Patent ID: 7091470

Claim:
A purge air flow passage structure for introducing purge air to a space at or near a surface of a light receiving part of an optical probe that detects radiation light of an object, comprising: an air flow passage disposed inside the probe for ventilating the purge air; a separation space, disposed in the air flow passage, that separates dust in the purge air from the purge air; and a filter, disposed in the air flow passage, upstream of the separation space, that collects dust in the purge air, wherein the filter comprises two or more shielding parts, wherein each of said shielding parts has a plurality of passing holes or slits formed in a width direction thereof, said shielding parts are successively arranged along a direction from an upstream side to a downstream side in the air flow passage to be separated from each other by a predetermined distance, and each of said shielding parts includes a wall portion facing upstream that is positioned such that the wall portion is struck by the purge air that passed through the passing hole or slit of the neighboring upstream shielding part.