A device for phase detection testing any kind of optical system, in particular an ophthalmic lens, comprises, along an optical path, an emission device which constitutes a light source, a holding station which receives the optical system under test, an array, a receiving device which receives the image observable downstream of the array, and a data processing device which exploits this image by phase detection. Between the light source and the holding station is a separator element which constitutes on the axis of the optical path a semi-reflecting surface. The receiving device is disposed to the side, in line with the separator element. The data processing device includes a ray plotting program and a calibration system allowing for theoretical and manufacturing aberrations due to the separator element.