The present invention provides a radiation source apparatus which can generate a DUV radiation beam having a wavelength of 193.4 nm efficiently. The radiation source apparatus according to the invention has first wavelength conversion means arranged to receive a first laser beam of a first fundamental wavelength and to generate a fourth-harmonic wavelength of the first fundamental wavelength, second wavelength conversion means arranged to receive the beam of the fourth-harmonic wavelength of the first fundamental wavelength (266 nm) and a second laser beam of a second fundamental wavelength and to sum-frequency mix the fourth-harmonic with the second fundamental wavelength radiation to generate a beam of second DUV radiation having a wavelength between approximately 232 nm and 237 nm, and third wavelength conversion means arranged to receive the beam of second DUV radiation and the third laser beam of a third fundamental wavelength and to sum-frequency mix the second DUV radiation with the third fundamental wavelength radiation to generate third DUV radiation having a wavelength between approximately 192.5 nm and 194.5 nm.