Hi MANG2004, u[=r,^YQ
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I am curious about your calculation, too. Plus, the definition of σ refers to 1/e^2 width not the FWHM. Even your calculation/defintion is correct. I have a doubt that it will work when the specualr incident ray angle is not normal......... u.m[u)HQ
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For thsoe who knows it better, please have some input. I am rather curious, too!! Thanks!!