A brief description is given of the rigorous formulism of Chen, which describes the diffraction by perfectly-conducting inductive grids. The formulism is used to prove several general properties of grids, including the relevant form of the Reciprocity Theorem. The theory is used to investigate the equivalent circuit model proposed by other authors for thin grids, and also to derive a monomodal model of the type first proposed by Chen. The latter model is shown to be useful even in the region where more than one spectral order propagates.
The accuracy of the formulism is established by the comparison of calculated results with a number of far-infrared measurements on grids. The use of grids as solar-selective elements is investigated. They are shown to be capable of providinga/e ratios of the order of 30–40, provided that they are always pointed towards the sun, and that the diffuse content of the illumination is low.