How is cut grading done on Princess cuts?

The American Gem Society Laboratories is the only major lab performing overall cut grading on Princess cuts. A true Ideal cut princess is one that has been certified as an AGS0. All A CUT ABOVE® Princess diamonds have AGS0 Platinum certifificates.

Read about light performance grading of princess cuts in this summary article on Cut Grading of Princess. The article contains a link to the full article for those looking for more technical details.