Dark Current
Definition and meaning of Dark Current in chemistry.
Dark current is the small electrical current that flows through a photodetector, such as a photomultiplier tube, photodiode, or charge-coupled device, even when no light is striking it.
In more detail
It arises from thermally generated charge carriers: thermionic emission of electrons from a photocathode in a photomultiplier tube, or thermal generation of electron-hole pairs in a semiconductor photodiode or CCD. Because it occurs independent of the analytical signal, dark current sets a background noise floor that limits the lowest concentration or intensity a spectroscopic instrument can reliably detect. It increases with temperature and detector bias voltage, so sensitive instruments (fluorimeters, low-light CCD cameras) often cool the detector thermoelectrically or cryogenically to suppress it. Analysts routinely measure and subtract a dark-current (blank) reading with the light source blocked before interpreting sample data.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Primary cause | Thermionic emission / thermal electron-hole pair generation |
| Typical magnitude | Picoamps to nanoamps, instrument-dependent |
| Common mitigation | Detector cooling and blank (dark) signal subtraction |
In a fluorescence spectrometer, closing the shutter to block all excitation and emission light still produces a small residual photomultiplier tube signal of a few nanoamps; this dark current is recorded as a blank and subtracted from subsequent fluorescence measurements.
Frequently asked questions
How do analysts correct for dark current?
They record a baseline reading with the light source blocked or shuttered, then subtract that dark reading from every subsequent sample measurement.
Why does cooling a detector reduce dark current?
Lowering temperature reduces the thermal energy available to spontaneously eject electrons from the photocathode or generate electron-hole pairs in a semiconductor, so fewer charge carriers form without incident light.