アブストラクト：
We have undertaken a detailed survey of a large sample z=3-4 Lyman alpha emitters (LAEs) whose intense [OIII]5007 emission indicates they are useful analogues of galaxies in the early universe. In this talk, we present the rest-frame optical and UV diagnostic emission lines. Using measures of xi_ion -- the number of Lyman continuum photons per UV luminosity -- from both Balmer and UV lines, we find a consistently harder ionising spectrum for LAEs than that inferred by similar techniques for z=2-4 continuum selected Lyman break galaxies, particularly for those LAEs with the faintest UV luminosities. We briefly discuss the implications for cosmic reionisation if the metal-poor intensely star-forming systems studied here are representative examples of those at much higher redshift.