The direct measurement of particle temperatures in particle-laden flows presents a unique challenge to thermometry due to the flow's transient and stochastic nature. Previous attempts to measure the bulk particle temperature of a dilute particle plume or particle curtain using intrusive and non-intrusive methods have been mildly successful. In this work, a non-intrusive method using a high-speed infrared (IR) camera and a visible-light camera to yield an indirect particle temperature measurement technique is developed and tested. The image sequences obtained from the IR camera allow for the calculation of the apparent particle temperature, while the visible-light image sets allow for the calculation of the plume opacity as a function of flow discharge position. To extract the true particle temperature, a post-processing algorithm based on Planck's radiation theory was developed. The results were validated through a series of lab-scale tests at the University of New Mexico using a test rig capable of generating particle curtains at various temperatures. The temperature profiles extracted from the methodology presented were compared to the temperature data measured during experimental measurements yielding agreement of the bulk particle temperature of the plume within 10% error. The methods described here will be developed further to estimate the heat losses from the falling particle receiver at Sandia National Laboratories.