We developed a powerful UV laser at a central wavelength varying from 260-283 nm. The laser system based on a frequency-trippled Ti:sapphire amplifier delivers mJ pulse energy within a duration of 1-10 ps (fwhm) with 1.5 nm spectral width (fwhm). The system is used to explore thermal emittance and quantum efficiency dependence on photon energy from metallic photocathodes. With pepperpot and solenoid scan technique we have measured the predicted theoretical limit for thermal emittance at room temperature (0.4 mm.mrad/mm rms laser spot size at 283 nm) for metallic photocathodes.