The relationship between the emission from the exciton reservoir and the population distribution of the lower polariton can be directly visualized by measuring the photoluminescence energy dispersion of the microcavity at a series of temperatures. Figure shows the PL dispersion at 200 K, where the increased PL intensity at larger in-plane wave-vectors indicates radiative pumping, as it suggests that a greater population of polaritons is concentrated at the energy overlap between the exciton reservoir emission (inset) and the lower polariton dispersion. This is characteristic of polaritons populated via radiative pumping, followed by inefficient relaxation toward �𝑘𝑘 ∥ ���⃗ � = 0, in agreement with previous reports . At 5 K (Figure ), the apparent bottleneck at larger in-plane wavevectors disappears, and the lower polariton branch gets populated at smaller �𝑘𝑘 ∥ ���⃗ � states. As discussed by Laitz et al., the disappearance of the apparent polariton bottleneck at lower temperatures is not a consequence of increased scattering events down the polariton dispersion, but rather a result of radiative pumping from the more defined exciton reservoir at lower temperatures. This is evident from the inset of Figure , where the steady state PL of the uncoupled perovskite film is shown to radiate at lower energies, directly feeding polariton states around �𝑘𝑘 ∥ ���⃗ � = 0. A more complete temperature dependence of the emission of the (PEA)2PbI4 bare film is shown in Figure , clearly showing the spectral structure that becomes defined at lower temperatures.