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How is PPD and/or PA+(++) calculated on SPF

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almost 3 years ago

So we all know the SPF represent the length of time one can stay out of the sun before one will burn. Which is usually blamed on UVB rays. But for those of us who are more concern with lightening or reducing dark spots and anti-aging, we might be more concern with UVA rays which SPF tells you nothing about. But Europe and Asia both came up with systems to clearly convey the UVA protection PPD (Persistent Pigment Darkening) and PA+(Protection Grade of UVA). What I want to know are these rating systems based of length of time or are they strength of protection from the UVA rays? How did they come up with the numbers since burning factor is not focus? and How does one use these numbers to determine what level to use lightening, anti-aging, or reduce tanning?
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