A beam generator for an aerial image generating apparatus includes a laser source for emitting a laser beam and a short wavelength beam source for generating a short wavelength beam by processing the laser beam such that the short wavelength beam is coherent with and has a wavelength shorter than that of the laser beam. A spectral unit includes a quartz plate and a spectral layer coated on a surface of the quartz plate. The spectral layer has a Brewster's angle greater than 70° with respect to the laser beam such that the short wavelength beam is reflected from the spectral unit without the laser beam, increasing the reflectivity of the shortwave beam while decreasing the reflectivity and absorptivity of the laser beam in the spectral unit.