Endoscopes
Endoscopes, which may be rigid or flexible, are devices which facilitate the examination of body cavities inaccessible to the human eye. They require a means of illumination and an image transmission system (Figure 16-1). Modern versions incorporate almost universally a bundle of glass fibers to conduct the illumination from an external light source to the tip of the instrument. Most rigid instruments utilize a train of lenses as an image transmission system but the fully flexible fiberoptic endoscope uses a specialized image-transmission glass-fiber bundle.
The illumination fiber bundle of a modern endoscope consist of 10-100,000 individual glass fiver, each of diameter 30-70 μ m.
Each fiber comprises a glass core surrounded by a thin outer coating of a glass of lower refractive index, which insulates the core from the effects of surface contamination, defects, and contact with adjacent fibers.
The fiber transmits light by multiple total internal reflection at the interface between the core and the cladding (Figure 16-2). Total internal reflection occurs, when the angle of incidence exceeds the ʻ critical angle ʼ. This results in a limited cone of acceptance with a semiangle α given by sin α = (n²-n1²)½ , where n and n1 are the refractive indices of the core and cladding, respectively. Although each reflection is total, transmission losses occur due to absorption by the glass and amount to approximately 30% per meter.
Glass fibers are extremely flexible by virtue of their fine diameters. The maximum bending which can be tolerated by the fiber without fracture is given by Rf=Ed/2σ , where Rf is the minimum radius of curvature, E is the modulus of elasticity, σ the ultimate failure stress, and d is the diameter of the fiber. Since glass is a brittle material, σ is critically dependent on the presence of small cracks on the surface. For bulk glass , E/2σ is typically 1000-1500 but the relative absence of surface flaws in drawn fiber gives values in the region of 10-30, so that a single fiber of 30μm diameter may be bent to a minimum bend radius of <1 mm.
Flexible Endoscopes
The modern fully flexible endoscope incorporates two types of fiber bundle. Illumination is provided from an external light source via a boundle of randomly assembled fiber (incoherent bundle), whereas a second type of fiber bundle (coherent bundle) is used to transmit the image formed by the objective lens to the eyepiece (figure 16-3). In a coherent bundle the individual fibers are carefully ordered, each having the same relative position on the front face of the bundle as on the back face. It is this ordering of the fibers which allows the image to be transmitted, since individual fibers only carry an average of the incident light and do not transmit image information. The outer cladding of the fibers and the spaces in between them do not transmit light, so the image is composed of spots of light, each the diameter of the inner core of the fibers and the accuracy with which they are ordered.
The calculation of the resolution of a fiber-optic system is complex; the size of the smallest resolvable object is dependent upon the working distance. Modern endoscopes when working at their shortest range will usually resolve finer detail than that resolvable by that naket eye.
Rigid Endoscope
There are three different types of relay systems used to transmit the image
from the objective to the ocular in modern rigid endoscopes.
a) Thin-Lens Relay System. In this system, doublet lenses corrected for chromatic aberrations are employed. The diameter of the individual lenses is greater than the lens thickness and the lens separations are usually 10-20 times that of the lens thickness. The image formed by each relay stage become the object for the next adjacent stage and so on for as many stages as are required by the length of the endoscope (figure 16-4a).
b) Rod-Lens Relay System. Initially developed by Hopkins, the lenses work in much the same way as a conventional thin-lens relay, but the thickness of each relay lens is typically ten times that of its diameter and the air gaps between the individual lenses are correspondingly smaller. This construction gives certain optical advantages, particularly in giving a brighter overall image (figure 16-4b)
c) Selfoc Relay System. A solid glass rod is used which has been treated so that the refractive index at the center of the rod is greater than that at the edge. This structure continuously refracts the light as shown in figure 16-4c. Generally, Selfoc system have a poor image quality in comparison to other relay systems, but they have advantages for small diameter systems (<2 mm diameter).
