1. A method for measuring a diameter of an optical fiber running along a theoretical axis of displacement, said method comprising the steps of: positioning two fixed measuring systems having optical axes which are substantially coplanar, perpendicular and intersect the theoretical axis perpendicularly and each of said systems having a magnifying optics associated with a strip of photodetectors; forming a magnified blurred image of the optical fiber on each strip of photodetectors; performing an interpolation calculation on the signals from said photodetectors for identifying the exact positions of the optical fiber edges on the blurred image; deducing from said positions the apparent diameter of the optical fiber as measured by one of the systems; correcting the diameter measurement by using the sideways displacement of the fiber along the axis of said one system as measured by the other system; comparing a predetermined threshold with the signals from said photodetectors; identifying an address of a particular photodetector situated substantially halfway along the blurred image of the optical fiber; and performing a linear interpolation on the signals from a predetermined number of photodetectors situated on either side of said particular photodetector.