During Hess-Lancaster testing, which eye should wear the red lens when evaluating the primary deviation?

• A. Fixating eye
• B. Paretic eye
• C. Both eyes
• D. Neither eye

Answer: (A)

In Hess-Lancaster, you dissociate the eyes with red-green filters to map each eye’s movements while one eye fixes. The primary deviation is the misalignment of the non-fixating (paretic) eye when the other eye is holding fixation. Placing the red lens on the eye that is fixating gives you a stable reference: that eye’s image is kept separate and constant, while the other eye’s movements are revealed on the chart through the opposite color. This setup makes it clear how far and in what direction the paretic eye deviates when trying to fixate, which is exactly what you want to measure. If the lens went on the deviated eye, fixation wouldn’t be as stable, and the resulting map would be harder to interpret. Therefore, the red lens should be over the fixating eye.