The spectral extinction file (usually named with a .ext file extension) is primarily associated with the Classic Extinction model that was first introduced in DIRSIG3.

The data is used to model the total extinction (absorption and scattering) of light within a volume. These files are used to model the extinction through objects ranging in scales from simple clouds (where path lengths are many kilometers) to a leaf or plate of glass (where path lengths are a few millimeters).

Format Details

The first line of the file is the number of spectral curves in the file.

Important The number of spectral curves must be 1.

The second line is the curve delimiter that marks the start of the curve data. Depending on the age of the file, the curve marker varies (which will be discussed later).

Important Even though there is only 1 curve in an extinction file, there is a marker for the start of the curve. Forgetting to include this line will throw off the parsing of the spectral extinction data following the marker.

The spectral curve data following the curve count varies depending on the age of the file. The units of the extinction coefficients are per kilometer.

Note Many users forget the units are per kilometer and import data with units of per meter. Although these latter units are more intuitive for many cases, the format of this file was defined many years ago when the data was used exclusively for large objects.
Note To be safe, users should supply a large extinction value (for example 1.00e+10) for wavelengths where the values are unknown. This will result in those spectral regions being optically opaque.

Old Format

The "old" format files had each curve sampled on fixed 100 wavenumber increments from 39850 to 350 wavenumbers, resulting in exactly 396 spectral points. The spectral samples are in decreasing order. This format is identified by the curve delimiter being the single value 0 (the curve number).

39850 1.00000e+10
39750 1.00000e+10
39650 1.00000e+10
[lines deleted for documentation purposes]
24850 5.08321e+06
24750 5.06721e+06
24650 5.03595e+06
[lines deleted for documentation purposes]
550 1.00000e+10
450 1.00000e+10
350 1.00000e+10

New Format

The "new" format was added during the DIRSIG3 era to allow the user to bring in curves with spectral resolution better than 100 wavenumbers. This format is identified by the curve delimited being the string CURVE_BEGIN. The units of the spectral samples are in microns and the samples are in increasing order.

 0.40000 2.59696e+06
 0.40084 2.64931e+06
 0.40169 2.70456e+06
[lines deleted for documentation purposes]
14.1335	1.000000+10
14.2092	1.000000+10
14.2857	1.000000+10
Note The spectral samples in a "new format" file do not need to be on regular intervals.