[12.4] Where a photochemical reaction exhibits reciprocity between irradiance (exposure dose rate, W/m2) and exposure duration over some range of time. In other words, a given response results from a given radiant exposure (J/m2) over a wide range of exposure times. Reciprocity may also fail. For very short duration, failure may be due to photon saturation.  For long duration, failure may be due to repair mechanisms or recombination.

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