promotions, browse how-to maintenance documentation, and have fun with a quiz for a chance to win prizes.
Shamir is collaborating with US optical laboratories nationwide, which produce their freeform or digital lenses, and have chosen to participate in the company’s #VisionFIRST initiative. The program was designed to provide complimentary prescription lens designs for safety eyewear or other lens types for healthcare workers and first responders.
“We are all in this together”, said Raanan Naftalovich, CEO, “The nation will return to normal, but first we need to ensure the safety of our healthcare professionals.” Shamir’s lens products are used in the optical industry today in the form of safety eyewear or occupational (small task) lenses, progressive or single vision lenses.
The company expects #VisionFIRST to contribute thousands of pairs of complimentary prescription eyewear, with Shamir digital designs, to first responders. “Our hope is to collaborate with as many Shamir Optical Lab partners as possible, increasing the opportunity to provide enhanced vision for those holding the front-line”, said Raanan. “All our digital lens products will be available to our optical laboratory customers so they and the eye care professional will have no constraints in finding a lens that will work for just about every first responder in need of eyewear”.
Over 20 partnered optical laboratories confirmed participation right from the start. Shamir anticipates up to 100 total optical laboratories nationwide will actively work with eye care professionals across the country, offering first responders personalized safety eyewear.
Participating #VisionFIRST labs can be found at www.shamirlens.com/visionfirst.
The research and development company in the field of optics says it wants to enliven the weeks of confinement with its own webinars that encourage to strengthen knowledge in a pleasant and productive way.
IOT has decided to publish weekly different webinars that seek to entertain and also teach and review various general and fundamental aspects of optics.
With the aim of offering a quality teaching adapted to as many users as possible, the webinars are being broadcast virtually and, in several languages, specifically, in Spanish, English, Italian and even Mandarin.
The call for these webinars is sent by e-mail, through which interested people can sign up with just a click. In addition, these courses are also promoted through the company’s social networks, thus offering an alternative way for all who are interested in them.
IOT recommends signing up as soon as possible for the webinar in which the user is interested because of the limited capacity.
This week the topic is Ophthalmic lenses for digital life.
April 30, webinar in Spanish / 12 PM ( GTM +5), Bogotá – 7PM (Madrid)
April 30, webinar in English / 11 AM (GMT +2) Madrid
April 30, webinar in English / 9AM PDT- 12 PM EDT