Measurement of fiber diameters with high precision

A method for measuring the diameter of a transparent filament comprising the steps of:

• (a) directing a beam of radiation at said filament so as to produce an interference pattern;
• (b) detecting said interference pattern, said detected interference pattern comprising a set of amplitude values;
• and
• (c) determining a value for the average spatial frequency of the detected interference pattern by performing discrete sequence Fourier transforms, a padded fast Fourier transform, a zoom fast Fourier transform, or a band-selectable Fourier transform on said set of amplitude values, said value for the average spatial frequency being determined with a precision of at least about 5.times.10.sup.-4 cycles/degree and being a measure of the fiber's diameter.

A method for measuring the diameter of a transparent filament comprising the steps of:

• (a) directing a beam of radiation at said filament so as to produce an interference pattern;
• (b) detecting said interference pattern;
• and
• (c) determining an average spatial frequency for the detected interference pattern by:
• (i) performing a fast Fourier transform on said detected interference pattern to produce a set of fast Fourier transform coefficients;
• (ii) reconstructing the interference pattern using a selected subset of said set of fast Fourier transform coefficients;
• and
• (iii) determining an average spatial frequency for said reconstructed interference pattern, said average spatial frequency being a measure of the filament's diameter.

A method for measuring the diameter of a transparent filament comprising the steps of:

• (a) directing a beam of radiation at said filament so as to produce an interference pattern;
• (b) detecting said interference pattern, said detected interference pattern being frequency modulated;
• and
• (c) determining an average spatial frequency for the detected interference pattern by:
• (i) removing at least some of the frequency modulation from said detected interference pattern to produce an at least partially demodulated pattern;
• and
• (ii) determining an average spatial frequency for said at least partially demodulated pattern, said average spatial frequency being a measure of the filament's diameter.

A method for measuring the diameter of a transparent filament comprising the steps of:

• (a) directing a beam of radiation at said filament so as to produce an interference pattern;
• (b) detecting said interference pattern, said detected interference pattern comprising amplitude values at a set of data points;
• and
• (c) determining an average spatial frequency for the detected interference pattern by:
• (i) reducing the amplitudes for at least some data points at both ends of the set of data points to produce a modified interference pattern;
• and
• (ii) determining an average spatial frequency for said modified interference pattern, said average spatial frequency being a measure of the filament's diameter.

A method for measuring the diameter of a transparent filament comprising the steps of:

• (a) directing a beam of radiation at said filament so as to produce an interference pattern;
• (b) detecting said interference pattern, said detected interference pattern comprising amplitude values at a set of data points;
• (c) transforming the amplitude values to the spatial frequency domain so as to produce a frequency spectrum comprising a set of Fourier coefficients;
• and
• (d) using the magnitudes of a subset of said Fourier coefficients to estimate the frequency at which a local maximum of said frequency spectrum occurs, said local maximum corresponding to the outer diameter of the filament so that the estimate is a measure of the filament's diameter.