In the Geometrical Optics interface, an absorbing medium has a complex-valued refractive index of the form n − iκ., where
n and
κ are real numbers and
κ > 0. As a ray propagates through an absorbing medium, the built-in variables that track ray intensity and power may decrease.
Noting that the intensity is proportional to E ⋅ E∗, the imaginary terms in the exponents vanish, leaving only the attenuation term,
In the physics interface Medium Properties node, you can control how the value of
κ is defined by selecting different options from the
Optical attenuation model list.
For Attenuation coefficient, enter the value or expression of the coefficient
α, defined as
The Optical attenuation model list also includes the following options:
To see this more concretely, suppose that internal transmittance data is available for 10 mm and 25 mm samples of a glass at λ0 = 600 nm, each reported to three digits. Suppose the reported values are
τi,10 = 0.998 and
τi,25 = 0.995. Because of roundoff, we may presume the actual value of
τi,10 to be between 0.9975 and 0.9985, and similarly for
τi,25 to be between 0.9945 and 0.9955. Substituting these lower and upper bounds into
Equation 3-6 then gives the following results:
If a material uses internal transmittance data and one of the options Extinction coefficient or
Attenuation coefficient is selected, the transmittance data will automatically be converted to this coefficient. If two or more sets of transmittance data for different sample thicknesses are defined for the material, the default behavior is to use the data for the thicker sample unless otherwise specified.