Coated narrow band green phosphor

A method of forming a coated phosphor, comprising:

• providing phosphor particles comprised of a phosphor material with composition (M)(A)2S4:
• Eu wherein:
• M is at least one of Mg, Ca, Sr and Ba;
• and
• A is at least one of Ga, Al, In, Y;
• and
• depositing a dense impermeable coating of an oxide material encapsulating individual ones of said phosphor particles by a gas phase process in a fluidized bed reactor;
• wherein said coated phosphor is configured to satisfy at least one of the conditions:
• (1) under excitation by blue light, the reduction in photoluminescent intensity at the peak emission wavelength after 1,000 hours of aging at about 85° C. and about 85% relative humidity is no greater than about 30%;
• (2) the change in chromaticity coordinates CIE(y), ΔACIE y, after 1,000 hours of aging at about 85° C. and about 85% relative humidity is less than about 5 ×10−3;
• (3) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least two hours at 85° C.;
• (4) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least one day at 20° C.;
• and
• (5) said coated phosphor does not turn black when suspended in a 1 mol/L silver nitrate solution for at least 5 days at 20° C.