Since the dimensions of the radiation source have considerable effect on the sharpness of the shadows, it is frequently desirable to determine the shape and size of the x-ray tube focal spot. This may be accomplished by the method of pinhole radiography, which is identical in principle with that of the pinhole camera. A thin lead plate containing a small hole is placed exactly midway between the focal spot and the film, and lead shielding is so arranged that no x-rays except those passing through the pinhole reach the film (See Figure 16).
The developed film will show an image that, for most practical radiographic purposes, may be taken as equal in size and shape to the focal spot (See Figure 17). If precise measurements are required, the measured dimensions of the focal-spot image should be decreased by twice the diameter of the pinhole.
Figure 16: Schematic diagram showing production of a pinhole picture of an x-ray tube focal spot.
The method is applicable to x-ray tubes operating up to about 250 kV. Above this kilovoltage, however, the thickness of the lead needed makes the method impractical. (The entire focal spot cannot be "seen" from the film side of a small hole in a thick plate.) Thus the technique cannot be used for high-energy x-rays or the commonly used gamma-ray sources, and much more complicated methods, suitable only for the laboratory, must be employed.
A focus-film distance of 24 inches is usually convenient. Of course, the time of exposure will be much greater than that required to expose the film without the pinhole plate because so little radiation can get through such a small aperture. In general, a needle or a No. 60 drill will make a hole small enough for practical purposes.
A density in the image area of 1.0 to 2.0 is satisfactory. If the focal-spot area is overexposed, the estimate of focal-spot size will be exaggerated, as can be seen by comparing the two images in Figure 17.
Figure 17: Pinhole pictures of the focal spot of an x-ray tube. A shorter exposure (left) shows only focal spot. A longer exposure (right) shows, as well as the focal spot, some details of the tungsten button and copper anode stem. The x-ray images of these parts result from their bombardment with stray electrons.
