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A Brief Word on Reciprocity

Peter A. Jenkins, MBA

The 3rd Law of Photobiology (a.k.a. the Reciprocity Rule and the Bunsen-Roscoe Law) states that “the effects of irradiation will be independent of irradiance and exposure time as long as radiant exposure is maintained”, i.e. the differences in power and irradiation time don’t matter, it’s the dose that’s important.

However, while reciprocity may hold true in many areas of photochemistry (the field in which Bunsen and Roscoe worked in the 19th century), as far back as 1992 questions were raised as to whether the 3rd Law applied to laser therapy.

Since then, numerous papers have provided evidence to suggest that reciprocity does not hold true for laser therapy, except over very narrow ranges. As Schindl et al state, “responses of cells and tissues to electromagnetic radiation usually involve a sequence of interacting biological reactions making a linear dose-time relationship less likely”.

In short, the dose is actually of less importance in determining the outcome of laser therapy than the power/intensity of the laser and the irradiation time.

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