Patent ID: 7262921

Claim:
An illuminating optical system for an endoscope, comprising: a light guide; and a gradient index lens having a refractive index distribution n(r) approximately expressed by Condition Equation (1), wherein the gradient index lens illuminates an observed region by light guided by the light guide, wherein the gradient index lens satisfies Condition Equations (2) and (3): n ( r )= N 0 (1−( A 2 r 2 /2)) (1) ( AR ) 2 ≦0.6 (2) 0.5 k+ 0.35 ≦AL/π≦ 0.5 k+ 0.65 (3) wherein N 0 designates a refractive index on an optical axis of the gradient index lens; r designates a distance from the optical axis along a radial direction of the gradient index lens; R designates an effective radius of the gradient index lens; L designates a length of the gradient index lens in a direction of the optical axis; A designates a constant coefficient; and k designates a nonnegative integer.