The controlled growth of organic layers on surfaces is still waiting for an in-situ reliable technique that would allow their quality to be monitored and improved. Here we show that the growth of a perylene monolayer deposited on Ag(1 1 0) at room temperature can be tracked with low energy atoms in a regime where the energy perpendicular to the layer is less than 0.1 eV and below the organic film damage threshold. The image processing required for this atom triangulation technique is described in detail. (C) 2015 Elsevier B.V. All rights reserved.