This International Standard describes the launch condition requirements used for measuring multimode attenuation in passive components and in installed cable plants. In this standard@ the fibre types that are addressed include category A1a (50 ?? /125 ??) and A1b (62@5 ?? /125 ??) multimode fibres@ as specified in IEC 60793-2-10. The nominal test wavelengths detailed are 850 nm and 1 300 nm. This standard may be suitable for multimode attenuation measurements for other multimode categories and/or other wavelengths@ but the source condition for other categories and wavelengths are not defined here. The purpose of these requirements is as follows: ? to ensure consistency of field measurements when different types of test equipment are used; ? to ensure consistency of factory measurements when different types of test equipment are used; ? to ensure consistency of field measurements when compared with factory measurements. This standard describes launch condition requirements for optical attenuation using sources with a controlled encircled flux (EF).