One of the specifications and chief features of an x-ray tube is its focal spot size, a very important factor in the resolution to be achieved during a radiologic examination. A small focal spot size will provide the maximum resolution. However, there are other factors in the construction of an x-ray tube, such as the heat dissipation within the target, which limits the minimum size of the target.
In order to determine the focal spot size of an x-ray tube, a small and precise pinhole is often used. Its size must be very small compared to the focal spot to be measured. Any of four precision pinholes can be used with our Model 07-623 and 07-622 test stands. The test stand height is adjustable in order to maintain the minimum magnification factor of two.
A fluoroscopic screen is part of the test stand and is used for centering the focal spot before exposing the film. The 0.010 mm pinhole diameter is for focal spot sizes from 0.5 to 0.10 mm; the 0.030 mm pinhole diameter is for focal spot sizes below 1.0 mm; the 0.075 mm diameter is for focal spots from 1.0 to 2.5 mm; the 0.1000 mm diameter is for sizes above 2.5 mm. The pinhole diaphragm is made from a 90:10 gold-platinum alloy.