BK7 Glass data from SCHOTT literature, plus fit k from T data
nm
NK
250	1.5537	2.3171e-05
260	1.5522	2.6453e-05
270	1.5507	3.0863e-05
280	1.5491	2.3689e-05
290	1.5476	1.0293e-05
300	1.5461	4.4082e-06
310	1.5446	1.9607e-06
320	1.5431	9.9918e-07
330	1.5416	6.1727e-07
340	1.5401	4.0505e-07
350	1.5385	2.3401e-07
360	1.537	1.333e-07
370	1.5355	5.6965e-08
380	1.534	2.4864e-09
390	1.5325	0
400	1.531	0
410	1.5296	0
420	1.5285	0
430	1.5274	0
440	1.5263	0
450	1.5254	0
460	1.5246	0
470	1.5237	0
480	1.5228	0
490	1.5221	0
500	1.5215	0
510	1.5209	0
520	1.5203	0
530	1.5197	0
540	1.5191	0
550	1.5185	0
560	1.5181	0
570	1.5176	0
580	1.5172	0
590	1.5167	0
600	1.5163	0
610	1.5159	0
620	1.5156	0
630	1.5152	0
640	1.5148	0
650	1.5145	0
660	1.5142	0
670	1.5139	0
680	1.5136	0
690	1.5134	0
700	1.5131	0
710	1.5128	0
720	1.5126	0
730	1.5124	0
740	1.5122	0
750	1.512	0
760	1.5118	0
770	1.5115	0
780	1.5113	0
790	1.5111	0
800	1.5109	0
810	1.5107	0
820	1.5105	0
830	1.5103	0
840	1.5101	0
850	1.5099	0
860	1.5097	0
870	1.5095	0
880	1.5094	0
890	1.5092	0
900	1.5091	0
910	1.5089	0
920	1.5088	0
930	1.5086	0
940	1.5085	0
950	1.5083	0
960	1.5081	0
970	1.508	0
980	1.5078	0
990	1.5077	0
1000	1.5075	0