Illuminating system for endoscope or borescope
BACKGROUND OF THE INVENTION 
     This invention relates to endoscopes or borescopes and is more particularly directed to an illumination system comprising a fiber optic light guide that carries light to the distal end of the endoscope or borescope insertion tube and a distributing lens system capable of providing wide angle illumination of a target. 
     In a typical endoscope or borescope, a lens system, comprised of one or more conventional spherical lenses, is disposed in the head of the endoscope or borescope for spreading the light carried by the fiber optic light guide. A spherical lens has a generally bell-shaped light distribution. Hence, when the illumination angle becomes wider to correspond with a wide viewing angle, the difference in illumination between the center and the edges becomes greater. One reason for this edge illumination dropoff is the fourth-power-of-cosine law that applies to spherical lenses. 
     In addition to this infirmity, positive spherical lenses tend to focus light, and can cause a dot or mesh pattern of illumination to appear on the target, from the light emanating from the optical fibers, when an even blanket of light would be desired instead. 
     A number of previous attempts have been made to resolve the problem of wide angle distribution of light. In U.S. Pat. No. 4,415,240, a cylindrical reflecting member is placed between the distal end of the fiber optic light guide and the lens distribution system. However, the lenses of that system are positive spherical lenses. U.S. Pat. No. 4,580,522 describes a spherical lens illumination system in which a bar-shaped member is embedded at the center of the exit end of the fiber optic light guide, which reduces the light intensity at the center of the target. However, no attempt is made in either case to adjust the light distribution properties of the lenses themselves, so these previously proposed systems have not been able to provide the desired flat illumination distribution for extremely wide illumination angles. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an illumination system for an endoscope or borescope which avoids the problems of spherical lens light distribution systems of the prior art. 
     It is a more particular object of this invention to provide a light distribution system which achieves wide angle illumination with even light distribution. 
     It is yet another object of this invention to provide an illumination system which employs a simple yet effective lens system, which has the advantages of low cost and ease of manufacture, and which can be made durable and resistant to abrasion. 
     In accordance with an aspect of this invention, the above objects are achieved with the optical illuminating lens of this invention which is employed in an endoscope or borescope in which a round fiber optic bundle carries light to the distal end of an endoscope or borescope for illumination of a target. This lens has a flat front surface and a compound rear optical surface. The rear surface is formed of a central circular flat surface, which is of smaller diameter than the diameter of the fiber optic bundle, and which define a central disc portion of the lens. Extending back from this central disc portion is a frustoconic or similar flared surface that surrounds the central circular surface portion. The flared surface extends back and outward from the circular surface portion beyond the diameter of the fiber optic bundle, and defines an outer portion of the lens. The rear surface fits against the fiber optic bundle with the lens central portion centered on the fiber optic bundle. This lens has the property that the outer portion distributes light from the bundle as a ring of light, while the central portion distributes the light as a spot to fill in the center of the ring. This creates a generally flat overall distribution. 
     In alternative embodiments, the front surface and the central portion of the back surface can be curved somewhat to effect a desired slight alteration of this distribution pattern, for example, to correct for parallax. Also, rather than being frustoconic, the flared portion may be somewhat toric. 
     This lens provides a substantially flat distribution of light over an angle of up to 120 degrees. Because of its simple and non-spherical shape, the lens can be of pressed glass rather than ground from a blank, thereby reducing manufacturing costs significantly. 
     The above and many other objects, features and advantages of this invention will be more fully understood from the ensuing detailed description of a preferred embodiment, which should be considered in connection with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic sectional view of a fiber optic bundle and distributing lens, which constitute the basic elements of the endoscope or borescope illuminating system according to an embodiment of this invention. 
     FIG. 2 is a chart of light intensity distribution of the embodiment of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawing, FIG. 1 shows an optical illumination system 10 according to a preferred embodiment of this invention, here formed of a fiber optic bundle light guide 12 which has a polished flat front surface 14, and a pressed glass distribution lens 16 which has a flat front surface 18 and a compound rear surface 20. The rear surface 20 consists of a central flat disc portion 22, which here has a diameter about forty percent of the diameter of the fiber optic bundle front face 14, and a frustoconic surface 24 which extends from the edge of the disc portion 22 back beyond the edge of the front face 14 of the fiber optic bundle light guide 12. The frustoconic surface 24 defines an outer region of the lens 16. The fiber optic bundle light guide 12 fits snugly against the surface 24 and leaves an air-filled space 26 between the front surface 14 of the light guide 12 and the rear surface 20 of the lens 16. Although in this embodiment the space 26 is filled with air, it could instead contain another medium which has a refractive index less than that of the lens 16 or of the fiber optic light guide 12. 
     As shown schematically in FIG. 1, the optical illuminating system 10 provides a target 28 or other object to be illuminated with a generally flat distribution of illumination 30 over an angle of about sixty degrees or more from the center line or axis. This is achieved as follows: 
     The outer or frustoconic portion of the lens 16 has a generally ring-shaped distribution pattern 32, as illustrated in FIG. 2, i.e., a generally bell-shaped annular distribution. The flat portion defined by the disc 22 provided a spot distribution 34, which fills in the center of the annular ring. The intensities of these two distributions 32 and 34 add to achieve the overall flat distribution 30. As mentioned previously, the intention of this embodiment was to render the system as simple as possible. This achieves simplicity of manufacture and at the same time increases the flatness of light distribution over an extremely wide angle. However, by varying the shape of the lens slightly, the light distribution can be skewed somewhat to compensate for parallax. 
     This construction also has the advantage that the conic depression defined by the surface 24 on the rear of the lens 16 automatically centers the axis of the fiber optic bundle light guide 12 onto the axis of the lens 16. 
     With this construction, an illumination angle of one hundred twenty degrees of even light distribution can be achieved for optical wide angle viewing in endoscopes or borescopes. The spreading of illumination in this fashion is repeatable. The wide angle distribution is effected at low cost and with a great ease of manufacture. Also, the pressed-glass lens 16 is highly durable, and is easily treated for scratch resistance. 
     While the invention has been described with respect to a single preferred embodiment, it should be recognized that the invention is not limited to that embodiment and that many modifications and variations would present themselves to those of skill in the art without departing from the scope and spirit of this invention as defined in the appended claims.