Multi-layer polyolefin films are commonly used in the packaging industry. Their mechanical and optical properties are of utmost importance. In this paper the thermo-optical behavior of bi-oriented single-layer blown films made by polypropylene/polyethylene (PP reach) blends was studied. Semi-crystalline polymers at room temperature show birefringence due to the presence and orientation of the crystalline phase. Upon heating, crystalline transformations mainly melting, can be quantitatively followed by the birefringence, hence the behavior of each component can be measured individually. The birefringence is defined as the optical path difference (OPD) normalized by the film thickness, and is normally obtained using a compensator in a hot stage polarized light optical microscope (PLOM). Readings are discrete values get after thermal equilibrium. In order to get them in real time a photocell was fitted to the optical path of the PLOM and the transmitted light intensity recorded during heating. This variable is a sin square function of the OPD and so the birefringence of the film. An in-line data collection system was developed, linked to the photocell and the thermo-optical behavior of the PP/PE blend recorded in real time. The expected reduction in the birefringence in the temperature range of the polyethylene melting point due to the fusion of its crystals was not observed. On the other hand when multi-layers films having some layers composed exclusively with pure polyethylene are used, the PE melting behavior of these single layers is clearly observed. One of the possible explanations is that the birefringence of the polyethylene crystals in the blend has an opposite signal of the polypropylene crystals and cancel each other. In order to better understand this phenomenon various thermal cycles are being developed and applied. A scientific explanation can lead to the development of test methods for the industrial quality control of commercial multi-layered polyolefin films.