The characterization of thin liquid films is relevant to many engineering applications, ranging from oil and chemical industry to refrigeration systems, to cooling of light water nuclear reactors. The total internal reflection method (TIRM) is an optical method known for decades for being able to non-intrusively measure film thickness of a wide range of fluids flowing over a transparent wall, but systematic studies on the accuracy of the method are still missing. In this work, TIRM is presented and all the main potential error sources related to the application of such measurement are thoroughly characterized. The analysis includes the potential impact of variation of the refractive index on the measured thickness, the extension of the experimental calibration range to a broader set of measurable thicknesses and the effect of the inhomogeneity of the film free surface on the measured thickness. This latter aspect was never investigated in detail before because of the inherent complexity of the involved physical phenomena, but an in-house developed ray-tracing simulation allows new insights into the problem. Overall, the present paper redefines the utilization limitations and the accuracy of TIRM.