A directional brain stimulation lead assembly provides a lead body and an insulating member defining one or more windows that selectively expose portions of electrodes carried by the lead body to produce a directional stimulation current field. A press-fit portion press-fitted to the terminal holding hole is pressure-forged in a direction orthogonal to an axis. For each of these scattering directions, the gathered light is separated into two polarized light components on the planes parallel and orthogonal to the scattering direction, the light intensity of each of the polarized light components is measured and a scattered light polarization ratio is calculated as a function of the measured light intensities of the polarized light components.