Absorbance is used for assays such as ELISA assays, protein and nucleic acid quantification or enzyme activity assays (i.e. in the MTT assay for cell viability). A light source illuminates the sample using a specific wavelength (selected by an optical filter, or a monochromator), and a light detector located on the other side of the well measures how much of the initial (100 %) light is transmitted through the sample: the amount of transmitted light will typically be related to the concentration of the molecule of interest.