Focus error systems based on the astigmatic principle relying on four quadrant photo detectors as the signal generation element have widely been used in many different applications. Nevertheless, its working principle is not particularly suitable for measurements involving more than one spot. In this work we propose a new method that uses a CCD as the detection device as well as a method of processing acquired data. A side effect of this change is the loss of the FES parameter key to these techniques. To overcome this issue a new parameter named focus signal is proposed. The thickness measuring process of a transparent sample with two reflective layers using this technique is also presented.Fil: Aguilar, Andrés. Universidad Tecnológica Nacional. Facultad Regional Delta. Investigación, Ciencia y Tecnología. CENES. Grupo de Nanofotónica; Argentina.Fil: Torga, Jorge. Universidad Tecnológica Nacional. Facultad Regional Delta. Investigación, Ciencia y Tecnología. CENES. Grupo de Nanofotónica; Argentina.Peer Reviewed