1. Field of the Invention
This invention generally relates to optical devices and a method for making such devices. More particularly this invention is related to devices for viewing the interior of a passageway in a selective region.
2. Description of Related Art
There are a variety of applications which require an individual to view the interior of a remote chamber. In an industrial environment, for example, it is desirable to view portions of the interior of a machine, such as a jet engine, that is otherwise inaccessible. In medical applications it is desirable to view a passageway in the body. Viewing scopes that enable one to view such remote passages are well known in the art. Such devices used for mechanical applications are generally called borescopes. For medical applications the viewing scopes take many forms and are known by different names. Generically the name "endoscope" means any slender, tubular optical instrument used as a viewing system for examining an inner part of the body. In practice such viewing scopes have a variety of names, such as endoscopes, laparoscopes and bronchoscopes. In the following discussion the phrase "viewing scope" is meant to include generically both (1) borescopes and (2) endoscopes in any form including laparoscopes, bronchoscopes, etc.
Viewing scopes generally have an eyepiece at a proximal end, an objective lens at a distal end and an image guide interposed between the eyepiece and the objective lens. The image guide uses two general forms of optics, namely: rigid optics and fiber optics. Image guides using rigid optics include relay or rod lenses to transfer an image from a distal end of the viewing scope to its proximal end. The following patents disclose examples of image guides using such rigid optics:
U.S. Pat. No. 4,036,218 (1977) Yamashita et al PA0 U.S. Pat. No. 4,993,817 (1991) Hoogland PA0 U.S. Pat. No. 4,776,668 (1988) Fujimoto PA0 U.S. Pat. No. 4,938,205 (1990) Nudelman PA0 U.S. Pat. No. 4,945,894 (1990) Kawashima PA0 U.S. Pat. No. 4,961,738 (1990) Mackin
Image guides using fiber optics generally interpose a bundle of optical fibers between the proximal and distal ends of the viewing scope. The following patents disclose image guides that incorporate such fiber optics:
The Fujimoto patent discloses a viewing scope with a coherent fiber optic bundle as an image guide. Each fiber comprises a pure quartz silica strand coated with a reflective layer. This patent discloses structure eliminating differential expansion between the fiber optic bundle and other portions of the viewing scope during flexure.
The Nudelman patent discloses an endoscope that includes a fiber optic bundle between distal and proximal ends. The fiber optic bundle is flexible between the ends.
The Kawashima patent discloses an endoscope in which image guide fibers transmit an image formed at the objective to a proximal end of the endoscope.
The Mackin patent discloses an angioplasty catheter that includes a fiber optic bundle for conveying an image between the distal and proximal ends.
Conventionally fiber optic bundles used in such image guides comprise a preferred three-glass fiber or two-glass fiber. A three-glass fiber has a glass core that acts as a light transmitter, a concentric reflective layer and a fusible, acid soluble outer layer. In conventional processing, a bundle of such fibers are assembled together and heated to fuse just the outer layer. Then the ends are isolated and the entire structure is immersed in an acid bath as a leaching solution to remove portions of the fusible glass between the ends so the bundle will be flexible along its length. It is difficult to control this leaching process. If the leaching continues for too long a time, the solution acid can etch through the outer glass layer and the reflective layer thereby detracting from the image by lowering its contrast. This leaching step effectively controls the minimum diameter of the individual fibers and the number of individual fibers that can be formed into a bundle of a given size. As the diameter of the fibers becomes smaller, the effort required to control the leaching process increases because the fusible layer becomes thinner. Consequently costs increase because the manufacturing is more difficult and because production yields decrease.
Optical fibers used in two-glass fiber optic bundles have a light transmitting core and a reflective outer cladding. Multiple drawing steps can reduce the light transmitting core to under 10 microns. During each drawing step a bundle of fiber passes through a die that reduces both its core and cladding and that causes the bundles and fibers within each bundle to adhere. When such fibers are drawn below 10 microns, however, the cladding becomes very thin and transmissive. Cross talk can occur and limit the contrast resolution of the viewed image. In some situations this requires the insertion of black glass or other barriers to prevent light from one fiber from transferring to another fiber through the thin cladding.
The basic criteria for selecting rigid optics or fiber optics for an image guide include spatial resolution as a primary criteria and contrast resolution, cost and ruggedness. Rigid optics provide the best spatial and contrast resolution. However, they are fragile and have the highest manufacturing costs. An image guide must be designed separately for each combination of a viewing scope length and diameter. Fiber optics, on the other hand, provide a rugged image guide and require less design effort and have lower costs. However, image guides using fiber optic bundles have limited spatial resolution. In viewing scopes of the type contemplated by this invention, the spatial resolution is limited to 5,000 to 30,000 pixels in an image. Fiber optics also provide somewhat lower contrast resolution than rigid optics and require an inventory of one bundle for each combination of viewing scope diameter and length.