In the past decade quantum cutting phosphors with a visible quantum yield over 100% were investigated. This article describes a method to calculate the maximum visible quantum yield for photon cascade emission of Er3+ using Judd-Ofelt theory. A maximum visible quantum efficiency of 112% was calculated. A significant part of the emission intensity is situated in the ultraviolet. If an ion can be co-doped that efficiently converts this ultraviolet into visible light, visible quantum efficiencies between 120% and 190% may be achieved. (C) 2004 Published by Elsevier B.V.