# Internal Medium

A (transmission) medium defines the properties of the matter through which light travels, when it is refracted at a material interface.

For example, a green glass medium will specify moderate absorption in the red and blue parts of the spectrum, so as to leave behind mainly green light; clear blue water will specify a small amount of absorption in the red and green parts of the spectrum so as to leave behind mainly blue light. If the medium contains many small particles, such as milk, then it will also specify other properties such as the scattering coefficient, etc.

• The object has to be a closed volume. This means it cannot have any holes leading to the interior of the mesh.
• All mesh faces must be facing outwards. Check the face 'normals'.

Precedence: Precedence is used to determine which medium is considered to occupy a volume when two or more media occupy the volume. The medium with the highest precedence value is considered to occupy the medium, 'displacing' the other media. The predefined and default scene medium, 'air', has precedence 1.

## Basic

Typical values for glass and water lie in the range 0.003 – 0.01 (see http://en.wikipedia.org/wiki/Cauchy%27s_equation for some coefficients)

 IOR Index of refraction. Should be >= 1. Glass has an IOR (index of refraction) of about 1.5, water about 1.33. The IOR of plastic varies, 1.5 would be a reasonable guess. Cauchy B Coeff Sets the 'b' coefficient in Cauchy's equation, which is used in Indigo to govern dispersive refraction. Units are micrometers squared. Setting to 0 disables dispersion. Note: the render can be slower to converge when dispersion is enabled, because each ray refracted through a dispersive medium can represent just one wavelength. So only set cauchy_b_coeff to other than 0 if you really want to see dispersion. Absorption Coefficient Spectrum Controls the rate at which light is absorbed as it passes through the medium. Subsurface Scattering Use this element to make the medium scatter light as it passes through it. Scattering Coefficient Spectrum Chooses the phase function used for the scattering.

## Phase Function

The phase function controls in which direction light is scattered, when a scattering event occurs.

 Uniform Takes no parameters Henyey Greenstein The Henyey-Greenstein phase function can be forwards or backwards scattering, depending on the 'g' parameter.

## Dermis

 Hemoglobin Fraction Controls the amount of hemoglobin present. Typical range: 0.001 – 0.1

## Epidermis

Medium for simulating the outer layer of skin.

 Melanin Fraction Typical range: 0 – 0.5 Melanin Type Blend Controls the amount of eumelanin relative to pheomelanim in the tissue. Typical range: 0 – 1