Pupillometers are used to determine the pupil opening in daylight and in twilight conditions.

This examination is of high relevance for all refractive surgery operations (laser/implantation). It indicates how large the treatment zone (in other words the optically usable laser ablation zone) must be in order for the twilight or night vision not to be compromised, or as little as possible. The objective is to avoid the appearance of so-called halos (luminous circles seen around a light source).
If the diameter of the pupil in the dark appears to be too large, this may be a reason not to carry out a refractive-surgery treatment by laser. The measurement is performed in a few seconds, in a darkened room.