IEC 60793-1-43:2015 establishes uniform requirements for measuring the numerical aperture of optical fibre, thereby assisting in the inspection of fibres and cables for commercial purposes. The numerical aperture (NA) of categories A1, A2, A3 and A4 multimode fibre is an important parameter that describes a fibre's light-gathering ability. It is used to predict launching efficiency, joint loss at splices, and micro/macrobending performance. The numerical aperture is defined by measuring the far-field pattern (NAff). In some cases the theoretical numerical aperture (NAth) is used in the literature, which can be determined from measuring the difference in refractive indexes between the core and cladding. Ideally these two methods should produce the same value. This second edition of IEC 60793-1-43, together with other standards in the IEC 60793-4X series, cancels and replaces the first edition of IEC 60793-1-43, published in 2001, and constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
- expansion of the scope to include A1, A2, A3 and A4 multimode fibre categories;
- addition of measurement parameters of sample length and threshold values, product specific to the variables that are now found in the product specifications;
- a new Annex B entitled "Product specific default values for NA measurement";
- addition of a new Technique 4 for measuring NA of A4d fibres;
- a new Annex A entitled "Mapping NA measurement to alternative lengths" that gives a mapping function to correlate shorter sample length measurements to the length suggested in the reference test method Naff. Keywords: numerical aperture of optical fibre, inspection of fibres and cables

This publication is to be read in conjunction with IEC 60793-1-1:2008, IEC 60793-1-21:2001 and IEC 60793-1-22:2001.