A mercury vapor discharge fluorescent lamp is provided having a phosphor layer that has a yttria-based red emitting rare earth phosphor. The phosphor layer is prepared from an aqueous phosphor slurry that is coated onto the inner surface of the glass envelope of the lamp, then baked in a lehring oven to vaporize or decompose non-phosphor components of the slurry to leave behind the finished phosphor layer. The phosphor slurry contains a non-ionizing thickener, preferably polyethylene oxide, but does not contain any calcium carbonate or carbonate salts. The phosphor layer includes a yttria-based red emitting rare earth phosphor.