Given the average surface current and the surface impedance, we could calculate the power dissipated as P D = 2 ρ ρ J ¯ ¯ 2 . However, because the actual extent of the metal in the grids is different between the capacitive and inductive grids and a flat sheet of metal, we need to introduce a factor η η which is the ratio of the area the grid to that of a flat sheet. For capacitive grids, η η = g / 2 a and for inductive grids η η = 1 / ( 1 − − 2 a / g ) . This modifies the power dissipated to be P D = 2 ρ ρ η η J ¯ ¯ 2 . Using the definition of skin depth, the unitless absorptivity, A = P d / P o where P o is the incident power, of the grid is