Abstract: The invention concerns a method whereby an optical signal representing the amount of ionizing light of at least a gas in the proximity of an object is displayed and/or recorded. Said method is characterized in that it consists in: gradually increasing (D1) the voltage (U) between the object and the conductor element up to a final value (U2) wherein the maximum brightness (L1) is observed; determining, as first characteristic value of the object, the value of the voltage (U2) as from which the brightness (L2) is not less than about 10% of the maximum brightness (L1) and in determining, as second characteristic value of the object, the voltage value as from which the brightness (L3) of the signal is not less than about 90% of the maximum brightness (L1). The installation comprises a sensor equipped with a flexible membrane defining a volume of confinement for a gas or a gas mixture wherein the ionization occurs.