We describe the design and the capabilities of the Photospheric Magnetic field Imager (PMI), a compact and lightweight vector magnetograph, which is being developed for ESA’s Lagrange mission to the Lagrange L5 point. After listing the design requirements and give a scientific justification for them, we describe the technical implementation and the design solution capable of fulfilling these requirements. This is followed by a description of the hardware architecture as well as the operations principle. An outlook on the expected performance concludes the paper.