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The Persistent Pigment Darkening (PPD) method is a method of measuring UVA protection, similar to the SPF method of measuring UVB light protection. Originally developed in Japan, it is the preferred method used by manufacturers such as L'Oreal.
The purpose of melanin is to absorb UV radiation and dissipate the energy as harmless heat, blocking the UV from damaging skin tissue.
UVA gives a quick tan that lasts for days by oxidizing melanin that was already present and triggers the release of melanin from melanocytes. UVB on the other hand yields a tan that takes roughly 2 days to develop because it stimulates the body to produce more melanin.