A surface of a metal hit by electrons emits a radiation. Its spectrum covers the range of visible light and the ultra violett. This radiation was first observed by Lilienfeld. We found that this radiation is not attributed to surface contaminations since it is also emitted from extremly clean metal surfaces. Spectrum, polarization, intensity, and temperature dependence of the radiation emitted by several metals were investigated. These measurements indicate that the Lilienfeld radiation can be interpreted as the transition radiation predicted by Ginsburg and Frank. The possibilities of excitation of bremsstrahlung and plasma radiation are discussed.