I have shown that the unified projection model proposed for central catadioptric cameras is also valid for fisheye cameras in the context of robotic applications. This model consists on a projection onto a virtual unitary sphere followed by a perspective projection onto an image plane. It exhibits many advantages. Firstly, this representation is compact in the sense that a unique parameter will be used to describe radial distortions. Secondly, this model is geometrically intuitive as the sphere provides radial distortions. Thirdly, the transfer function has an analytic inverse. Finally, many algorithms have been designed for central catadioptric cameras using the USM for instance for camera calibration, visual servoing or attitude estimation.