Patent ID: 7667211

Claim:
An illumination light detecting optical system which is used for an optical apparatus comprising a light emitting element which emits excitation light; an insertion part of a long-narrow-shape; a fluorescence luminescence component arranged near a top of the insertion part; an excitation light guiding means which leads the excitation light emitted from the light emitting element to the fluorescence luminescence component; and a light detecting means which detects return light which is a part of the illumination light, wherein the illumination light detecting optical system comprises a first pillar-shaped transparent component for irradiating illumination light, which is arranged coaxially at a forward position of the fluorescence luminescence component; an illumination light-extracting means which extracts the illumination light out of the illumination light which is emanated from the fluorescence luminescence component to a forward direction, and enters into the inside of the first pillar-shaped transparent component, which enters at an angle smaller than a total reflection angle, into the side surface of the first pillar-shaped transparent component from the first pillar-shaped transparent component; and a deflecting means by which the extracted illumination light is directed toward the light detecting means.