Returns or sets the emissivity of the interface given by index in the spectrum object given by spectrum.

Please note:

Spectra are counted 1, 2, 3, ...

Emissivity positions are counted according to the following scheme:

0 (external surface of the glazing)

1 (surface of the first pane pointing to the interior)

2 (surface of the second pane pointing to the exterior)

3 (surface of the second pane pointing to the interior)

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