Illumination optical system for endoscope

1. An illumination optical system for an endoscope provided in an elastic insertion tube of an endoscope, comprising:

two light guides disposed in the insertion tube and arranged in a first direction to sandwich a center of the insertion tube therebetween;

an observation window disposed on a tip end face of a tip portion of the insertion tube;

a cap having a circular outer shape and comprising a transparent material for letting the illumination light to pass therethrough, the cap being provided on a front of the end faces of the two light guides,

two concave lens parts having negative powers, the two concave lens parts being disposed to sandwich the observation window at positions facing the end faces of the two light guides on the tip end face of the insertion tube,

wherein:

on the tip end face of the tip portion of the insertion tube, the end face of each of the two light guides has a smaller width in the first direction than a width thereof in a second direction perpendicular to the first direction;

each of the two concave lens parts comprise a recessed part on a surface of the cap facing the end faces of the two light guides;

each of the two concave lens parts has a larger negative power in the first direction than a negative power thereof in the second direction; and

optical axes of the two concave lens parts are decentered from optical axes of the two light guides, respectively;

wherein when w (unit: mm) represents a width of each of the end faces of the two light guides in the first direction, r (unit: mm) represents a radius of curvature of each of the two concave lens parts in the first direction, s (unit: mm) represents a decentering amount of the optical axis of each of the two concave lens parts toward a center of the insertion tube with respect to the optical axis of a corresponding one of the two light guides, and n d represents a refractive index at d-line of material of the cap, the illumination optical system satisfies a relationship:

2×10 −3 <( n d ×w×s 2 )/ r< 13×10 −3

of illumination light which has propagated through each of the two concave lens parts after being emitted from each of the two light guides, an optical path of light which has propagated through a center of each of the two light guides and has been emitted from a center of each end face of each of the two light guides is inclined outward in the first direction with respect to an axis direction of the insertion tube.

2. The illumination optical system for an endo scope according to claim 1 , wherein when d (unit: mm) represents a distance, in the first direction passing through a center of the tip end face of the insertion tube, between an outer edge of the cap, the illumination optical system further satisfies a relationship:

15

×

10

-

6

<

(

n

d

×

w

×

d

×

s

3

)

/

r

<

200

×

10

-

6

.

3. The illumination optical system for an endo scope according to claim 1 ,

wherein the cap is configured such that an outer diameter of the cap becomes smaller toward a tip of the insertion tube.