The flux of impurity atoms into plasmas from limiting surfaces is considered. It is shown how the flux of an impurity released from a surface can be derived from spectroscopic measurements along a line-of-sight directed at the surface. A theoretical atomic level population model is developed to obtain the 'ionization per emitted photon' quantities which link the spectroscopic measurement to the flux. Metastable states and finite density plasma effects are taken into account and observations at visible wavelengths are emphasized. Detailed studies and calculations are performed for C+1, C+2, O+1, O+2, Cr, Cr+1, Fe, Fe+1, Ni and Ni+1. Tabulations and graphs of relevant quantities are provided. The application of the theory to impurity influxes in the JET Tokamak is described.