What is the purpose? for reflectance, it is only along one direction. Fields to other directions are bounced back due to periodicity.
If it is not periodic, you can integrate the far field power inside a cone. But non-periodic structures do not have fixed reflectance, as the result will change depending on source span.
Please rerfer:
Ansys Insight: 周期结构结果分析：透射反射率、散射、远场、角分布、近距场分布、位相提取等 综合帖
Ansys Insight: 非周期结构散射的远场分析：散射截面、远场分布、斜入射 综合帖
Ansys Insight: 为什么非周期结构一般不用平面波照明，不评定反射率和透射率？
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