bores 134 and 136 of the uv propagation section 132 prism compartment 140 desirably has purging inlet 144 and purging outlet 146 so that the entire uv compartment can be purged a translation stage 154 is provided for mounting the set of prisms or optional dichroic mirrors to change the position of the incident beams on the prisms or mirrors in case damage occurs on the prisms or to the mirror coating to increase the lifetime of the wavelength separator the beams propagating from the uv crystal expand along the uv crystal beam path that is the length of uv propagation section 132 according to the invention beam propagating from said uv beam outlet 124 has a beam diameter greater than 1 0 mm preferably 1 0 mm to 2 0 mm the preselected length l of said main compartment is sufficient that said uv beam has a diameter to uv beam outlet 124 of 1 0 mm or greater the length l of the main compartment is chosen to allow sufficient beam expansion of the uv beam to the desired beam diameter on its two passes along the length of the main compartment in the bore 134 and 136 of the propagation section l is at least 25 cm in length preferably at least 30 cm desirably 25 to 100 cm in length the uv beam path extends through the uv propagation section along the outside of the main compartment and desirably along it length l a uv beam outlet 124 in optical communication with the uv beam propagating from the uv propagation section bore 136 is located at the end of the uv generation compartment 114 and preferably aligns with bore 136 the resulting uv.