Optical materials. a. Low optical absorption materials, as follows: a.1. Bulk fluoride compounds containing ingredients with a purity of 99.999% or better; or Note: 6C994.a.1 controls fluorides of zirconium or aluminium and variants. a.2. Bulk fluoride glass made from compounds controlled by entry 6C004.e.1 of Annex I of the Dual-Use Regulation; b. ‘Optical fibre preforms’ made from bulk fluoride compounds containing ingredients with a purity of 99.999% or better, specially designed for the manufacture of “fluoride fibres” controlled by 6A994.b.