Some Considerations of Focal Spot Sizes

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Abstract
It is well known that the focal sizes of diagnostic rotating anode tubes are larger than the nominal sizes. This state of affairs was well established before rotating anode tubes were manufactured in this country, and has been accepted by common usage universally since the war. This regrettable state of affairs calls for some universally acceptable solution and the purpose of this paper is to put forward a proposal which we think will be practicable and acceptable to all concerned. Firstly, it is necessary to clear up some common misconceptions regarding focal sizes; the most common being that foci are what they are because manufacturers do not know how to do any better. This is most certainly not true. Reputable X-ray manufacturers design and manufacture rotating anode tubes with foci within predetermined close limits. The fact is that the present sizes for foci in common use were determined by the demand for X-ray apparatus working at high currents. We believe that it was this almost insatiable appetite for higher and higher loadings which compelled the manufacturers to enlarge their foci. It should be pointed out that these foci, although larger than nominal, have been used successfully for about 15 years, not because users had no alternative, but because they chose to use them. Surely as most rotating anode tubes are double focus tubes, users for many years have had the option to use the smaller focus for all diagnostic work; in fact, of course, nothing of the sort has occurred.

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