Method and apparatus for measuring the thickness of one or more layers of a multi-layer film obtained by blow extrusion process

An apparatus for measuring, in a multi-layer film obtained by blow extrusion process and formed of one or more layers of a first material and one or more layers of a second material, the total thickness (L1) of the first material and/or the total thickness (L2) of the second material, said apparatus comprising:

• an electromagnetic radiation sensor arranged to provide a first measurement signal (Sx) representative of the total thickness of the film;
• a capacitive sensor arranged to provide a second measurement signal (Scap) which is the sum of the signals given by the first and second materials of the film, wherein the signal given by each material of the film is a function of the thickness (L1, L2) of said material;
• and
• processing means for calculating, from said first and second measurement signals (Sx, Scap), the total thickness (L1) of the first material and/or the total thickness (L2) of the second material.