Abstract: An electro-optical instrument means for measuring the relative angle of roll about an optical axis extending between a sending unit and a receiving unit. The measurement is independent of relative displacements in pitch and yaw within the dimensional limits of the apparatus. The sending unit comprises a pair of light sources behind respective apertures in a mask and differently modulated. The apertures are imaged onto a divided photodetector on the receiving unit. Each half of the detector receives modulated light from both sources. The differently-modulated photocurrents are separated electrically, and the roll angle computed from the ratios of the separated components. These ratios are unaffected by relative displacements of the two units in pitch and yaw.