An attenuation adjustment device is disclosed that includes a plurality of members configured to cast penumbras in a radiation beam illuminating a patterning device in a lithography apparatus. Furthermore, an attenuation control device is provided to adjust the members in such a manner as to control the attenuation of the patterned radiation beam projected onto a target portion of a substrate across the cross-section of the patterned radiation beam. The attenuation control device includes a detector configured to provide an output indicative of the position of each member in dependence on detection of a beam of detecting radiation reaching the detector after attenuation by the member.