Thermodynamics of daylight-pumped lasers

We cast the problem of very-low-threshold, daylight-pumped lasers in a general thermodynamic framework. We calculate that the requirements to reach threshold are that the Stokes shift of such a laser be greater than 13.3kT and that the absorption ratio of the pump/emission band and the geometrical aspect ratio both be greater than exp (13.3). We describe some luminescent material systems that might be able to satisfy these rigid requirements.