Understanding NRT- Reading 1- 2 of 2- Radiogaphic Testing A
Understanding nrt reading 1- 2 of 2- radiogaphic testing a
Understanding nrt reading 1- 2 of 2- radiogaphic testing a
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THE RECIPROCITY LAW<br />
In the sections immediately preceding, it has been assumed that exact<br />
compensation for a decrease in the time <strong>of</strong> exposure can be made by<br />
increasing the milliamperage according to the relation M1T1 = M2T2. This<br />
may be written MT = C and is an example <strong>of</strong> the general photochemical law<br />
that the same effect is produced for IT = constant, where I is intensity <strong>of</strong> the<br />
radiation and T is the time <strong>of</strong> exposure. It is called the reciprocity law and is<br />
true for direct x-ray and lead screen exposures. For exposures to light, it is<br />
not quite accurate and, since some radiographic exposures are made with the<br />
light from fluorescent intensifying screens, the law cannot be strictly applied.<br />
Errors as the result <strong>of</strong> assuming the validity <strong>of</strong> the reciprocity law are usually<br />
so small that they are not noticeable in examples <strong>of</strong> the types given in the<br />
preceding sections. Departures may be apparent, however, if the intensity is<br />
changed by a factor <strong>of</strong> 4 or more. Since intensity may be changed by<br />
changing the source-film distance, failure <strong>of</strong> the reciprocity law may appear to<br />
be a violation <strong>of</strong> the inverse square law.<br />
Charlie Chong/ Fion Zhang
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