Patent ID: 7040766

Claim:
An illumination optical system for guiding a light emitted from a light source to an object to be illuminated along an optical path comprising: a first reflecting mirror reflecting a part of a light which is moving in a forward direction along the optical path to move away from the light source; and a second reflecting mirror, disposed entirely between the light source and the first reflecting mirror, reflecting a part of a light which is reflected from the first reflecting mirror and moving in a reverse direction along the optical path to approach the light source; wherein the first reflecting mirror has an opening that allows the other part of the light moving in the forward direction to pass therethrough and reach the object to be illuminated, and the second reflecting mirror has a window that allows the light emitted from the light source to pass therethrough, whereby an optical cavity is constituted by reflecting surfaces of the first and second reflecting mirrors that substantially face each other and cause light waves to resonate therebetween.