some cases it can exceed 100% (i.e. when more than one electron is created per incident photon). EQE mapping : Conventional measurement of the EQE will give the efficiency of the overall device. However it is often useful to have a map of the EQE over large area of the device. This mapping provides an efficient way to visualize the homogeneity and/or the defects in the sample. It was realized by researchers from the Institute of Researcher and Development on Photovoltaic Energy (IRDEP) who calculated the EQE mapping from electroluminescence measurements taken with a hyperspectral imager. Spectral responsivity Spectral responsivity is a similar measurement, but it has different units: amperes per watt (A/W); (i.e. how much current comes out of the device per unit of incident light power). Responsivity is ordinarily specified for monochromatic light (i.e. light of a single wavelength). Both the quantum efficiency and the responsivity are functions of the photons' wavelength (indicated by the subscript λ). To convert from responsivity (, in A/W) to QEλ (on a scale 0 to 1): where is the wavelength in nm, h is the Planck constant, c is the speed of light in vacuum, and e is the elementary charge. Note that the unit W/A (watts per ampere) is equivalent to V (volts). Determination where = number of electrons produced, = number of photons absorbed. Assuming each photon absorbed in the depletion layer produces a viable electron-hole pair, and all other photons do not, where t is the measurement time (in seconds), = incident optical power in watts, = optical power absorbed in depletion layer, also in watts. See also Counting efficiency DQE (imaging) Solar cell efficiency References Engineering ratios Photodetectors Physical quantities Quantum electronics Spectroscopy