Handheld device used to measure heterophoria at near. It consists of a septum and two slit apertures, one for each eye. One eye sees a double tangent scale (vertical and horizontal) calibrated to read in prism dioptres, while the other eye sees a white arrow pointing upward and a red arrow pointing horizontally to the left. As the two retinal images are quite different there is no attempt at fusion and the eyes stay in the passive position. The arrows seen by the left eye point to the numbers seen by the right eye. The numbers represent the vertical and horizontal components of the phoria, which can be read directly by the observer (Fig. M2).
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann