Abstract: The invention concerns a device for the determination of the absolute polarisation of alkali metal atoms. The device can inter alia be used for the diagnosis of alkali metal polarisation in a polariser for inert gases. The device includes a detection laser, which together with a &lgr;/4 plate creates circularly polarised light. A sensor for the measurement of the light absorbed by the sample of the polariser is provided. A Helmholtz coil pair serves for the creation of a static magnetic field B0. An oscillating magnetic field is created with an RF coil. A gradient coil serves for the creation of a linear magnetic field gradient. Various optical components are present. An electronic processor effects the determination of the absolute polarisation of the alkali metal atoms. All the said components with the exception of the Helmholtz coils are firmly mounted onto a platform which can be moved parallel to the direction of the B0 field.