Optics as follows : a. Optical filters: a.1. For wavelengths longer than 250 nm, comprised of multi‑layer optical coatings and having either of the following: a.1.a. Bandwidths equal to or less than 1 nm Full Width Half Intensity (FWHI) and peak transmission of 90% or more; or a.1.b. Bandwidths equal to or less than 0.1 nm FWHI and peak transmission of 50% or more; Note: 6A994 does not control optical filters with fixed air gaps or Lyot ‑type filters. a.2. For wavelengths longer than 250 nm, and having all of the following: a.2.a. Tunable over a spectral range of 500 nm or more; a.2.b. Instantaneous optical bandpass of 1.25 nm or less; a.2.c. Wavelength resettable within 0.1 ms to an accuracy of 1 nm or better within the tunable spectral range; and a.2.d. A single peak transmission of 91% or more; a.3. Optical opacity switches (filters) with a field of view of 30 degrees or wider and a response time equal to or less than 1 ns; b. “Fluoride fibre” cable, or optical fibres therefor, having an attenuation of less than 4 dB/km in the wavelength range exceeding 1,000 nm but not exceeding 3,000 nm.