  Light thermalised at room temperature in an optically pumped, dye-filled
microcavity resembles a model system of non-interacting Bose-Einstein
condensation in the presence of dissipation. We have experimentally
investigated some of the steady-state properties of this unusual state of light
and found features which do not match the available theoretical descriptions.
We have seen that the critical pump power for condensation depends on the pump
beam geometry, being lower for smaller pump beams. Far below threshold, both
intracavity photon number and thermalised photon cloud size depend on pump beam
size, with optimal coupling when pump beam matches the thermalised cloud size.
We also note that the critical pump power for condensation depends on the
cavity cutoff wavelength and longitudinal mode number, which suggests that
energy-dependent thermalisation and loss mechanisms are important.
