• A model that uses physical properties that can be looked up in the CRC Handbook of Chemistry and Physics (indices of refraction, reflectivity, conductivity, etc.)
  
  
  
• Parameters that that have clear physical analogies (how rough or polished a surface is)
• Models that are predictive (the simulation attempts to model the real scene)
• Models that conserve energy
• Complex surface substructures
  (crystals, amorphous materials, boundary-layer behavior)
• If it was easy... everyone would do it.