I am in the process of designing an external PSU but maths not being my strong point, I need some help with a formula.

This formula is for working out the efficiency with an active on state to see if it would comply with the latest EU regulations.

The page in question is:

Efficiency Standards for External Power Supplies | DigiKey

The formula is as follows (this is based on a 1W to 49W PSU), according the the page above this formula is to calculate the efficiency as a decimal number, what I don't understand & they make no mention of, is what is ln? & what would Pout be?

0.0834 × ln(Pout) - 0.0014 × Pout + 0.609

Sure hope somebody can help with this, its probably obvious but I just can not see it