The transmitted light intensity of a series of ionic magnetic fluid samples based on maghemite nanoparticles was investigated using the typical birefringence experimental setup. The field dependence of the transmitted light intensity evolves from a peak-like profile to a typical second-order Langevin profile as the nanoparticle volume fraction reduces from 2.4% to 0.9% volume fraction. The onset of the peak-like profile as the nanoparticle volume fraction increases is accounted for by two points, namely the transmitted light first scales with a sine function and second the birefringence signal appears in the argument of the sine function. The observed negative differential transmitted light is explained as one brings the above-mentioned sine dependence into the nowadays used model for the analysis of birefiringence in magnetic fluids. (c) 2005 Elsevier B.V. All rights reserved.