The spectral emissivity file (usually named with a .ems file extension) is used by various DIRSIG optical properties, but is primarily associated with the Classic Emissivity model that was first introduced in DIRSIG3.

The file can contain one or more emissivity curves, where multiple curve files can be employed by the texture mechanism of the Classic Emissivity to introduce spectral variability within a given material class.

Format Details

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

The next 91 lines are spectrally constant, angular weighting values for the emissivity for angles ranging from 0 to 90 degrees (inclusively, resulting in 91 values).

Note The angular weighting values were introduced when early versions of DIRSIG focused on broadband LWIR simulations. These weighting values cannot be assumed to hold across all wavelengths. Therefore, most emissivity files will have these values filled with 1.0.

The curve data following the curve count and angular weighting header varies depending on the age of the file.

Old Format

The "old" format files had each curve sampled on fixed wavenumber samplings:

  • Every curve was sampled at 100 wavenumber increments from 39850 to 350 wavenumbers (cm-1), resulting in exactly 396 spectral points.

  • The spectral samples are in decreasing order.

  • The curves are separated by a line containing the a single value denoting the curve number (start with curve 0).

The following example is for a file containing 2 curves.

[lines deleted for documentation purposes]
39850 0.6865
39750 0.6654
39650 0.7123
[lines deleted for documentation purposes]
550   0.9098
450   0.9102
350   0.9187
39850 0.7465
39750 0.6954
39650 0.7524
[lines deleted for documentation purposes]
550   0.9198
450   0.9012
350   0.9076

New Format

The "new" format was added during the DIRSIG3 era to allow the user to bring in curves with spectral resolution better than the fixed 100 wavenumber sampling enforced by the "old" format:

  • The primary indicator that a file is a "new" format file is that the curve delimiter is the string CURVE_BEGIN.

  • The units of the spectral samples are always in microns and the samples are in increasing order.

  • The spectral samples do not need to be on regular intervals and each curve can be sampled at different spectral locations.

The following example is for a "new" format file containing 300 curves.

[lines deleted for documentation purposes]
 0.40000 0.992053
 0.40084 0.991947
 0.40169 0.991833
[lines deleted for documentation purposes]
14.1335	1.000000
14.2092	1.000000
14.2857	1.000000
 0.40100 0.985854
 0.40051 0.985616
 0.40217 0.985213
[lines deleted for documentation purposes]
14.1835	0.998100
14.2329	0.999030
14.2587	1.000000