The Falling Particle Receiver (FPR) at the National Solar Thermal Test Facility (NSTTF) is a testbed for promising receiver technologies offering solutions to the temperature and irradiance limitations exhibited by gas and molten salt receivers, since the particle curtain is directly irradiated without the need of containment. Until recently, the heat loss of the NSTTF 1 MWth FPR was not fully characterized. One of the challenges of the FPR characterization is the intricate flow conditions that the particle curtain experiences due to its cavity design with a single open aperture, to allow the direct irradiance. Recently, particle plumes expelled from the FPR during operation were observed. While this phenomenon affects the FPR heat loss and needs to be closely monitored, it is extremely difficult to operate any kind of sensors near the aperture of the FPR. This work describes the development of a methodology using a high-speed IR camera, located ≥ 5 meters away from the aperture, to estimate the opacity of a particle plume, which in turn can be used to extract the average particle temperature of a region of interest with a known background temperature. Experiments performed at the University of New Mexico using four different flow configurations and three different temperatures (200, 450, and 750°C) were conducted to determine the relationship between the plume opacity in the visible range and the “particle-pixel” opacity obtained from thermograms in the IR range. We present a “particle-pixel function” that describes the combined impact of an unknown number of particles at a specific temperature on a thermogram pixel value with an initial value equal to the background temperature. The novelty of this function is that it provides a reasonable estimate of the plume opacity using thermograms obtained from the IR camera; hence a bulk particle temperature can be obtained. Future development of this methodology will make it possible to compute the advective losses from the FPR and provide a first order approximation of the convective losses for the system.