# Need Help for DIFFUSE FRACTION values

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 December 6, 2007, 14:12 Need Help for DIFFUSE FRACTION values #1 Zoran Guest   Posts: n/a Hello, I have to find values for the "Diffuse Fraction" that are needed in my CFX-Radiation simulation. Diffuse fraction is defined as: ratio of the diffuse reflectivity to the sum of the diffuse and specular reflectivity. DF = rho_d / (rho_s + rho_d) <= 1.0 with rho_s + rho_d = 1 The materials are: - polished steel - plexiglas - ZnSe - 3M black velvet paint (blackbody) I made an estimation considering the surface roughness and the emitted radiation wavelength @ 380 K. When the roughness is much smaller than the wavelength, specular reflection is dominant, otherwise diffuse reflection. BUT what is the order of magnitude??? My assumptions are: DF_steel = 0.1 --> rho_s / rho_d = 9 DF_plexi = 0.05 --> rho_s / rho_d = 19 DF_ZnSe = 0.05 DF_blackpaint = 1 --> rho_d = 1, rho_s = 0 Are these values somehow plausible? What are the values you use for your radiation simulation??? A keyword for this could be "Bidirectional Reflectance Distribution Function" (BRDF) Can someone help please?! Thanks in advance! Zoran

 August 16, 2009, 12:59 answer for seting up of reflectivity #2 New Member   limin Join Date: May 2009 Posts: 21 Rep Power: 10 diffuse reflectivity = diffuse fraction(1-internal emissivity) specular reflectivity = 1- diffuse fraction absorptivity= diffuse fraction* internal emissivity

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