A small bit of cardboard with a 2-mm pin hole can be taped to the spectacle lens to form a peep sight through which gaze must be maintained. This allows reproduction of frontal plane gaze position with an accuracy of one degree. In turn, this allows accurate reproduction of alignment measurements, which otherwise would vary with change of gaze position in incomitant strabismus.
Scott AB, de Groot JA. An Accurate Gaze Position Indicator for Strabismus Measurement. Arch Ophthalmol. 1973;90(3):225-226. doi:10.1001/archopht.1973.01000050227008