Figure 1.2 A major step in advancing technology or endoscope development was placing the exposed wire filament in an enclosed bulb. This improved the quantity of light and decreased the risk of tissue burning. This picture is of a standard endoscope bulb connected to an ACMI 120° cystoscope. The bulb is 4 mm long and 2 mm wide.
George Kelling reported visualizing the abdominal contents of a dog by using a cystoscope in 1902 (Kelling 1902; Barringer 1947). H.D. Jacobaeus first described thoracoscopy in human medicine in 1910 and proposed the term laparoscopy for examining the abdominal cavity (1910). Laparoscopy was first reported in the United States in 1911 utilizing a proctoscope to visualize the gall bladder (Bernheim 1911).
What we consider to be modern endoscopy started with the paradigm changing development of “fiberoptics,” the technology that allows light transmission through bundles of very fine flexible glass fibers. Attempting an accurate historical record of the invention and development of fiberoptics is beyond the scope of this publication. In looking at the easily available material, there is more disagreement than consensus with different important time points and different names associated with significant events. Suffice it to say that fiberoptic technology does exist and has developed over a long period of time. The concept of guiding light by total internal reflection was first demonstrated in the 1800s and it was over 150 years before the combination of technologies required to make a functional flexible gastroscope occurred in the 1950s. This allowed a large bright light source to be used outside the body with transmission of adequate light while minimizing transmission of heat into the body. The term “Cold light source” was coined to describe this configuration. This also allowed an image to be transmitted from inside the patient to the outside where it could be seen by an observer.
The principal of “fiberoptics” is based on the phenomenon of refraction or light traveling at different speeds in different media so that light entering one end of a glass fiber is reflected internally until it is emitted at the opposite end of the fiber. Refraction is defined as the bending of light when passed from one medium to another medium possessing a different refractive index (ri = velocity of light in vacuum/velocity of light in substance) (Figure 1.3). When the angle of light hitting an interface of two materials with different refractive indices exceeds the critical angle of incidence, it is reflected back into the original medium (Figure 1.4).
Figure 1.3 Representation of a light beam being bent as it passes from one medium to another of lower refractive index. The darker medium has a higher refractive index (where light travels slower) than the lighter medium that has a lower refractive index (where the light travels faster). If the light wave goes through the interface of the two media at an angle, one edge of the light wave “ab” goes through the interface first and the other edge “eg” goes through the interface later. In the time that it takes the edge “fg” to reach the interface between the two materials, the other side of the wave has traveled the distance “bc.” The segment “bc” is longer than “fg” because light travels faster in the second less dense medium. This causes a bending or refraction of the light wave. The angle light that hits the interface is “α”.
Figure 1.4 As the angle of incidence of the light waves “α” increases, so does the angle of the refracted light and the light beam will be bent to varying degrees, dependent upon the angle at which it hits the medium of lower refractive index. When “α” equals “c,” the refracted light travels along the interface of the two media. This angle is known as the “critical angle of incidence.” When the angle of incidence of the light beam hitting the interface is greater than the critical angle of incidence, the light reflects back into the original medium.
Light entering the end of a glass fiber will be transmitted through the fiber if its surface is clean and it is surrounded by a substance of a lower refractive index (Figure 1.5). This is known as “total internal reflection” of light. Each fiber is clad with a substance with a lower refractive index than the core of the fiber to keep light within the individual fibers. The fibers are very small so that they are flexible, and many fibers are assembled to create a flexible fiber bundle. If a fiber is not clad properly, if there is any foreign matter on the fiber, or if the fiber touches adjacent fibers, light will leak at those points, total internal reflection does not occur, and light is lost through the sides of the fiber (Figure 1.5). Total internal reflection of light is not total and in reality, not all light that enters one end of a fiber will exit at the other end. The amount of light lost is dependent on the length of the optical path, which is determined by the length of the fiber and the number of internal reflections. Even with a properly clad fiber, a small amount of light is lost at each internal reflection and with thousands of internal reflections per meter, the amount of light lost may be significant. The smaller the fiber and the greater the length, the more light is lost. Light is also lost at the surfaces of both ends of the fiber and light falling between fibers or on the cladding.
Figure 1.5 Total internal reflection of light in a fiberoptic glass fiber occurs if it is clean and is surrounded or “cladded” with a substance of a lower refractive index. The top drawing shows proper cladding of a glass fiber to minimize light loss along the fiber producing total internal reflection of light. The bottom drawing is without cladding or with impurities in the fiber glass allowing loss of light where it hits the surface of fiber.
There are two types of fiber bundles: incoherent and coherent. Coherent fiber bundles are arranged so that the individual fibers are at the same location at both ends of the fiber allowing an image to be transmitted from one end of the bundle to the other end (Figure 1.6). Flexible endoscopes use coherent bundles to transmit images from the distal tip of the endoscope to the eyepiece. Each individual glass fiber transmits a small part of the total image and with each fiber at the same position at each end of the bundle, an image is transmitted from the tip of the endoscope to the eyepiece. Fiber bundles of this type are called image guide bundles and are composed of smaller diameter fibers with little cladding to produce an image with better resolution. The fiber pattern is visible in the transmitted image with the quality of the image dependent on the size of the