  We report on the development of an instrument for the measurement of the
Encircled Angular Flux (EAF) and on establishing its metrological traceability
at the required level of uncertainty. We designed and built for that purpose
two independent EAF measuring instruments, both based on the analysis of the
two-dimensional far field intensity profile observed at the output of an
optical fibre, using either CMOS or CCD cameras. An in depth evaluation of the
factors influencing the accuracy of the measurements was performed and allowed
determining an uncertainty budget for EAF measurements, which was validated by
a first series of inter-comparisons. Theses comparisons were performed between
the two independent EAF measuring systems, using a 850 nm LED coupled into a
gradient index fibre as a test object. We demonstrated a very good equivalence
between the two systems, well within the absolute measurement uncertainties
that were estimated at the 10-3 level. Further inter-comparisons using light
sources coupled to step-index, large core and small core multimode fibres are
still ongoing, with the aim to confirm the performances of the instrument under
various illuminating conditions.
