At radio wave‐lengths the use of mirrors and lenses in the measurement of radiators, by methods which are customary in physical optics, is possible only if errors, due to incidental diffraction from the relatively small apertures of such apparatus, can be corrected by calculation. Theory is developed in this paper from which these calculations can be derived for linear radiators. A new relation which expresses the Fresnel field in terms of the distant field is fundamental to the method. Satisfactory agreement has been obtained between the predicted and measured distant fields of several electromagnetic horns whose near fields were obstructed; the dispersion between measured and predicted values is given as under one‐half decibel but the details of the experiment are not reported.