Origin of the well-known blue-green emission of spray pyrolysed ZnO thin films has been discussed on the basis of variation of the properties due to different treatment of the samples such as ion beam irradiation and doping. 120 MeV Au ions and 80 MeV Ni ions were used for ion beam irradiation while indium was used for doping studies. It was assumed that, Au ions might have produced huge amount of defects while the defects might be less in Ni irradiated films due to the difference in energy and mass number. Photoluminescence studies of pristine sample showed only one emission at 517 nm at room temperature while the irradiated films showed a decrease in intensity of this emission. Indium doping also reduced the intensity of this emission. But additional emissions were also observed in these films. Based on the observations, we proposed that the blue-green emission was due to the transition from conduction band to the level due to oxygen antisite (O-Zn).