Abstract: The present invention relates to an improved device for use in the detection and identification of various misalignment conditions of the eye, for example heterophorias, strabismus and the like. The device, in its preferred embodiment, includes a unidirectional occluder having an occluding disk which occludes an individuals sight in one direction through the disk while allowing direct observation of the occluded eye from the other side of the disk. Preferred embodiments include a self contained hand held support having a handle region and an occluder region, the occluder region being of the size effective to occlude vision of a fixation target through one or both eyes, the disk formed of a reflective transparent material mounted within the occluder region, a light source positioned on one side of the disk so as to illuminate the side of the disk when energized, and a mechanism contained within the support for energizing the light source, the energizing mechanism including a switch mounted on the support.