This part of IEC 61300 defines the encircled angular flux measurement of multimode waveguide light sources, in which most of the transverse modes are excited. The term "waveguide" is understood to include both channel waveguides and optical fibres but not slab waveguides.The applicable fibre types are the followings:- A1 specified in IEC 60793-2-10;- A3 specified in IEC 60793-2-30;- A4 specified in IEC 60793-2-40.