RLUs

Relative Light Units (RLUs) are a unit of luminescence intensity used in luminometric assays. RLUs are produced by detectors in luminometers or plate readers that convert emitted photons from a chemical or enzymatic reaction into electronic signals, which are reported as digital values. The RLU scale is instrument-dependent and values are not absolute across different instruments or settings.

RLUs arise in assays that generate light, such as luciferase reporter tests, chemiluminescent immunoassays, and ATP-based bioluminescence assays. In these assays, light output depends on factors including substrate concentration, enzyme activity, incubation time, temperature, and illumination duration. Because RLUs are relative measurements, they are most informative when compared within the same experimental run and under consistent instrument settings.

Data interpretation typically involves subtracting background RLUs measured in negative controls or blanks, and, when possible, normalizing to a standard or to a related measurement such as protein content or cell number. In many workflows, a standard curve or reference samples are used to convert RLUs to a more interpretable quantity, though in some contexts RLUs remain a relative readout.

Applications of RLUs include monitoring gene expression with luciferase reporters, quantifying protein or antigen levels in chemiluminescent assays, and assessing cell viability or metabolic activity in luminescent assays. Limitations include lack of cross-instrument comparability and potential signal saturation at high signal levels. Proper controls and consistent assay conditions are essential for reliable interpretation of RLUs.