A sample analyzer has a support for an assay sample vessel, a detector, and a shutter assembly. The assay sample vessel contains an assay sample within an assay sample reservoir. The detector has an optical axis aligned with the assay sample reservoir, so as to detect luminescence from the assay sample. The shutter assembly includes an illuminator and is positioned between the detector and the support, intersecting the optical axis, such that the illuminator causes luminescence of the assay sample. Thus both illumination and detection occur on the same side of the assay sample vessel.