A special ophthalmic trial lens in the form of a rod or series of rods (grooves/cylinders) that changes the size, shape, and colour of an image to dissociate the eyes in the evaluation of eye muscle dysfunction [method to evaluate the alignment or deviation (i.e., heterophoria or heterotropia) of a pair of eyes]. The device is usually made of red glass and intended to be hand-held or placed in a trial frame.