Optical windows based on a multi-period master-slave nested ring array of concentric rings are suited for electromagnetic shielding. A metal grid of the ring array has basic rings, concentric sub-ring pairs, secondary sub-rings, filling rings, concentric modulation ring pairs, and modulation sub-rings. Basic rings and concentric modulation ring pairs form a two-dimensional orthogonal array. External rings of concentric modulation ring pairs are externally tangentially connected to basic rings. Concentric sub-ring pairs and filling rings are arranged within basic rings, secondary sub-rings are arranged within concentric sub-ring pairs, and modulation sub-rings are arranged within concentric modulation ring pairs. Where rings are tangentially connected, wires overlap or metal ensures reliable electrical connections between connected rings, thus all rings are conductive. The metal grid structure significantly reduces non-uniformity of grid high-order diffracted light intensity distribution, causing stray light distribution caused by diffraction to be more uniform and imaging to be less affected.