An inspection system, a lithographic apparatus, and a method are provided. The inspection system includes an illumination system, an optical system, a shutter system, an objective system and a detector. The illumination system is configured to generate an illumination beam. The optical system is configured to split the illumination beam into a first sub-beam and a second sub-beam. The shutter system is configured to independently control a transmittance of the first sub-beam and the second subbeam. The objective system is configured to receive the first sub-beam and the second beam from the optical system and direct the first sub-beam and the second sub-beam towards a substrate having a target structure. The detector is configured to receive an image or a diffracted image of the target structure.