Due to the large volume of email I receive regarding the Venus absorption renders, I’ve decided to set up a quick tutorial on VRay absorption which may answer some of your questions. There are several tutorial files below available for download. In order to use them, you must have 3dsmax 4.2 and Vray Advanced v1.09.03.
Absorption is used for simulating translucency. Unlike Sub-Surface Light Scattering (SSS, 3S), the light is absorbed within the subject instead of bouncing about and contributing to internal illumination.
Absorption within VRay is produced through a combination of Glossiness Refraction, Fog Color/Multiplier, and Index of Refraction (IOR). You do not need to check the translucency checkbox because we do not wish to contribute to subsurface light scattering. In addition, global illumination does not have to be enabled for this example. The screen capture of VRay settings below describes the differences between these variables in reference to the featured image above. While lower fog multiplier values increase the fog’s transparency, higher glossiness values decrease the diffusion (blurriness) of the refraction. This is apparent when you compare the first sphere on row A with the first sphere on row B. Both spheres have a fog multiplier of 0.1 but because sphere A has a refraction glossiness value of 0.7 in contrast to sphere B’s value of 0.2, the refraction is therefore sharper and internal objects (such as the illuminated bar) are more visible.
Index of Refraction (IOR) also plays an important role in absorption as it produces different effects. While any value will work for absorption, a value of 1.0 (which is used in the example below) produces no refraction and treats the object as transparent. (This is not the default VRay value and often the reason why you may be getting strange absorption results.)
The final and probably most important point to remember is that you must backlight your subject in order for absorption to be visible. In doing so, you are illuminating your subject from behind and can therefore see the absorption in effect. The example in the featured image has one spotlight located directly behind the spheres.
The tutorial for example A & B can be downloaded from here.
The tutorial for example C can be downloaded from here.