Quantum Efficiency
The proportion of photons striking a photosite that are actually converted into measurable signal.
Formula / rule
QE = electrons / photons
Measured in
%
In depth
Quantum efficiency sets the ceiling on a sensor's sensitivity: no processing can recover photons that were never registered. Modern back-illuminated sensors reach 60 to 80 per cent in the green region, close to the practical limit, which is why recent low-light gains have come from noise reduction and stacking rather than from raw sensitivity.
Examples in practice
A sensor at 70 per cent quantum efficiency gathers a third more signal than one at 52 per cent under identical light.