FOCAL SPOT CHARACTERISATION

Radiografic pinhole camera procedure according to EN 12543-2:

The quality and the resolution of X-ray images are highly dependent from the characteristics of the focal spot, especially from its size and from the two-dimensional allocation of the intensity. The focal spot is defined as X-ray emitting area on the target of the X-ray tube and below the perspective of the pinhole camera. This standard attends to the exposure of focal spot dimensions larger then 0.2 mm for X-ray sets up to 500 kV tube voltage.

Measuring fixture:

Pinhole camera: The aperture of the camera has to be made from tungsten or a higher absorbing material. Subject to the current focal spot size the aperture of the camera needs to have the following dimensions:
Focal spot size:

0,2..1,0 mm

> 1,0 mm
Diameter "P":

30 + 5 µm

100 + 10 µm
Height "H":

75 + 10 µm

500 + 10 µm

  Note: The angle between reference and aperture axis has to be adjusted to a value smaller 3°.

Geometry of aperture Allignment of pinhole camera

  

Reproduction geometry at the pinhole camera:

 The distance between aperture and the focal spot has to be adjusted that way that the change of the geometrical enlargement above the extent of the current focal spot does not exceed 5 % in reference direction.

By no means the distance is permitted to be smaller then 100 mm. The X-ray film has to be positioned vertically towards the reference axis. At the same time, according to the picture on left, a distance has to be set that assures the geometrical enlargement to be applied.

  

Reproduction demand for focal spot radiographs:

 
Focal spot size:

0,2..1,0 mm

> 1,0 mm
Minimum Enlargement: n/m: 3 : 1 1 : 1
Load data: The X-ray tube voltage has to average 75 % of the nominal voltage but may not exceed 200 kV. The X-ray tube current has to be set to the maximum value possible for the voltage chosen.
Measurement: The X-ray film has to be evaluated with the aid of a film viewer with a permanent luminance of 2000 to 3000 cd/m. The inspection takes place visually with the help of a magnifying glass (magnification x10) with a build-in scale with 0.1 mm resolution. The focal spot size to be specified is defined as the visible extension of the blackened area, divided by the magnifying factor.
Evaluation: Every focal spot size is characterised through its lenght l in direction of the X-ray tube axis and its width w transverse to it. The bigger of both dimensions is appointed as "Focal Spot Size d". A specification of the focal spot size "d" on the X-ray tube has to correspond to this standard.

 

 

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