Active lava surfaces emplaced during effusive eruptions are typically observed in the form of lava flows, lakes, or domes. There are numerous volcanoes on Earth that regularly produce active lava including Kīlauea volcano in Hawai'i, which had a continually-active lava flow field from the 1980s until the end of the last eruptive phase in 2018. During the last decade of that period, it also had a persistent lava lake at the summit. This study used a new ground-based Miniature Multispectral Thermal Camera (MMT-Cam) imaging system to acquire highspatiotemporal data of active lava surfaces at Kīlauea volcano in 2018. The goal was to quantify the short-term variability of the radiative properties (e.g., emissivity, kinetic temperature, and heat flux) of molten lava during cooling and propagation using concurrent in situ multispectral thermal infrared (TIR) measurements. As expected, there was a strong positive correlation between kinetic temperature, fraction of exposed melt, and heat flux. Importantly, there is also a negative (or inverse) correlation found for the emissivity of lava surfaces with temperature during cooling immediately following emplacement. The temporal results reveal low- and high- frequency (10s of minutes to seconds) changes in radiative properties, up to 25% and 5% variability, respectively. The spatial analysis provided insights into emplacement dynamics and activity potential through the interpretation of the improved heat flux measurements. For example, these data highlighted lava lake over- turning, subsurface supply pathways, and lava flow breakout zones. Additionally, the first, calibrated in situ emissivity measurements of actively-emplaced lava indicated a lower efficiency of radiant heat flux from molten lava surfaces prior to a viscoelastic crust forming, which results in lower calculated heat fluxes than previous estimates. This implies that prior heat budget calculations and lava flow propagation models may have overestimated heat flux by at least 20% from the molten lava prior to cooling and crust formation. Therefore, the results of this study allow for more accurate calibration of prior calculations and models, resulting in more accurate values and a reduction in the uncertainty of hazard models reliant on these values.