Radiography in Modern Industry - Kodak

If the milliamperage remains constant and the x-ray intensity is varied by changing the focus-filmdistance, the compensating changes shown in Table VI should be made in the exposure time.Table VI gives a rough estimate of the deviations from the rules given in the foregoing sectionthat are necessitated by failure of the reciprocity law for exposures with fluorescent intensifyingscreens. It must be emphasized that the figures in column 3 are only approximate. The exactvalues of the factors vary widely with the intensity of the fluorescent light and with the density ofthe radiograph.When distance is held constant, the milliamperage may be increased or decreased by a factor of2, and the new exposure time may be calculated by the method shown in "Time-DistanceRelation", without introducing errors caused by failure of the reciprocity law, which are serious inpractice.LogarithmsSince logarithms are used a great deal in the following section, a brief discussion of them isincluded here. Some handbooks and intermediate algebra texts give a more detailed treatment.Before discussing logarithms, it is necessary to define the term "power". The power of a numberis the product obtained when the number is multiplied by itself a given number of times. Thus, 10= 10 x 10 = 1000; 5 = 5 2 x 5 = 25. In the first example, 1000 is the third power of 10; in thesecond, 25 is the second power of 5, or 5 raised to the second power. The superscript figure 2 isknown as the exponent. Fractional exponents are used to denote roots.The common logarithm of a number is the exponent of the power to which 10 must be raised togive the number in question. For example, the logarithm of 100 is 2. The logarithm of 316 equals2.50, or log 316 = 250; the logarithm of 1000 equals 3, or log 1000 = 3. It is also said that 1000 isthe antilogarithm of 3 or antilog 3 = 1000.Radiography in Modern Industry 63