6.C.5."Laser" materials as follows:

a. Synthetic crystalline "laser" host material in unfinished form as follows:
1. Titanium doped sapphire;
2. Not used since 2012
b. Rare-earth-metal doped double-clad fibres having any of the following:
1. Nominal "laser" wavelength of 975 nm to 1,150 nm and having all of the following:
a. Average core diameter equal to or greater than 25 µm; and
b. Core 'Numerical Aperture' ('NA') less than 0.065; or
Note 6.C.5.b.1. does not apply to double-clad fibres having an inner glass cladding diameter exceeding 150 µm and not exceeding 300 µm.
2. Nominal "laser" wavelength exceeding 1,530 nm and having all of the following:
a. Average core diameter equal to or greater than 20 µm; and
b. Core 'NA' less than 0.1.
Note 6.C.5.b. includes fibres assembled with end caps.
Technical Note
For the purposes of 6.C.5.b., the core 'Numerical Aperture' ('NA') is measured at the emission wavelengths of the fibre.