Assuming that the skin depth of the metal being used in the grids is much smaller than the thickness of the grid, the real part of the surface impedance of the metal is ρ ρ = 1 / δ δ σ σ where σ σ is the conductivity of the metal and δ δ is the skin depth of the metal. With a reflected wave Γ Γ ( ω ω ) , the change in the magnetic field amplitude across the grid is 2 Γ Γ ( ω ω ) because of surface currents on both sides of the grid. The average surface currents on both sides of the grid are J ¯ ¯ = Γ Γ ( ω ω ) ∗ ∗ c / 4 π π .