If you specify a complex-valued refractive index and are solving for ray intensity or power (see the section Intensity, Polarization, and Power), you can make the intensity and power vary along the ray path. If the medium properties are uniform, the intensity and power usually follow exponential decay (for an absorbing medium) or growth (for a gain medium). You can switch between absorbing and gain media by changing the sign of the imaginary part of the refractive index.

An alternative way to specify the imaginary part of the refractive index is to enter a value or expression for the internal transmittance for a given thickness of the material. For example, many of the optical glasses from the Optical Material Library (see the Material Libraries section) provide tabulated values of the internal transmittance as a function of wavelength.

Usually the real and imaginary parts of the refractive index in a homogeneous medium are material properties. Sometimes, however, the absorption is due to the presence of small imperfections or particles in a medium. Some examples include dust, water droplets, or smoke in air; bubbles or sediment in water; or colloidal suspensions. In such cases, you can include the losses due to absorption and scattering by these particles using the dedicated Scattering Domain node.