We are used to seeing clearly through glass because we are often surrounded by glass that is very thin. Whether it is the rear window of our car, a wine glass, an aquarium or a window in the office: we can see through it effortlessly. We come across thick glass somewhat less frequently. At most, we might be familiar with the glass blocks in the hallways of old houses from the 1930s or the Paris metro system. The world behind this glass looks wavy and blurry.
The reason that glass can be clear or translucent is a matter of form and formula. Since ordinary window glass is usually only a few millimeters thick, you can see right through it. However, glass that is 10 to 20 centimeters (3 to 7 inches) thick starts to exhibit a greenish hue. If you go even thicker, such as a 2-meter-wide (7-foot-wide) block of glass, it becomes almost impossible to see through. The reason for this is that the soda-lime glass used to make most windows isn’t very high quality, and so the light quickly degrades while traveling through it. However, there are many applications where high-quality light transmission is very important, for instance in medical applications.
Optical glass fibers make it possible to transmit light precisely thanks to their purity and structure. The greenish discoloration of glass usually comes from impurities introduced during the manufacturing process. Glass isproduced by melting sand at temperatures of over 1000° Celsius. In the process, this sand can encounter various other substances. Traces of iron, for example, turn the glass greenish. Very clear glass requires very pure sand. This is easier said than done since even contact with an iron shovel could contaminate the sand enough to impair the final quality of the glass.
SCHOTT’s optical glass fibers for light transmission have an extremely pure core glass in which impurities of all kinds are avoided. This is how SCHOTT glass fibers get their name: “PURAVIS®”.
Another reason it is possible for light to travel through glass fibers effortlessly is that they are made of two materials with different refractive indices. The outside material is known as the cladding and the inside as the fiber’s core. When light hits the boundary between the core and cladding at a certain angle, it is reflected in a way that keeps it inside the fiber. Thus, the light is transmitted through the core of the optical fibers to the end of the fiber bundle where it shines in high quality.
In endoscopy, optical fibers are used to make the inside of the human body visible. This is done by passing light into the body. Thanks to this technology, the tissue inside the body can be examined in minimally invasive surgical procedures, requiring only tiny incisions for the patient. In addition, image guides made of glass optical fibers capture the image and send it back out of the body. In applications such as robot-assisted surgery, it is even possible to send light very precisely through glass fibers in lengths of up to seven meters (20 feet).
For early cancer detection, for example, it is crucial that high-quality bright white light is transported into the body, allowing for high-resolution images to be transmitted from the body. This is important because every detail, including the natural color of the tissue, is relevant for a good diagnosis. It would be unacceptable for the light and images travelling through the glass fibers to be distorted, giving us the same greenish and milky hue which we encounter in architectural glass blocks: A color shift to green could lead to an incorrect diagnosis.
In industrial applications, glass optical fibers are often used in the form of image guides. The fibers’ outstanding optical properties allow us to see, with their help, into challenging environments that cameras and sensors cannot withstand. Glass fibers remain unaffected by heat, electricity, magnetic fields and vacuums.
Glass is versatile: Although we are fascinated by the fact that pure core glass can be used to illuminate the inside of the human body precisely, we are grateful that frosted glass does not allow anyone to peek inside our bathroom.