EEPROM memory matrix in which column lines are alternatively used as detector lines. A precharge voltage is applied to selected detector lines together with the relevant column line in each case before read-out of the memory columns. If a detector line loses its precharge level during the read-out of the memory cells, light incidence is assumed and a corresponding alarm function is triggered. Preferably column lines adjacent to the column lines that are respectively selected for the data transmission are connected as detector lines.