In germanium-based light detectors for scintillating bolometers, a SiO_2 anti-reflective coating is often applied on the side of the germanium wafer exposed to light with the aim to improve its light collection efficiency. In this paper, we report about a measurement, performed in the temperature range 25-35mK, of the light-collection increase obtained thanks to this method, which resulted to be of the order of 20%. The procedure followed has been carefully selected in order to minimize systematic effects. The employed light sources have the same spectral features (peaking at ~630nm wavelength) that will characterize future neutrinoless double beta decay experiments on the isotope ^8^2Se and based on ZnSe crystals, such as LUCIFER. The coupling between source and light detector reproduces the configuration used in scintillating bolometers. The present measurement clarifies the role of SiO_2 coating and describes a method and a set-up that can be extended to the study of other types of coatings and luminescent materials. (C) 2013 Elsevier B.V. All rights reserved.