ASTM E2903 Effective Focal Spot Size of Mini and Micro Focus X-ray Tubes

ASTM E2903 test method gives instructions for measuring the effecting size (dimensions) of industrial X-ray tube’s micro and mini focal spots. The resolution of X-ray images dramatically depends on the characteristics of the focal spot. The values stated in SI units are to be regarded as standard.

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    The ASTM E2903 method is advantageous as it allows users to do geometric calculations to determine the practical focal spot dimensions of the X-ray source. The results of this method are used for maintaining the desired maximum magnification possible or degree of geometric unsharpness, or both, for a given radiographic imaging application.

    Test Procedure:

    The ASTM E2903 method is based on indirect measurement of the focal spot size by determining the geometric unsharpness, then using a geometric calculation to determine the effective focal spot dimensions. For this purpose, edges are imaged either on a film or a radioscopic or digital radiographic device using a relatively high geometric magnification.

    Specimen size:

    The specimen’s focal dimension must be from 5 μm up to 300 μm with 225 kV tube voltage.


    The following formula is used in ASTM E2903 to determine the geometrical magnification,

    ML, W  = DL,W /Deal 

    where: DW and DL are the diameters of the test object in the width and length direction and Dreal is the real diameter.


    The ASTM E2903 method is advantageous as it allows users to do geometric calculations to determine the practical focal spot dimensions of the X-ray source.


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