Apparatus for measuring the thickness of a moving film utilizing an adjustable numerical aperture lens

An apparatus for measuring the thickness of a film, said apparatus comprising:

• low coherence source means for generating a light signal;
• first light coupling means for directing said light signal at said film and for collecting light reflected from said film to generate a collected light, signal, said first light coupling means comprising an optical fiber and a lens having an adjustable numerical aperture for focusing the light leaving said fiber onto said film;
• second light coupling means for coupling said collected light signal to an interferometer to generate an interference light signal comprising the coherent sum of first and second light signals, said first light signal comprising said collected light signal and said second light signal comprising said collected light signal offset in time with respect to said first light signal;
• means for varying said offset in time;
• and
• means for detecting said interference light signal and said offset for different values of said offset, wherein said means for detecting said offset comprises:
• means for generating a reference coherent light signal having a frequency different from the frequency of said collected light signal;
• means for combining said reference coherent light signal with said collected light signal;
• and
• means for detecting a reference fringe pattern in said interference light signal.