Laboratorio de Optica Integrada, Facultad de Ciencias, Universidad de Oviedo, Spain Received 11 October 2004, revised 6 May 2005, accepted 3 June 2005 Published online 29 July 2005 PACS 02.30.Zz, 07.05.Hd, 42.79.Gn, 78.20.Bh, 78.20.Ci, 82.39.Wj A modified dark-lines method of prism-coupling technique is utilized for the experimental determination of the effective index of propagating modes in a glass planar waveguide. We use to make the waveguides a silver-sodium ion exchange in a nitrate solution and sodalime glass as substrate (microscope slides). The measurements were accomplished by direct HeNe laser beam incidence and sensing the reflected light by a Thorlabs Dec110 optical detector linked to a Protek500 digital multimeter. A LabView virtual instru-ment was implemented for the automation of the measurement process. The effective indexes measured have been used to calculate the refractive index profile by IWKB method. A comparison with other results shows that our experimental setup is suitable for slab waveguide modes characterization.