Purpose: The purpose of the research is to achieve the same degree of blackening of the finished radiograph with different exposure factors by means of measurements, and to check various information from the available literature which argue that in order to achieve the same level of blackening, the 15 % principle has to be followed. . Working methods: We began with the literature review, followed by measurements on a water phantom. We preserved the same level of blackening and contrast while changing the exposure factors. We recorded the dose rate and measured the exposure index. Results: The results show derogations in the application of the ± 15 % rule, mostly in cases where the value of the acceleration voltage increases by 15 % of the initial value, while the product of the peak current and the exposure time is reduced by half its initial value. Variations occur less frequently in cases where the value of the accelerating voltage is reduced by 15 % of its initial value, while the product of the peak current and exposure time is increased by half of the initial value. Discussion and conclusion: The derogations from the ± 15% rule occur because the dose is increasing exponentially with kVp. For that we updated the recommended values of the acceleration voltage, using the percentage that would be appropriate in practice in order to provide a constant degree of blackening while changing the exposure factors.
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