Conductors (metals) and dielectrics (glasses, liquids, ...) are materials that both reflect and transmit light. They are defined by their index of refraction (how the light is deflected or reflected when entering the material) and their index of extinction (how much of the light is absorbed by the material). Both indexes can be measured and they can be simulated in Predict Engine.

Conductors and dielectrics are both designed with an index of refraction and an index of extinction/absorption. The choice of the type (Dielectric or Conductor) is an optimization setting for Predict Matter :

• if a material is dielectric, the light can be transmitted inside the material : the path tracer will try and find a path through the material,

• if a material is conductor, the light cannot be transmitted inside the material : the path tracer will only compute paths reflected by the material.

The formula that defines how much of the light has been absorbed after travelling a distance z in the volume is known as the Beer-Lambert law:

I(z) = I0 * exp(- z * alpha),

where I(z) is the intensity after a distance z, I0 is the intensity of the light entering the volume, and alpha is the absorption coefficient.

Example : to model a material that absorbs 5% of the light every 1cm,

alpha = - 1 / z * ln(I(z) / I0),

where z = 1e-2m, and I(z)/I0 = 95% = 0.95 : alpha = 5.1.