The invention relates to the field of analytical chemistry, in particular to multi-well assay plates having a unique flatness for use in high throughput (bio)chemical and biological assays which rely on luminescence detection. Provided is a multi-well assay plate a top layer including walls defining a plurality of adjacent sample wells for receiving assay samples, and a bottom layer defining the bottom of the wells, having a bottom surface facing away from the wells and wherein said bottom surface is provided with a grid structure forming a plurality of mock wells at the plate bottom, characterized in that the total volume of the mock wells is less than the total volume of the sample wells.