A projection optical unit images an object field in an image field. The photomask is comprised of a transparent substrate, a pattern on that transparent substrate, a plurality of main mask keys on side edges of the transparent substrate, and a plurality of auxiliary mask keys, each spaced apart from a corresponding main mask key. According to another approach the ratio of self-antigen to autoantibody is measured by exposing a sample suspected of containing self-antigen and autoantibody to a bead set associated with monoclonal antibody specific for the self-antigen and a bead set associated with the self-antigen.