A method is presented for the experimental determination of the modulation transfer functions of x-ray focal spots. The apparatus described is sufficiently flexible to permit the measurement of these functions under' various conditions of x-ray tube operation and at various positions and directions in a plane perpendicular to the direction of the x-ray beam. The results of measurements on four tubes used in the United States are discussed.Data on two tubes are given in detail with the theoretical functions expected from a focal spot having uniform emission and the dimensions specified by the manufacturer.Phase transfer functions me calculated from the measurements and their significance in radiography is discussed.MTF of the focal spot from the MTF of a group of components in the system.

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