A local purging tool for purging a portion of a surface of a wafer with purging gas is disclosed. The purging tool includes a purging chamber configured to contain purging gas within a cavity of the purging chamber, a permeable portion of a surface of the purging chamber configured to diffuse purging gas from the cavity of the chamber to a portion of a surface of a wafer, and an aperture configured to transmit illumination received from an illumination source to a measurement location of the portion of the surface of the wafer and further configured to transmit illumination reflected from the measurement location to a detector.