A 1 mm shift on Hirschberg/Angle lambda represents approximately how many prism diopters of deviation?

• A. 18 pd
• B. 22 pd
• C. 25 pd
• D. 28 pd

Answer: (B)

In the Hirschberg test the position of the corneal light reflex relative to the pupil provides a quick estimate of how misaligned the eyes are. The angle lambda concept helps relate that reflex shift to the amount of prism diopters needed to correct the deviation. A widely used rule of thumb is that about 22 prism diopters correspond to a 1 mm shift of the corneal reflection on the cornea. So when you see a 1 mm displacement, it suggests roughly 22 pd of deviation. This is an approximate conversion and used because the reflex moves in proportion to the angular misalignment of the eyes. If the reflex shift were smaller, it would indicate less deviation; a larger shift would indicate more.