JP2733217B2 - Illumination optical system for endoscope - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an illumination optical system for an endoscope. 2. Description of the Related Art In a conventional endoscope illumination optical system of this kind, a cover lens is generally arranged in front of an exit end face of a light guide, that is, on an object side to diffuse light emitted from the light guide to a wide range. However, light emitted at a wide angle from the light-emitting end surface of the light guide diffusely reflects on the inner surface of the outer peripheral portion of the cover lens and disappears, or is totally reflected by the inner surface of the outer peripheral portion toward the center of the visual field and the light guide. This causes a problem that the light distribution to the periphery of the visual field becomes insufficient and uniform illumination over a wide range cannot be obtained. Therefore, in order to solve such a problem, for example, it is considered that the surface on the light guide side of the cover lens (concave lens) is a composite surface including a central portion of a spherical shape and a peripheral portion of a conical shape. Was. However, these prior arts merely change the direction of the principal ray of the light emitted at a wide angle from the exit end face of the light guide, and do not change the direction of the lower ray. The total internal reflection on the inner surface of the outer periphery of the cover lens has not been solved yet, and therefore, it has been insufficient to obtain uniform illumination over a wide range. The present invention has been made to solve such a problem, and an object of the present invention is not to provide an illumination optical system for an endoscope which can sufficiently obtain uniform illumination over a wide range of an observation object. It is assumed that. In order to achieve the above object, the illumination optical system for an endoscope according to the present invention has a cover lens disposed on the object side of the illumination light exit portion, and the cover lens is entirely negative. The refractive power of the cover lens, the object-side surface of the cover lens is formed of a convex aspheric surface having a larger curvature at the periphery than at the center, and the surface at the light guide side is smaller than the curvature at the center. Are characterized by being formed from a concave aspheric surface having a smaller curvature at the peripheral portion. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to a first embodiment shown in FIG. 1. Reference numeral 1 denotes a light guide, and 2 denotes a front portion of an emission end face 1a of the light guide 1. It is a cover lens which is a concave lens arranged on the object side).
The object-side surface 2a of the cover lens 2 is formed of a convex aspheric surface including an aspherical (large curvature) peripheral portion 2a 'and a planar (0 curvature) central portion 2a ". The surface 2b of the cover lens 2 on the side of the light guide 1 is formed of a concave aspheric surface having a peripheral portion having a smaller curvature than a central portion. First, the light emitted from at a wide angle is weakened at the light guide 1 side surface 2b of the cover lens 2 because the peripheral portion has a smaller curvature than the center, so that the degree of light diffusion is reduced. The lower light beam B blocked by the edge 3 of the cover lens 2 is also emitted as illumination light from the object side surface.
Since the lower light beam B is not blocked by the edge portion 3, the disadvantage that the lens diameter (outer diameter) of the cover lens 2 must be increased in order to force the lower light beam B to be emitted is solved, and as a result, the cover is The outer diameter of the lens 2 can be reduced. The principal ray A and the lower ray B passing through the light guide 1 side surface 2b of the cover lens 2 are:
Both can pass through the peripheral portion 2a 'of the object side surface 2a to illuminate the peripheral portion of the visual field. As described above, by forming the peripheral portion 2a 'of the object side surface 2a of the cover lens 2 into a convex aspherical shape having a curvature larger than that of the central portion 2a ", the tangent plane of the peripheral portion is horizontal from the direction perpendicular to the optical axis. In this direction, the incident angle of the principal ray A and the lower ray B with respect to the tangent plane can be raised, and the total reflection on the inner surface of the outer peripheral portion and the total reflection caused by exceeding the critical angle of total reflection in the past. According to the structure of the first embodiment, it is possible to prevent a decrease in the amount of peripheral light due to irregular reflection derived from the surface.
As shown in FIG. 2, a substantially trapezoidal light distribution characteristic is obtained without increasing the outer diameter of the cover lens, and uniform illumination over a wide range can be sufficiently obtained. FIG. 3 shows a second embodiment, in which the shapes of the object side surface 2a and the light guide 1 side surface 2b of the cover lens 2 disposed in front of the emission end face 1a of the light guide 1 are the same as in the first embodiment. is there. Further, the cover lens 2 in the present embodiment is formed of an inhomogeneous medium lens whose refractive index is higher at the peripheral portion than at the central portion. Therefore, in addition to the operation and effect of the first embodiment, the light emitted from the central portion of the emission end surface 1a of the light guide 1 can be redirected toward the peripheral portion as the light passes through the cover lens 2. In addition, it is possible to brightly illuminate the periphery of the visual field. The cover lens 2 is manufactured by a glass mold or a plastic mold, and as a result, the manufacture is easy. As described above, the endoscope illumination optical system according to the present invention has a practically important advantage that uniform illumination over a wide range can be sufficiently obtained.

[Brief description of the drawings] FIG. 1 is a sectional view of a first embodiment of the present invention. FIG. 2 is a graph showing light distribution characteristics of the first embodiment. FIG. 3 is a sectional view of a second embodiment of the present invention.

Claims (1)

(57) [Claims] In an illumination optical system for an endoscope in which a cover lens is arranged on the object side from an illumination light emitting unit, the cover lens has a negative refractive power as a whole, and the object-side surface of the cover lens has a central portion. Is formed from a convex aspheric surface having a larger curvature at the peripheral portion than the curvature of
An illumination optical system for an endoscope, wherein a surface on a light guide side is formed of a concave aspheric surface having a smaller curvature at a peripheral portion than at a central portion. 2. 2. The illumination optical system for an endoscope according to claim 1, wherein the medium of the cover lens is formed of a heterogeneous medium lens having a higher refractive index at a peripheral portion than at a central portion. 3. The illumination optical system for an endoscope according to claim 1 or 2, wherein the cover lens is formed by a glass mold or a plastic mold.