Recent studies have shown that thermographic phosphor particles are suitable tracers for simultaneous temperature and velocity imaging in gas flows. For the development of this thermographic particle image velocimetry technique, the choice of phosphor is a key consideration and new phosphor materials need to be identified and their luminescence properties thoroughly characterised. This paper describes a method for investigating phosphor particles for gas thermometry. The method consists of performing spectroscopic experiments on particles dispersed in the gas, while simultaneously measuring the number density of tracer particles using a particle counting system, which is based on high-resolution Mie-scattering images. The measurement of the seeding density allows a direct comparison of different phosphors on a “per particle” basis and estimation of the effect of the particles on the gas thermal properties. The effect of parameters such as the excitation fluence, gas composition and temperature on the luminescence emission intensity can be studied independently of the seeding density. As a demonstration, this system is used to investigate micron-size BAM:Eu²⁺ particles. The necessary seeding density for precise temperature measurements is determined, and a nonlinear dependence of the luminescence emission intensity on the excitation fluence is identified.