Optical properties

what is the meaning of the REFLECTIVITY and EFFICIENCY in the optical properties of the surface?
G4double reflectivity[num] = {0.3, 0.5};
G4double efficiency[num] = {0.8, 1.0};

G4MaterialPropertiesTable *myST2 = new G4MaterialPropertiesTable();

myST2->AddProperty(“REFLECTIVITY”, ephoton, reflectivity, num);
myST2->AddProperty(“EFFICIENCY”, ephoton, efficiency, num);

The outcomes of the REFLECTIVITY and EFFICIENCY will depend on the optical surface model applied to the volume. Details regarding the surface models and boundary processes can be found here in the application developers guide.

Both these parameters are energy-dependent so they can be fine-tuned to be different wavelengths of the optical photons in a simulation. In general REFLECTIVITY defines the probability of reflecting so is a value between 0 and 1. Whereas EFFICIENCY is the probability of being absorbed at the surface, also taking values between 0 and 1. With 1 being 100% absorption. Transmission occurs if neither absorption or reflectance occurs. Although all of this is all dependent on the model and you could read the G4OpBoundaryProcess.cc and G4OpBoundaryProcess.hh source code to gain a greater understanding by looking at theReflectivity and theEfficiency variables.

I hope this provides some information into what these properties do. If you want further reading for optical surface parameters I recommend the following:
Peculiarities in the Simulation of Optical Physics with Geant4
Section 7.2.5 of E. J. Leming thesis

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