Recently millimeter wave imaging absorbs many attentions. Especially much research work has been done on the millimeter focal imaging because of its advantage of real time imaging. This paper analyzes extended hemispherical lens, which can be used in focal imaging. An integrated antenna array is placed at the back plane of the lens to receive the power focused by the lens. This structure can eliminate the effect of the surface wave exited in the thick dielectric substrates at millimeter wave wavelengths. Low loss and compact are its merit. Assuming an electromagnetic wave is incident on the lens surface, the field distribution in focal space of the lens is analyzed by using Stratton-Chu formula and ray tracing method. It will provide much information about the imaging performance of the lens. Experiments have been carried out for two types of incident waves, one is normally incident plane wave and another is Gaussian-beam. Good agreement has been found between experiments and calculation results. The lens antenna also can be used to eliminate the transition between paraboloidal antenna and integrated front-end in conventional integrated millimeter wave systems so that the system loss can be decreased and performance can be improved. Furthermore, the lens can be used to couple with a quasi-optical system. The results presented here will provide useful guidelines for the above mentioned applications.