The transmission of light through the photonic lantern was investigated by injecting light of different numerical apertures (NAs). Light at 550nm was collimated and projected through an adjustable iris. The light was then focussed using a plano-convex lens (Thorlabs LA1951-A, f = 25.4mm) onto the multimode port of the lantern, which was attached to a 20 m length of multicore fibre. The NA of the injected light was controlled by adjusting the size of the iris. The total throughput, the magnitude of which includes contributions from the multicore fibre propagation loss, input coupling loss and Fresnel reflections at the input and output of the device, was calculated from the power before the photonic lantern (Power in) and the power after the multicore fibre (Power out). The average propagation loss in the fibre was evaluated to be approximately 0.1 dB / m, while the Fresnel reflection was 0.18 dB / facet. This file contains the raw data obtained during these measurements, in terms of both Power in and Power out, as a function of iris diameter.