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Why do billions of people still not have glasses?

It's thought 2.5 billion people could benefit from technology first developed in the 1200s.

At Lockheed Martin, for example, it used to take a technician two painstaking days to measure 309 locations for certain fasteners on a particular curved panel.

But according to Shelley Peterson, the aerospace company's head of emerging technologies, the same job now takes little more than two hours.

What changed? The technician started wearing glasses. But not just any old glasses: specifically, the Microsoft Hololens.

It looks like a bulky set of safety goggles. And it layers digital information over the real world. In this case, it scans the curved panel, makes its calculations, and shows the technician exactly where each fastener should go.

Productivity experts are gushing about the potential benefits of augmented-reality devices such as the Hololens and Google Glass.

When Google first demonstrated its smart glasses in 2012, their prospects seemed quite different. They were seen as a consumer device, something that would let us check Instagram and take videos without the hassle of reaching for our phones.

They did not catch on. The few people who ventured out in public wearing Google Glass attracted the dismissive soubriquet "glassholes".

Google soon realised it had misidentified its target market, so reinvented the glasses for the workplace. Many jobs, after all, involve frequent pauses to consult a screen that tells us what to do next.

With smart specs, we can see those instructions while we keep working. It saves a vital few seconds in getting information from internet to brain.

50 Things That Made the Modern Economy highlights the inventions, ideas and innovations that helped create the economic world.

It is broadcast on the BBC World Service. You can find more information about the programme's sources and listen to all the episodes online or subscribe to the programme podcast.

A thousand years ago, information travelled rather more slowly.

In Cairo, in the 1010s, the Basra-born polymath Hasan Ibn al-Haytham wrote his masterwork, the Book of Optics, but it took two centuries for his insights to be translated beyond Arabic.

Haytham understood vision better than anyone before him.

Some earlier scholars, for example, had said the act of seeing must involve some kind of rays being emitted from the eye. By careful experiment, Haytham proved them wrong: light comes into the eyes.

Before Haytham, optical devices had been cumbersome: the Roman writer Seneca magnified text using a clear glass bowl of water. But the gradual spread of knowledge inspired new ideas. Some time in the late 1200s came the world's first pair of reading glasses.

Who made them is lost to history but they probably lived in northern Italy. Venice, in particular, was a hub of glassmaking at the time - problematically so, as buildings in Venice were made of wood and the glassmakers' furnaces kept starting fires.

In 1291, the city's authorities banished the entire trade to the neighbouring island of Murano. By 1301, "eyeglasses for reading" were popular enough to feature in the rulebook of the Guild of Venetian Crystal Workers.

But historians' biggest clue to the origin of eyeglasses comes from a sermon in 1306 by one Friar Giordano da Pisa. The invention was now 20 years old, he told his congregation in Florence. As Alberto Manguel notes in A History of Reading, the friar declared glasses to be "one of the most useful devices in the world".

He was right. Reading strained the eyes at the best of times: medieval buildings weren't famed for their big windows and artificial light was dim and expensive.

As we age, it becomes harder to focus on close-up objects; middle-aged monks, scholars, notaries and merchants were simply out of luck. Friar Giordino was 50. One can imagine why he appreciated his spectacles so much.

But they were useful only to the small minority who could read. When the printing press came along, glasses reached a bigger market. The first specialist spectacle shop opened in Strasbourg in 1466.

Manufacturers branched out from concave lenses, which help people see close-up. They learned how to grind concave lenses as well, which help people focus on things far away.

Put concave and convex lenses together and you have the basic ingredients for a microscope or a telescope. Both inventions emerged from the spectacle shops of the Netherlands around the year 1600, opening whole new worlds to scientific study.

Nowadays we take glasses for granted - in the developed world, at least. A survey by the College of Optometrists suggests about three-quarters of people in the UK wear glasses or contact lenses or have had surgery to correct their vision. It's a similar story in America and Japan.

In less developed countries, however, the picture is very different - and only recently did we get a clearer view of it.

Historically, the World Health Organization has collected data on people who have really serious problems with their vision only.

Many more can see well enough to muddle through daily life but would still benefit from spectacles. But how many? The world's leading lens-maker, Essilor, decided to find out, one assumes not for entirely selfless reasons.

In 2012 came the answer: around the world, some two and a half billion people need glasses and don't have them. That's an eye-popping figure but serious people think it's credible.

And many of those people may have no idea glasses could help them.

In 2017, researchers tested the vision of hundreds of tea-pickers aged 40 or over on a plantation in Assam. They gave a simple $10 (£8.20) pair of reading glasses to half of those who needed them. Then, they compared how much tea was picked by those who wore the glasses and those who didn't.

Those with the glasses averaged about 20% more tea. The older they were, the more their tea-picking improved. The tea-pickers are paid by how much tea they pick. Before the study, not one owned glasses. By the end, hardly any wanted to give them back.

How widely we can extrapolate from this study is hard to say: picking tea may reward visual acuity more than some other jobs.

Still, even conservative estimates put the economic losses from poor eyesight into the hundreds of billions of dollars - and that's before you think about people's quality of life or children struggling at school.

One randomised trial concluded giving children glasses could be equivalent to an extra half year of schooling.

And the need is growing. Presbyopia, or long-sightedness, comes with age; but among children there's now a global epidemic of myopia, or short-sightedness. Researchers aren't sure why, though it may have to do with children spending less time outdoors.

What would it take to correct the world's vision? Clearly, more eye doctors would help - the number varies widely from country to country. Greece, for example, has one ophthalmologist for roughly every 5,000 people; in India, it's one per 70,000; in some African countries, it's one per a million.

But while serious eye problems demand skilled professionals, people whose needs are more easily fixable could be reached by other workers.

In Rwanda, a charity trained nurses to do sight checks; researchers found they did them well over 90% of the time.

Could teachers identify students struggling to see? I've worn glasses since primary school, when my teacher saw me squinting at the blackboard and told my mother to take me to an optician.

Another study suggests they could. After just a couple of hours of training, teachers at schools in rural China could spot most children who would benefit from wearing glasses.

It shouldn't be rocket science to roll out 13th-Century technology.

One wonders what Friar Giordano would make of a world in which we build spacecraft in augmented reality but haven't yet helped a couple of billion people fix their fuzzy views of actual reality. He'd probably tell us where to focus. BBC

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