If you already have a light-recycling setup for the phosphor - maybe you could additionally use brightness-enhancement films to use for light recycling in angular space. Just like in liquid crystal screens, BEFs could refract light rays with high exit angles back towards the phosphor, giving you a collimation of about 45° for rectangular, linear prism films.
But of course, conservation of optical étendue ('optical entropy' so to say) can't be cheated and absorption and scattering losses will be the trade-off for better collimation.
But of course, conservation of optical étendue ('optical entropy' so to say) can't be cheated and absorption and scattering losses will be the trade-off for better collimation.