Do Multiple Scattering - Enable to use fast approximation of light dispersion in smoke. This option is not supported for Arnold renderer.
In FumeFX, light that affects fire and smoke can come from two different sources: external lights, or fire itself. These two sources have separate strength multipliers and a common falloff multiplier, which controls how much light is dispersed to surrounding smoke and fuel. It is usually calculated much faster than an illumination map and cannot be cached.
Maximum Depth - This parameter sets the level of simplification for fire as an illumination source. The higher the maximum depth parameter is, the faster is will be computed. Decrease this value if you need more accuracy - for example, if you notice sudden illumination changes in some areas.
Fire Strength - This sets Fire illumination strength.
Smoke Strength - This sets Smoke illumination strength.
Falloff - This value sets the rate for Falloff of the scattered light through the medium. Use smaller numbers to evenly illuminate smoke.
Note: Values for this parameter are not normalized; they can be very big or small numbers, depending on shader settings, simulation and scene size. You will have to experiment a little bit to balance falloff with overall light. Decreasing falloff substantially will also increase the total amount of light in the smoke, so you may sometimes need to decrease strength.