EFILE (Electric Field Induced Lyman-α Emission) is a new way of measuring weak electric fields in vacuum and in a plasma. It is based on the emission of Lyman-α radiation (121.6nm) by a low-energy metastable H atom beam due to Stark-quenching of the 2s level induced by the field. In this paper, we describe the technique in detail. Test measurements have been performed in vacuum between two plates polarized at a controlled voltage. The intensity of emitted radiation, proportional to the square of the field modulus, has been recorded by a lock-in technique, which gives an excellent signal to noise ratio. These measurements provide an in-situ calibration that can be used to obtain the absolute value of the electric field. A diagnostic of this type can help to address a long standing challenge in plasma physics, namely the problem of measuring electric fields without disturbing the equilibrium of the system that is being studied.