Abstract: A method of preparing a halophosphate phosphor of controlled particle size with includes the step of compacting the calcium hydrogen orthophosphate di-hydrate crystals. The steps in the method include precipitating the calcium hydrogen orthophosphate di-hydrate crystals from a fluid formed by mixing a calcium chloride solution or a calcium nitrate solution and a diammonium phosphate solution, separating the precipitated di-hydrate crystals from the remainder of the fluid, compacting the di-hydrate crystals, converting the di-hydrate into de-hydrate crystals, preparing a raw mix containing the de-hydrate crystals, and firing the raw mix to form the halophosphate phosphor. The di-hydrate crystals may be separated from the remainder of the fluid by filtering, in which case, the compacting is preferably achieved by pressing the crystals against the filter.