|The photograph shows the anode of a
stationary anode tube, taken using a pinhole camera. X-rays cannot be focused,
but an image such as this can be produced using a sheet of lead, with a
small hole in it. The lead plate is positioned about half way between
the X-ray tube and the film. In this example, the pinhole camera was
positioned at about 45º to the anode. The area of the target bombarded
by the electron beam is rendered an maximum density black, but the
photograph has been reversed in this example.
About 20% of the electrons striking the target, bounce off, to land elsewhere on the anode. These electrons give rise to soft radiation, which forms the image of the ghostly outline of the anode shown here.
The effect of this extra-focal radiation reaching the film is to reduce the contrast of the image, and so manufacturers of high-end equipment produce beam collimators that incorporate additional blades positioned close to the tube window, in order to reduce the extra focal radiation to a minimum.
In order to get a reasonably sharp image a lead plate with a small conical hole is needed. For best results the plate needs to be a little bigger than the film used. A fast film/screen combination will reduce the exposure time, although exposures in the order of minutes will be needed. The kV must be kept low, in order to prevent too much radiation penetrating the lead plate.