Color display system utilizing phosphors having an electron retarding surface layer

Claim 1

In a color display system for producing colored images: a viewing screen comprising particles of a first phosphor which emit light of a first color when excited by a beam of electrons having a velocity of at least a first predetermined value, and particles of a second phosphor selected from the group consisting of zinc sulfide and zinc sulfide-cadmium sulfide, said second phosphor having a surface layer consisting essentially of zinc sulfide which constitutes a partial barrier to electrons whereby the second phosphor particles have a higher electron energization threshold than that of said first phosphor particles, said surface layer formed in situ on the particle surface by chemical reaction of said second phosphor particles having a zinc oxide layer with sulfur in the vapor phase, said second phosphor particles emitting a second color light when least a second predetermined value which is greater than said first value, and means for generating a beam of electrons to excite said phosphor particles.