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This document specifies a method for the measurement of effective focal spot dimensions above 0,2 µm of X-ray systems by means of the edge method applied to digital images taken from hole type or disk type gauges if no phase contrast is observed. The imaging quality and the resolution of X-ray images depend highly on the characteristics of the effective focal spot, in particular the size and the two-dimensional intensity distribution as seen from the detector plane. This document provides instructions for determining the effective size (dimensions) of standard, mini and micro focal spots of industrial X-ray tubes for users in applications where the pin hole method of ISO 32543-1 is not practicable. This determination is based on the measurement of a profile of an image of a hole or disk type gauge. The method as described in this document can be used for long term monitoring of focal spot sizes without a pin hole camera. The accuracy of this method is lower than the one of ISO 32543-1, ISO/DIS 32543-3 and the future standards in preparation, for measurements of the effective focal spot size, using ASTM hole plate IQIs (as described in ASTM E 1025, E 1742), due to its manufacturer tolerance of ±10 % of the hole diameter. It can be used by manufacturers if special hole gauges, manufactured with lower tolerance, as described in 5.2.1 are applied (see Figure 1).