Any milliamperage multiplied by an exposure time setting that gives the same milliamperage second outcome should give the same relative density to an image. However, this law is dramatically affected by the image receptor response curve, especially when it is not a 45° linear curve. In radiographic intensifying film and screen technologies, the reciprocity law does not hold at long exposure times because of the reversal of the D log E response curve.
Bunsen and Roscoe established the equation E = It, which signifies that the exposure is equal to the product of intensity and time. For instance, an exposure of 20 mA·s can be achieved through various combinations, such as 200 mA for 0.1 sec or 20 mA for 1.0 sec. In reality, there exists an infinite number of combinations of intensity and time that can yield an equivalent 10 mA·s and produce the same effect on the film.