## A.2 Using your own optical constants

If you wish, you can use your own optical constants with IMD. To add you own optical constants, you must follow these two steps:

1) Create an nk file, according to these conventions: the file must consist of three columns of data (lambda [in angstroms], n, k). You can include comments (preceded by semicolumns in the first column) at the beginning of the file, but there can be no comments or blank lines anywhere else in the file. (Make sure there are no blank lines at the end of the file!) You should also follow the file-naming convention described in the previous section. When creating plots, IMD can display the reference portion of the file name as a subscript. (See the `File->Global Plot Preferences->Material Names` option.) Also, any numbers in the Material name are displayed as subscripts, e.g., the "2" in SiO2 will be displayed as a subscript.

2) Tell IMD where to find the file you've created. This is done by selecting `Materials/Optical Constants->Optical Constants Search Path` from the IMD menu bar. A widget will appear that allows you to add and subtract directories to the IMD optical constants search path; you can also adjust the order of directories searched, as IMD will search the directories in the order listed, and will use the first file found that matches the material name you specify. Note that the default optical constants search path can be changed as desired by editing the`imdsiteconfig.pro` file. See Appendix B.1 for details.

If you wish, you can also create a file called `AAACATALOG.TXT,` to be kept in your personal optical constants directory, so that you will be able to use the `Materials/Optical Constants -> Browse Optical Constants Database` option to view the contents of your files using XNKPLOT directly from IMD. Follow the same format that's used in the `AAACATALOG.TXT` file in `nk.dir`,

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