Saturday, 21 December 2013

History: Lens Making-- Beyond than a Grind

In 1268, the English scientist Roger Bacon wrote that people with weak eyes could read small print by reading through a piece of glass "shaped like the lesser segment of a sphere, with the convex [rounded] side toward the eye."

Although the advice was good, it could be ignored by the general population for the simple reason that the general population was illiterate. Even if they could read, books were so rare and so jealously guarded that the general public couldn't get their hands on them, let alone their eyes.

Nearly 200 years later, in the mid-1440s, a goldsmith in Mainz, Germany, named Johannes Gutenberg changed all that. He and his partner, Johann Fust, took a new and somewhat radical idea called movable type and started the world's first publishing empire. A few decades later people who had been raised not knowing what a book was found themselves complaining that even though they now had plenty of books, they couldn't read the fine print in them.

Why did it take the invention of movable type and the creation of mass-produced books and pamphlets to spark the development of lenses? Think about it. There are very few things you need to stare at—for hours at a time—that are smaller than the print on this page.

All of a sudden, Roger Bacon's commentary about looking through "the lesser segment of a [glass] sphere" became more than just a curious bit of trivia. It became the cornerstone of a brand-new industry created to grind the lenses that all of those lately literate people with weak eyes and a new library needed to read their books.

A Guild for Glasses

Early magnifying lenses were held closer to what was being read than to the eye. It was just a matter of time, though, before someone realized that the lens would also work' if you held it near the eye. Then someone figured out that you could use a different lens for each eye.
But the important part was the lens.

Many people knew just how important, and the lens-making industry grew to the point that in 1629, England's King Charles I gave a charter to the Worshipful Company of Spectaclemakers of London, which had formed "for the better order, rule and government of those using the Art and Mistry [sic] of Spectaclemaking."

According to the rules established by the Worshipful Company—also known as a guild—a person could become an apprentice at age 16. After nine years of work—years spent learning everything from grinding the lenses to making the frames—an apprentice could submit a sample of eyeglasses to the guild's examining board. If the glasses were good enough, the apprentice became a master and could open his or her own shop. It was one of the few guilds that allowed women members.

The emphasis then was on making spectacles that fit their wearers' heads, not their eyes. As far as the "prescription" was concerned, spectacles were sold according to age. It was assumed, for example, that all 30 year olds with eye trouble would have similar vision problems, and when one came in for glasses, the choice of lenses was limited to those deemed suitable for 30year-old eyes. Sometimes the person who needed the glasses would order them by mail, asking only for a pair of glasses suitable for a specific age and size.

Over the years, the process of fitting glasses to individual visual needs developed along with the process of making them. Most of the developments and progress in the craft of lens making took place in Europe. So did the developments in lens using. Along with the best lenses came the best telescopes, cameras, and microscopes to use those lenses. In fact, the United States got nearly all of its optical glass from Europe until the outbreak of World War I. Once the war began and America decided to team up with England against the Germans—Europe's premier lens makers—American companies had to learn how to make their own.

The Art of Making Glass

The basic process of making glass is neither hard nor mysterious. The first glassmakers very possibly "discovered" their craft after watching what happens to a sand beach when lightning strikes it. The "hard ice" that formed out of the fused sand served as their inspiration. They then probably experimented with supplying their own heat and mixtures of sand and other components to first create colored ceramic glazes. Their craft eventually evolved into glass making. It is worth noting that the word glaze refers to both windows and to the coating put on pottery.

But just as there is a big difference between playing the guitar and playing it well, there is also a big difference between making window glass and making optical glass.

Today, optical glass is made by machines according to complicated and often secret formulas that call for the blending of special sands with minerals, chemicals, and even some rejected optical glass from other batches. These mixtures are cooked for more than a day in fire pots that have to be preheated for several days before even receiving the mixture.

The mixture is cooked until it turns into a liquid as thick as mud or molasses. It is then poured on a preheated iron table and rolled out to a specific thickness. After the mixture is rolled out, the table is moved—by conveyor belt—into another section of the oven, where it is slowly allowed to cool. It is still much too hot for anyone's hands to come close to it, let alone work on it.

One of the first lessons learned by glassblowers and anyone else who works with glass is that you can't tell how hot a piece of glass is by looking at it. Glass hot enough to give you a severe burn doesn't look any different from room-temperature glass.

Once the glass cools, automatic shears cut out individual pieces, which are pressed into lens blanks. Still traveling by conveyor belt, the blanks go on to another furnace, where they are cooked and cooled yet again.

Getting the glass to the right temperature is only part of the process. The glass has to be heated and cooled for a specific period of time, and when it is allowed to cool down, it must do so slowly. If molten glass is cooled too quickly, it becomes very brittle.

Once cooled, the individual lens blanks are then inspected, machine polished to meet the specific requirements for that batch of lenses, inspected yet again, and then shipped to the offices of the modern descendants of the Worshipful Company of Spectacle makers and into the hands of the anxiously waiting master craftsmen employed therein.

Personalizing the Product

It's up to the optician to make sure that the lenses you need and want will fit into the frames you picked. But there is more involved than just cutting the lenses to fit the frames and popping them in. Each lens must have the right "strength" or prescription. And for the prescription to work as intended, the lens must be carefully positioned.

Alignment for Astigmatism

Lens placement is especially important to people with an astigmatism—people whose eyeballs are not perfectly round. Their glasses must compensate for this odd shape, so the entire curve of the lens must be both tuned to and turned to the curve of the eyeball.

How Lens Prescriptions Are Filled

Some 6-foot-tall men wear size 6 shoes, others wear size 16. The vast majority are somewhere in the middle.

In the same way that it would be impossible to come up with a one-size-fits-all shoe that would guarantee a perfect fit to every man in the world, it would be equally impossible to make a one prescription-fits-all pair of glasses.

In fact, they can't even come up with a one-size-fits-all lens blank.

As you read earlier, it is the light-bending curvature of the lens that determines its power or prescription. Each lens has two curves, one on the front surface and one on the back surface. There are virtually millions of different possible combinations of curves.

To simplify matters, lens manufacturers generally make about six different blank lenses, each with a different "base curve." These blanks are made by the millions and sold to finishing labs, which then grind the front surface of each lens into a curve that will fit an individual prescription.
While most lens manufacturers make five or six different lens blanks, nearly all prescriptions can be made using a lens with one of the three common base curves. The other lens blanks are kept in stock for the same reason that shoe stores routinely carry size 13 shoes. Some people need them.

But what about a man who needs a size 15, or a size 3? Special lens blanks, like special shoes, can be ordered from custom manufacturers.

Imagine making a plaster cast of your feet and then standing in the wet plaster until it dries so that the cast matches your footprint like a second skin. Imagine standing in that cast and sensing how smooth it feels. Now imagine turning around and standing in it with your feet facing the opposite way. The cast still fits the contours of your feet, but it doesn't fit the way you have your feet in them.

It's pretty much the same with an astigmatism. The lens has to be precisely positioned in the frame to fit the irregular curvature of the eye. The optical "top" of the lens must be at the optical "top" of the glasses.

Bifocals and Trifocals

The process is a bit more complicated for people with bifocals—glasses with two prescriptions on each lens. Bifocal wearers usually look through the top portion of the glasses for general walking around and "seeing" and through the bottom portion for reading.

While some bifocal lenses are made from two different pieces of glass fused together, others are made out of one piece that has been ground into two separate prescriptions.

The whole process is taken a step further with trifocals. Each lens of these glasses has three separate prescriptions.

The most common type of trifocal has a general "seeing" lens on top, a "reading" lens on the bottom, and a "middle distance" lens in—you guessed it—the middle.

Some people, however, have trifocals made with the general "seeing" lens in the middle and "reading" lenses at the bottom and at the top. This type of lens would be handy for a person who has to read things above his head as well as below. A piano player, for example, might need this type of lens so he could read the music on the music stand by glancing up from "reading" the keys.

Many people dislike wearing bifocals—and trifocals—if there is a clear and distinct line between the different pieces of lens. They feel that just being seen in bifocals makes them look old. They also complain about the "jump" in vision as their eye travels from one lens to the other.

Off-the-Rack Reading Glasses

Those of you who have never worn glasses have a rude awakening ahead at around age 42 (give or take a few years). You may find that you suddenly (or gradually) have trouble reading fine print—the sort found in a telephone directory—or that you have to hold reading material farther away than normal to see it clearly.

Some people say that their arms have grown too short.

If this already sounds familiar, you have presbyopia, a condition in which we lose our ability to change the shape and, therefore, the focusing power of the lens inside our eye.

A simple solution is reading glasses, also known as magnifying glasses. You've probably seen them in the drugstore, on the rotating racks.Even many people who wear contact lenses may wear magnifying glasses over them for reading.

Magnifying glasses are numbered according to their magnifying power. The numbers generally run from + 1.00 to + 3.00, and increase in increments of 0.25. Both lenses will be the same power.

If you started out nearsighted, farsighted, or with a fair amount of astigmatism, you probably need prescription reading glasses, bifocals, or trifocals. But many people find the drugstore glasses very adequate.

People considering store-bought magnifying lenses typically have two questions. "Are the lenses of inferior quality?" and "Will I hurt my eyes if I pull too strong or too weak a prescription off the shelf?" The answer to both questions is no.

Some bifocals are made without that line. The two prescriptions blend together in a seamless piece of glass. While this hides the dreaded "bifocal line," it does create an area between the two lenses where vision is distorted, since the "blended" area fits neither the prescription for seeing nor the one for reading.

Plastic Lenses

No matter what prescription you need, you can probably get it in either glass or plastic. The choice is up to you.

Plastic is lighter, easier to work with, less likely to fog, and more resistant to shattering. The strongest lenses available today are made from polycarbonate, the plastic used to make bulletproof windows. Polycarbonate lenses can be coated, tinted, and ground to any prescription. They weigh about half as much as glass lenses.

The light weight of plastic is a major bonus, especially if you are required to wear the thicker sort of glasses that your more tactless acquaintances might refer to as "Coke bottle lenses." There are some drawbacks, however. Plastic lenses are more expensive to make and are more likely to scratch.

Plastic lenses also need special care. While you can clean a glass lens with any piece of soft cloth, plastic lenses should be washed with soap and water and wiped clean while they are still wet. If you wipe them while they are dry, you will generate a static electricity field on the lenses. This static charge will attract dust and dirt that will get the lenses dirty again that much more quickly.

Tints, Coatings, and Other Options

Some lenses—both glass and plastic—are colored as they are made by adding dye to the lens mixture. This can be a problem, though, for people with thick lenses or lens sections, because the color darkens as thickness increases.

Polarized lenses are made by adding chemicals to the glass or plastic when they are being made. Sometimes polarized plastic is laminated to a glass lens.

Polarized lenses reduce glare from water, snow, car windows, and other reflective surfaces by allowing only light coming in at specific angles to pass through. That is why tilting your head or turning polarized lenses changes the amount of visible light.

Other lenses are coated after they are made. There are five types of lens coatings: dielectric, color, non-reflective, scratch resistant, and mirror.
  • A dielectric coating is used to block out both UV and infrared radiation.
  • Color coating a lens does just what the words imply.
  • non-reflective coating can be applied to the back of the lens, the side closest to the eye. When light strikes the lens from the back—from behind the person wearing them—the light passes out through the front of the lens and is not reflected back into the eyes.
  • A good scratch-resistant coating can make a pair of plastic lenses almost as scratch resistant as glass ones. A scratch-resistant coating could also be applied over other coatings to prevent them from being scratched or rubbing off the lens.
  • A mirror coating prevents a great deal of light from getting through the lens and turns a pair of glasses into a one-way mirror. No one can see in, and the person looking out through them is protected from infrared radiation. Mirrored lenses are actually the coolest lenses to wear—in terms of heat, not style—and should be worn outdoors in intense glare from either direct sunlight or sunlight reflected from ice, water, or snow.
Photochromic lenses also block out sunlight, but without the use of any coating. Sometimes referred to as photosensitive or sun-sensitive lenses, these "automatic sunglasses" darken when exposed to bright light. When the light decreases, the lenses lighten up again.

It is not the brightness of the light that makes them darken, however; it's the UV light that is part of normal sunlight. They stay dark as long as they are exposed to ultraviolet light.

Many people wear photochromic lenses instead of sunglasses. But if you are going to be outdoors in the bright sunlight a great deal, you might want to get a regular pair of sunglasses as well. The latter will block out more light and do so as soon as you put them on.

Other drawbacks of photochromic lenses are that they tend to get darker in cold weather than they do in hot weather, and they can't reach full darkness while you are driving because the windshield blocks some of the UV rays needed to trigger the darkening process.

These lenses also take time to lighten up. Entering a dark room from bright sunlight could be a problem until both your eyes and the lenses adjust. And they never go completely clear; they always have a slight tint to them. This could be a problem for older adults who usually require more light in order to see.

Frames

No matter which type of lens you pick, you will also have to choose the frames to put them in.
Frames are usually made out of metal or plastic and come in innumerable sizes, styles, and colors. Some even have sequins.

If you are indecisive, there are numerous fashion consultants—some trained and some self-appointed—who can tell you which type of frame is "in" and which type is "out"; which frames are right for your face, your hair, or your occupation; what colors bring out your best features or match your teeth ... whatever. Some of these fashion consultants might even know what they're doing.

In terms of style and color, get the glasses you like. It's perfectly natural to ask people their opinions, but the final choice is yours. While you aren't going to be looking at yourself wearing them, you are the one who will be wearing them. If you get talked into frames that you dislike, they'll spend more time in their case than on your nose, and the glasses won't do you any good.
Whatever style you choose, they should be sturdy and well constructed with smoothly working hinges and screw joints.

Your glasses should fit lightly on the ears, nose, and temples, so that you're hardly aware that you are wearing them. But while they should be light, they must also be snug. They won't do your eyes or your checking account any good if they keep falling off your face.

They should also be suitable for what you're going to be doing while you are wearing them. A pair of eyeglasses for a person who spends most of his time at a desk might be different from those needed by someone who wears them while climbing mountains, playing basketball, riding bucking broncos, or looking through a telescope.

By the way, when your glasses are not on your nose, they belong in their case. That's why your optician gave you one. With eyeglasses routinely costing more than $100 a pair, keeping them unprotected in your purse, pocket, or pack is an invitation to an expensive accident. Most eyeglasses that are broken meet that fate when they are not being worn rather than when they are.

So much for choosing the right frame material, color, and style, and how to keep your glasses safe. Before any style choices can be made, you first have to determine which size you need.
The size frame you need depends on the size of your head and the distance between your eyes. An optician will determine your P. D. (pupillary distance) by measuring the distance between the pupils of both eyes. That measurement is important because the optical center of each lens should be right in front of the pupil. The farther apart your eyes are, the farther apart the optical centers of the lenses must be.

People with large heads and eyes that are relatively far apart usually need larger lenses and larger frames. And as with so many other things in life, not all styles are available in all sizes.

The final considerations that need to be mentioned are the weight and thickness of the lenses. Thick and heavy lenses need frames that are sturdy enough to support them.