When we talk of microplate reading and luminescence, Rlu stands for the Relative Light Units. The amount of bioluminescence is measured by the luminometer and the result is given in units known as the relative light units. The ATP bioluminescence reaction is linear, the more ATP present means the more light will be present. The measure of ATP can therefore be determined indirectly by determining the measure of Rlu in living organisms and even organic materials. RLU measurements vary from system to system. RLUs are not standardized. Therefore, they should not be compared across instruments.