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When a ray of light hits a trasparent object, one part of the energy is reflected, other part is refracted and a third is absorbed. You have three color parameters and LuxRender only has two, kr and kt, so it never can be physically correct..

Once you have specified what gets reflected and what passes through, whatever remains must be what gets absorbed. So you only need two parameters.

You just forgot the energy that is turned into heat, and plants also use this energy for other things and human skin turns UV energy in vitamin D and so on...

And a black object? Light goes in and nothing goes out.

These are the properties for glass surfaces, a bit more than 2.

Glass

name                    description  
glass::Kr              The reflectivity of the surface  
glass::Kt              Fraction of light transmitted through the surface  
glass::index        The index of refraction for the glass surface 
glass::cauchyb   Cauchy B coefficient  
glass::film           The thickness in nanometers of the thin film coating (0.0 disables) on the surface  
glass::filmindex  The index of refraction of the thin film coating on the surface 
 

Software: OS X 10.8 - Poser Pro 2012 SR2 - Luxrender 1.0RC3 - Pose2Lux
Hardware: iMac - 3.06 GHz Core2Duo - 12 GB RAM - ATI Radeon HD 4670 - 256 MB