Plasmonics based electrochemical microscopy (PECM) has been introduced to image local electrochemical reactions optically. The algorithm used to convert the plasmonic signal to current, however, is not applicable to thin layer electrochemistry, which is important for microfluidic detection and biosensing applications. Here we introduce a PECM algorithm for both semi-infinite and thin-layer electrochemical geometries. The new algorithm expresses plasmonic signal as local concentration of redox species on the electrode surface, which, together with the diffusion and Nernst equations, determines the current. We validate the algorithm experimentally by fabricating a thin-layer electrochemical cell, consisting of two-parallel electrodes separated with various distances. (C) 2016 Elsevier B.V. All rights reserved.