CR39 is a type of plastic. Most manufacturers of discount eyeglasses offer their cheapest pairs with CR39 lenses. This is because CR39 is extremely cheap. The process of making them is easy and very affordable for optical companies. Believe it or not, CR39 is actually better optically than Polycarbonate. The difference is incredibly subtle, but it's there. However, many people choose other lens materials anyway because of the down sides of CR39. Because it is a type of plastic, CR39 is not very durable at all. If the lenses are stressed or bent in any way, or if the eyeglasses are twisted by accident, the chances of the lenses breaking or chipping are not terrible. CR39 also tends to be a bit heavier and thicker than a Polycarbonate lens of the same prescription would be. If you are looking into buying prescription sunglasses, CR39 might actually be a good idea. It tends to tint, and hold tint better than other lens materials. In fact, most non-prescription sunglasses are made with some type of plastic lenses.
Polycarbonate is comparable to an acrylic material, and it has many uses. It has actually been used to make bomb proof shelters. Polycarbonate is incredibly durable. It can practically be bent in half without breaking. Which is why it is the law for kids eyeglasses to be made only with Polycarbonate. Compared to other lens materials, it is also thinner and lighter. For the most part, wearing a pair of eyeglasses with Polycarbonate lenses does not weigh your face and head down, or give you headaches. Another great quality it possesses is scratch resistance. No matter what lens material you have, scratches are always possible. However, Polycarbonate is known to be very resistant to obtaining a permanent scratch.
Trivex Introduced in 2001 by PPG, as the only lens material other than polycarbonate to pass FDA Impact Resistance Test (@ 1mm CT), the High Velocity Impact Test, and meet ANSI Z87.1 '89 standards, Trivex has been slowly increasing in both popularity and availability. While Trivex has a slightly lower refractive index (1.53 compared to 1.58), it's specific gravity, lowest of all lens products, which makes it the lightest of any lens material available today. Like polycarbonate, Trivex also has inherent UV protection. However, unlike polycarbonate, Trivex is optically superior. Further distinguishig itself, Trivex is ideal for drill mounting. The tensile strength of Trivex makes it highly resistant to cracking around drill holes, so much in fact, Younger Optics guarantees its Trivex products (Trilogy) for life, against stress fractures and drill mount cracking.
Hi-Index lenses come in several indices of refraction. The higher the number the thinner your lenses will be. They come in a 1.60,1.67, and the thinnest of them all The 1.74. The High index lenses except for the 1.74 are good for drill type lens mountings as well. The 1.74 in higher prescriptions can significantly reduce thickness, magnification and minification.
Tuesday, March 2, 2010
Fascinating facts about the invention of Bifocals by Benjamin Franklin in 1760. BIFOCAL GLASSES:
Glass lenses, for use as magnifiers or for starting fires, date to about 300 BC, but the first eyeglasses to aid or correct vision were almost certainly invented in 1280 in Florence, Italy by the Dominican friar Alessandro della Spina and / or his friend, the physicist Salvino degli Armati. Prescribed for far-sightedness, the glasses had convex lenses and were worn by Armati, who had injured his eyes while performing light refraction experiments and discovered that it was possible to enlarge the appearance of objects by looking through two pieces of convex glass.
It was in the early fourteenth century that concave lenses were used to correct near-sightedness. In fact, Pope Leo X was depicted wearing glasses, with concave lenses, in a 1517 painting by Raphael. Whereas early eyeglasses were made of polished quartz, by the sixteenth century developments in glassmaking made it possible to mass produce them from glass.
Bifocals, the combination of both concave and convex lenses for both types of vision correction, a top lens for distant viewing and a lower lens for reading, were developed around 1760 by the American statesman and inventor Benjamin Franklin.
While working as US ambassador to France in 1779, Franklin had ordered a pair of spectacles from the English optician, Sykes, who had a business on the Place du Palais-Royale. The fact that Sykes wrote to Franklin explaining that his spectacles had been delayed due to the lenses having broken three times during cutting has been taken as evidence that the ambassador was ordering something out of the ordinary. The price of 18F, which he paid for the spectacles, was certainly higher than was usual at the time.
In a letter of August 1784, written to his friend George Whatley, Franklin declared himself 'happy in the invention of double spectacles, which serving for distant objects as well as near ones, make my eyes as useful to me as ever they were'.
But we were dismayed to learn Franklin's legend may not be all it's cracked up to be:
The Dollonds were, it seems, producing spectacles of this, or a similar, type, as a bespoke service, but were not convinced that the idea had commercial potential.
Bifocal spectacles were described in Franklin's in a further letter to Whatley, dated 23 May 1785. In this he again referred to 'my double spectacles' and provided a sketch. The letter continued: 'The same convexity of glass, through which a man sees clearest and best at the distance proper for reading, is not the best for greater distances. I therefore had formerly two pair of spectacles, which I shifted occasionally, as in travelling I sometimes read, and often wanted to regard the prospects. Finding this change troublesome, and not always sufficiently ready, I had the glasses cut and half of each kind associated in the same circle. By this means, as I wear my spectacles constantly, I have only to move my eyes up or down, as I want to see distinctly far or near, the proper glasses being always ready.'
Some historians, however, have suggested that in this letter to Whatley, Franklin refers to an experiment carried out more than 20 years earlier in London. A letter from the newspaper editor, John Fenno, to his wife dated 8 March 1789, supports this theory. In it Fenno describes a meeting with Franklin in which the elderly statesman had mentioned wearing the spectacles over many years: 'He informed me that he had worn spectacles for 50 years; Split bifocal spectacles c.1780s-1790seach eye appeared to be formed of two pieces of glass divided horizontally - he informed me that he had always worn such'.
The Fenno letter also fits in with the fact that Franklin suffered from hyperopia, requiring spectacles for this condition as early as the 1730s. This would have made him likely to have benefited from bifocals by the time he arrived in London in 1757.
Before leaving the US, Franklin clearly had an interest in optical developments. He imported vision aids and advertised 'spectacles of several sorts' in the Pennsylvania Gazette. This may well have led him to experiment with bifocals at an earlier date.
Yet the only portrait of Franklin in which he is depicted wearing bifocals is one by Charles Willson Peale, dated 1785, and public awareness of the invention only came about in the early 1790s, following Franklin's death.
Regardless of the inventor the spectacle lens frame worn still are avaialable today. Here are some of those Vintage looking eyewear.