Luminescence is measured by detecting the photons (light) a sample emits, converting them into an electrical signal, and expressing the result in relative light units (RLU). Unlike fluorescence measurements, many types of luminescence don’t require an excitation light source, which simplifies the optical setup but demands a highly sensitive detector because the signals can be extremely faint. The core instrument for the job is a luminometer, though microplate readers with a luminescence mode serve the same function for high-throughput work.
What Luminescence Actually Is
Luminescence is a broad term for any process where a material emits light without being heated to incandescence. The trigger for that light emission varies, and the trigger determines what kind of luminescence you’re dealing with. Photoluminescence (including fluorescence and phosphorescence) happens when a substance absorbs light at one wavelength and re-emits it at a longer wavelength. Chemiluminescence converts chemical reaction energy directly into light. Bioluminescence is a subset of that, driven by biological enzymes like luciferase. Electroluminescence is excited by an electric field, and triboluminescence occurs when a material is mechanically stressed, scratched, or crushed.
The measurement approach shifts depending on the type. Fluorescence requires an excitation light source and emission filters to separate the incoming light from the emitted light. Pure chemiluminescence and bioluminescence produce their own photons with no excitation source needed, so the detector simply sits in the dark and counts what arrives. This distinction matters because it affects which instrument you need and how you set it up.
How a Luminometer Detects Light
The heart of most luminometers is a photomultiplier tube (PMT). A PMT is a vacuum tube containing a light-sensitive cathode (typically made of a potassium-caesium alloy), a chain of electrodes called dynodes, and an anode held at roughly 1,000 volts. When a photon strikes the cathode, it knocks out an electron. That electron accelerates toward the first dynode, where two or three secondary electrons are released on impact. Those electrons hit the next dynode, releasing still more, and so on down the chain. By the time the cascade reaches the anode, a single incoming photon has generated several million electrons, producing a measurable electrical pulse.
PMTs can operate in two modes. In photon-counting mode, each pulse corresponds to one detected photon, giving you a count per unit time. In analog mode, the tube outputs a continuous current proportional to the total light hitting the cathode. Photon counting is more sensitive for very dim signals; analog mode handles brighter samples without saturating.
Portable or miniaturized instruments sometimes replace the PMT with a silicon avalanche photodiode. These solid-state detectors don’t need the high-voltage power supply a PMT requires, making the device smaller and cheaper, though typically less sensitive for the faintest signals.
Understanding Relative Light Units
Most luminescence measurements are reported in relative light units (RLU). RLUs have no fixed physical meaning. The number you get depends on the PMT’s efficiency, the voltage applied, the operating mode, and the instrument’s internal processing. One luminometer might return values between 1 and 10 million for a given sample, while a different model reads the same sample between 0.0001 and 100.
This means RLU values are only comparable within the same instrument under the same settings. You cannot take an RLU reading from one lab’s luminometer and directly compare it to a reading from a different brand. For this reason, luminescence assays almost always include internal controls: a blank (no analyte), a negative control, and a positive control or standard curve run on the same plate, in the same session, on the same machine.
Choosing the Right Plate and Reader
For microplate-based assays, plate color has a significant impact on data quality. White microplates, often colored with titanium dioxide, are the standard choice for luminescence. The white walls reflect emitted light back toward the detector, boosting signal strength. They also reduce well-to-well crosstalk, where light from a bright sample bleeds into an adjacent well and inflates its reading.
Black plates are better suited for fluorescence work because the dark walls absorb stray light and cut background noise. Using a black plate for a luminescence assay would absorb much of the emitted signal, lowering your signal-to-background ratio. If you’re running a dual-readout assay (luminescence plus fluorescence), white-walled plates with clear bottoms offer a compromise, though you’ll want to validate that crosstalk is acceptable for your application. When highly active and inactive samples sit next to each other, crosstalk from a poorly chosen plate color can produce false readings in the inactive wells.
Flash Versus Glow Assay Timing
The timing of your measurement after adding substrate is one of the most common sources of error in luminescence assays, and it depends on whether you’re running a flash-type or glow-type reaction.
Flash-type assays produce an intense but short-lived burst of light. The signal peaks within seconds of adding the reagent and drops off rapidly. This makes them more sensitive overall, but you need to read the signal immediately. That means using an automated injector built into the plate reader so the reagent is dispensed and the measurement taken in one continuous step. Pipetting the reagent by hand introduces a delay between wells, causing significant sample-to-sample variation because each well is measured at a different point on its decay curve. If you’re using a flash assay without an injector, your data will be inconsistent.
Glow-type assays generate a stable signal that persists for roughly an hour, decaying slowly enough that you can add reagent to an entire plate, then read it without worrying about well-to-well timing differences. Glow assays are less sensitive than flash assays, but they’re far more forgiving and practical for most laboratories that lack built-in dispensers.
Temperature and Signal Quenching
Temperature is a major factor in luminescence intensity. As a general principle, higher temperatures quench luminescence, meaning the signal gets weaker. Research on luminescent nanocrystals shows a roughly 10-fold decrease in emission intensity when warming from about minus 73°C to room temperature (27°C), with a corresponding 5-fold drop in the time the signal lasts. The underlying reason is that heat increases molecular vibrations, which dissipate the excited-state energy before it can be released as light.
For practical lab work, this means your samples and reagents should be at a consistent, known temperature before you start reading. A plate that has been sitting at room temperature for 20 minutes will give different readings than one pulled straight from a 37°C incubator. Most protocols specify equilibrating the plate to room temperature for 10 to 15 minutes before measurement. If your luminometer has a temperature-controlled chamber, use it.
Background Subtraction and Controls
Every luminescence measurement includes some background signal: electronic noise from the PMT, stray photons, and light produced by the reagents or plate material themselves. Subtracting this background is essential for getting a meaningful result.
The standard approach is straightforward. Include several blank wells on your plate that contain everything except the analyte of interest (buffer, substrate, reagent, but no sample). Average the RLU readings from those blanks, then subtract that average from every sample well. This corrected value represents the light actually produced by your analyte. From there, a signal-to-noise ratio (the sample reading divided by the standard deviation of the blanks) tells you how confidently you can distinguish your signal from random noise. A ratio below 3 is generally considered indistinguishable from background.
For quantitative work, you’ll run a standard curve: a series of wells with known concentrations of the analyte, measured alongside your unknowns. Plotting RLU against known concentration gives you a curve from which you can interpolate the concentration of each unknown sample.
Common Applications and Sensitivity
One of the most widespread uses of luminescence measurement is the ATP bioluminescence assay for detecting microbial contamination in food safety, water quality, and surface hygiene testing. The firefly luciferase enzyme reacts with ATP (a molecule present in all living cells) and emits light proportional to the amount of ATP present. More microbes means more ATP, which means a brighter signal.
The sensitivity of these assays depends on the sample type and preparation method. In food safety testing, detection limits range from about 100 colony-forming units per gram in ground meat to roughly 10,000 per gram in processed products like cooked, cured meats. On surfaces like beef carcasses, bacterial loads as low as 100 organisms per square centimeter have been detected, with results available in as little as 15 minutes. That speed is the main advantage over traditional culture-based methods, which take 24 to 48 hours.
In molecular biology, luciferase reporter assays measure gene expression by linking a gene of interest to the luciferase enzyme. When the gene is active, the cell produces luciferase, and the light output after adding substrate reflects how strongly that gene was turned on. Dual-reporter systems use two different luciferases (one as the experimental signal, one as an internal control) to normalize for differences in cell number or transfection efficiency between wells.
Calibrating for Absolute Measurements
Most luminescence work relies on relative measurements, comparing treated samples to controls. But when absolute photometric values are needed, calibration traces back to physical standards maintained by national metrology institutes. NIST, for example, bases its photometric units (the candela for luminous intensity, the lumen for luminous flux) on an absolute cryogenic radiometer, which measures optical power with very high accuracy. Working standards, typically calibrated incandescent lamps operated at precise color temperatures around 2,730 to 2,856 K, are recalibrated annually to keep measurement uncertainties low.
For routine laboratory luminometers, manufacturers often provide a built-in light source or a reference standard (a small radioactive or chemiluminescent source with a known, stable output) to verify that the instrument’s sensitivity hasn’t drifted between service intervals. Running this check before a critical experiment takes seconds and can save you from collecting an entire dataset on a degraded detector.