Optical materials, as follows: (a) Zinc selenide (ZnSe) and zinc sulphide (ZnS) ‘substrate blanks’, produced by the chemical vapour deposition process and having any of the following: 1. A volume greater than 100 cm 3 ; or 2. A diameter greater than 80 mm and a thickness of 20 mm or more; (b) Electro-optic materials and non-linear optical materials, as follows: 1. Potassium titanyl arsenate (KTA) (CAS 59400-80-5); 2. Silver gallium selenide (AgGaSe 2 , also known as AGSE) (CAS 12002-67-4); 3. Thallium arsenic selenide (Tl 3 AsSe 3 , also known as TAS) (CAS 16142-89-5); 4. Zinc germanium phosphide (ZnGeP 2 , also known as ZGP, zinc germanium biphosphide or zinc germanium diphosphide); or 5. Gallium selenide (GaSe) (CAS 12024-11-2);