(a) Field of the Invention
The present invention relates to an optical system for endoscopes provided with a relay lens comprising an inhomogeneous lens whose refractive index reduces progressively in going from a center portion toward a periphery.
(b) Description of the Prior Art
Since an inhomogeneous lens has a refracting power in medium per se and retains an imaging function only in the configuration of a tiny rod-shaped member even though its end surface is not ground to a spherical surface, it is used as an image transmitting optical system for extremely fine endoscopes. An example of the optical system for endoscopes provided with the image transmitting optical system of the type is given in FIG. 1. In this figure, reference numeral 1 denotes an objective lens unit composed of two plano-convex lenses whose convex surfaces are arranged in such a manner as to face to each other and forming an image M.sub.1 of an object M on a plane of exit thereof. To this plane of exit is cemented an inhomogeneous rod lens 2 having a length of nP (P is the pitch of an inhomogeneous medium which brings about, for example, twice relay functions, namely, imaging functions with a length of 1P and n is the positive integer) and on an exit end face of the inhomogeneous rod lens is cemented a cover glass 3 keeping dirt off the exit end face and preventing an object image M.sub.2 formed on the exit end face from being viewed together with dirt. The optical arrangement on the rear side of the cover glass 3 comprises a relay lens unit 4 for erecting the object image M.sub.2 formed by the inhomogeneous rod lens 2 and correcting chromatic aberration of magnification and axial chromatic aberration which are generated through the inhomogeneous rod lens 2, an aperture stop 5 arranged in an pupil position of the relay lens unit 4 and allowing the reduction of flare as well as the adjustment of a depth of field and the uniformity of brightness in a visual field, a field lens 6 arranged on the rear side of the relay lens unit 4 and adjusting the position and size of an eye point ExP, a field stop 7 disposed in an imaging position of an image M.sub.3 formed through the relay lens unit 4 and the field lens 6, an eyepiece 8 arranged behind the field stop 7 and magnifying the image M.sub.3 defined by the field stop 7, and a cover glass 9 disposed behind the eyepiece 8.
In such an optical system, although individual optical components are arranged so that the object image produced through the objective lens unit 1 is formed on an entrance end face and exit end face of the inhomogeneous rod lens 2, a problem has been encountered that it is difficult for the inhomogeneous rod lens 2 to prevent the periphery of the entrance and exit end faces from being damaged into fine flaws (a so-called burr) when the end faces are polished and as such, in the above arrangement, the burr will be viewed together with the object image and will bring troubles of observation. Further, another problem has arisen that, during the assembly of the optical system, if minute dust adhering to a lens frame and the like and dirt such as cutting chips of metal parts are attached to the entrance end face of the inhomogeneous rod lens 2, they will be viewed together with the object image and will cause observation to be difficult. In addition, there also has been a problem that since the relay lens unit 4 consists of two lens groups, the manufacturing cost of the entire optical system is high.
An optical system disclosed in, for example, Japanese Utility Model Preliminary Publication No. Sho 55-147013, proposed with the intention of solving these problems makes use of an inhomogeneous rod lens 10, as shown in FIG. 2, with a length of (n/2)P somewhat different (the rod lens shown is some shorter than (n/2)P from that of FIG. 1 and is designed so that the image M.sub.2 transmitted through the rod lens is formed in front or rear of the exit end face of the inhomogeneous rod lens 10 and so that the image is directly observed through the eyepiece 8. According to this structure, although the dirt and flaws on the exit end face of the inhomogeneous rod lens 10 are not viewed, a problem has been posed that the dirt and flaws on the entrance end face are still viewed along with the object image and as such observation is difficult. Also, there has been a problem that since the structure fails to include any optical system for correcting the chromatic aberration produced through the inhomogeneous rod lens 10, the image will suffer considerable deterioration. Further, although it is considered that the aperture stop is disposed on the side of the objective lens unit 1, namely, at a point A indicated in FIG. 2 because there is no space sufficient to place the aperture stop on the rear side of the inhomogeneous rod lens 10, it has been extremely difficult for the accuracy and making of parts to provide a doughnut-shaped stop in the objective lens which is no more than 1 mm in diameter.