Photoelectric devices: Quantum efficiency

The quantum efficiency of the response is typically of the order of 0·1 or 10%, meaning that, on average, one photoelectron is emitted for every ten photons which are incident on the sensitive surface. Using this information, it is very easy to predict the current to be measured according to the energy arrival rate. For example, if the photoelectric cell is receiving 200 photons s⁻¹ (i.e. the photon rate just detectable by eye), the number of photoelectrons emitted is 20 s⁻¹. Now the charge on an electron is equal to 1·6 × 10⁻¹⁹ coulomb (C) and, therefore, the total charge liberated is
 

20 × 1·6 × 10⁻¹⁹ C s⁻¹ = 3·2 × 10⁻¹⁸ A.
 

This, of course, is an extremely small current and it would require large amplification before it could be registered by some recording device.