Appropriate characterisation methods are necessary to probe the photoluminescence properties of phosphor particles, in order to develop new and more efficient materials and broaden the scope of their application. In this work, experiments show that the relative luminescence signal level of different phosphor particles obtained from studies of bulk powder bears no relation to the relative signal of the same materials when probed as dilute liquid dispersions. Characterisation of particles suspended in a liquid is shown to deliver appropriate spectroscopic data including the photoluminescence emission intensity per particle, allowing the rapid evaluation of phosphors using milligram quantities of powder. This characterisation technique is relevant to applications involving optically thin arrangements of particles as found in, for example, medical bioimaging, thin coatings for lighting, solar cell sensitisation and temperature measurements in fluids. Quantitative benchmark results are reported for the thermographic phosphors BaMgAl₁₀O₁₇:Eu²⁺, ZnO, Mg₄FGeO₆:Mn⁴⁺, (Sr,Mg)₃(PO₄)₂: Sn²⁺, Y₃Al₅O₁₂:Pr³⁺, La₂O₂S:Eu³⁺ and Y₃Al₅O₁₂:Dy³⁺.