To obtain Vacuum ultraviolet (VUV) phosphors with quantum efficiencies high
er than 100%, the concept of downconversion is used. In a downconversion pr
ocess a VUV photon is split into two visible photons by making use of energ
y transfer between different rare earth ions. Two examples of downconversio
n couples are discussed, viz. the Gd3+-EU3+ couple and the Er3+-Gd3+-Tb3+ s
ystem. (C) 2000 Elsevier Science S.A. All rights reserved.