<html><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><META name="Author" content="Novell GroupWise WebAccess"></head><body style='font-family: Tahoma, sans-serif; font-size: 13px; '>Hello all, I am attempting to factor in the transmittance of optical photons through different light guides. I suppose I will have to change the absorption length parameter in the Materials.xml file, but it's not quite the same. I want to be clear with how GATE uses the absorption length. So: <propertyvector name="ABSLENGTH" unit="m" energyunit="eV"> <ve energy="1.84" value="50"/> <ve energy="4.08" value="50"/> </propertyvector> Does this mean that for an optical photon of 1.84eV, it will travel about 50m before it is absorbed? Is this an average depth value? Is it based on a Gaussian probability distribution? Transmission of optical materials is normally given as a fraction of the incident light, at a given wavelength of light and for a given thickness of material. For example, say 92% transmittance of 405nm photons for a 1.0mm thick lead glass lightguide. Can anyone offer any insight as to how GATE uses these ABSLENGTH values? <div style='clear: both;'>Cheers, Devin Van Elburg ------------------------ Physics w/ c/ Biomedical Physics Lakehead University</div> </body></html>