The Benefits of Small Focal Spot Size in Radiography – NDT

The focal spot size has a direct correlation to image resolution and the ability to detect or see small defects.

Geometrical Un-Sharpness

The geometrical un-sharpness (Ug) results in blurring of details in a X-ray image as the source X-rays originate from a surface rather than a single focal point (fig. 1).  Figure 2 shows in more detail how radiation from the source reaches the detector/film.  As can be seen, the edges of the object will produce a blurred area in the detector, resulting in geometric un-sharpness.

Comparison of focal spot size on image sharpness NDT radiography
Figure 1. Comparison of focal spot size on image sharpness.

Geometric un-sharpness is determined by:

  1. The size of the focal spot (f)
  2. Distance between object and the source (a)
  3. Distance between the object and detector/film (b)

Where Ug = f x (b/a)

geometric unsharpness in radiography
Figure 2. Schematic illustrating how un-sharpness (Ug) is calculated.

Reducing Geometric Un-sharpness

One way of decreasing Ug is to decrease the focal spot size (f) from say 3mm down to 1mm, which results in a 3-fold improvement in the geometric un-sharpness.

Reducing Exposure Time

Dose rate (l) is inversely proportional to the square of the distance from the source to the detector per:

l = 1/d2

Similarly to Ug, the exposure time is proportional to the focal spot size and the dose rate.  So, by decreasing the spot size from 3mm to 1mm, the exposure time will be one-third as long, allowing you to take more shots in a given time.


Yxlon manufacture a range of tube heads with small focal spot size of 1.0mm.  As shown above, using a small focal spot system will increase the image sharpness/resolution enabling you to more accurately see smaller features.  These systems will also reduce your exposure time increasing your workflow, efficiency and throughput and hence your earning capacity.


Yxlon portable radiography unit - tube head
Weight (kg)2629
Height (mm)708774
Focal spot size (EN 12543)1.0mm1.0mm
High voltage adjustment (kV)40-22550-300
mA adjustement0.5-5.50.5-4.5
MAX X-ray power (W)900900
Beam angle (°)30x6030x60
Leakage radiation (mSv/h)5.05.0
Temp. range (°C)-20 to +50-20 to +50
Cont. exposure 35°C, MAX kV/mA (hr)Min. 1Min. 1
Water-cooled option availableYesYes