LW-6041 - The Myth of Reciprocity

Reciprocity is the notion that if you increase irradiance in a weathering test, you can decrease your exposure time by a reciprocal amount and still get the same results. Said another way, reciprocity states that the amount of degradation in a weathering exposure is strictly proportional to total light dosage (i.e. irradiance x time), regardless of how much time it took to deliver that dosage. For example, you get the same total dosage from an irradiance of 1.1 W/m 2 for 1,000 hours as you get from an irradiance of 0.55 W/m 2 for 2,000 hours. So one might assume that the two exposures would produce the same degradation. However, this is almost never the case.

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