 A modern formulation of Schwarzschild's law is given as

where "E" is a measure of the "effect of the exposure" that leads to changes in the opacity of the photosensitive material (in the same degree that an equal value of exposure "H = It" does in the reciprocity region), "I" is illuminance, "t" is exposure duration and "p" is the "Schwarzschild coefficient".However, a constant value for "p" remains elusive, and has not replaced the need for more realistic models or empirical sensitometric data in critical applications.When reciprocity holds, Schwarzschild's law uses "p" = 1.0.