Quantum efficiency is a unitless number that measures the sensitivity of the photocathode to light.  It is the ratio of the number of electrons emitted to the number of incident photons.  This property depends on the wavelength of light being used to illuminate the photocathode.  For many applications, QE is the most important property as the photocathodes are used solely for converting photons into an electrical signal.
Quantum efficiency may be calculated from photocurrent (), laser power (), and either the photon energy () or laser wavelength () using the following equation.