All About Glass

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Getting the Whole Picture: Bundled Glass Fibers

All About Glass

Glass rods could transmit light, but could they transmit an image? A professor at a French agricultural college found himself faced with that question in the 1890s while he was tinkering with an early version of television.

Henri C. Saint-Rene needed to find a way to transmit an image onto his display. A bundle of glass rods might work. Each glass rod would act as a pixel—a picture element—and together they would transmit an entire image.

Other inventors eventually came up with the same idea, but none tested it. Finally, in 1930, Heinrich Lamm, a German medical student, proved that it worked. While trying to design a flexible gastroscope for peering inside the body, he put a light with a V-shaped filament at one end of a bundle of aligned thin glass fibers. When he looked at the other end, he saw a faint “V.” Success! Lamm had transmitted the first fiber-optic image.

On the outside looking in

coated glass fibers

Imaging trying to swallow a rigid tube filled with lenses. It’s not a pleasant thought—but doctors sometimes needed to peer down a patient’s throat into the stomach. A flexible gastroscope, made from a bundle of glass fibers, would be easier to use, but the development of one that is practical had eluded scientists since the 1930s.

The more than 20,000 fibers had to be perfectly aligned; otherwise the image was scrambled. If the fibers became scratched or dirty, or if they touched one another, light escaped and the image was faint. Was coating the fibers the answer? The coatings that had been tried didn’t work very well.

Lawrence Curtiss, a college student in Michigan, suggested coating the glass fibers with a different type of glass. His professor was sure the idea wouldn’t work. But Curtiss proved him wrong. In 1957, he made the first flexible gastroscope, from fibers that were both aligned and coated with glass.

Around the bend

total internal reflection

Daniel Colladon, a Swiss physicist, was an unlikely showman. But for years, European audiences were delighted with the illuminated water fountains that were based on his discovery. In 1841, Colladon had been experimenting with streams of water when he noticed that the light he used to highlight the streams followed the water’s path, even around bends.

An Irish civil engineer, John Tyndall, described the phenomenon at the Royal Institution in London in 1854. The water traps most of the light, guiding it along by total internal reflection. The light can’t escape because it’s reflected whenever it hits the boundary between the water and the surrounding air at a shallow angle.

When the effect was demonstrated using a bent glass rod instead of a stream of water, the %%ground%% work was laid for guiding light to and from other inaccessible places.

Total Internal Reflection: The smooth surface where the stream of water and the air meet acts like a mirror. When the light strikes it at a shallow angle, it reflects light better than any man-made mirror. Total internal reflection traps the light in the stream—and will trap it the same way in a glass fiber.

Published on October 20, 2011