An ELISA reader is a laboratory instrument that measures the absorbance, fluorescence, or luminescence of samples in a microplate to quantify the amount of a substance like an antigen or antibody. It is a key component in performing an Enzyme-Linked Immunosorbent Assay (ELISA), which is a common diagnostic test for diseases, in vitro diagnostics, and research applications like drug discovery, cancer detection, and food science. The reader works by passing light through a sample and measuring how much light is absorbed or reflected, with a higher intensity of color indicating a higher concentration of the target molecule.