It is presented a criteria for selecting the optimum apertureradius for the one beam Z-scan technique (OBZT), based on the analysis ofthe transmittance of the aperture. It is also presented a modification to theOBZT by directly measuring the beam radius in the far field with a rotatingdisk, which allows to determine simultaneously the non-linear absorptivecoefficient and non-linear refractive index, much less sensitive to wave frontdistortions caused by inhomogeneities of the sample with a negligible lossof signal to noise ratio. It is demonstrated its equivalence to the OBZT. © 2013 Optical Society of America.