Pupilometer

Pupilometer is the name used for two different devices -- one that measures the distance between pupils, and one that measures the pupil's response to visual stimuli.

Pupillary distance measurement
In the most common sense, a Pupilometer is a tool for measuring pupillary distance (PD). It is used for fitting eyeglasses  so that the lenses are centered in the visual axis. This is the most common nomenclature. It may also be used to verify a PD measurement taken from a millimeter ruler placed across the bridge of a patient's nose for distance and near focus.

Pupil Response
Alternatively, a pupilometer is a type of pupil response monitor -- a monocular device measuring the amount of dilation of the pupil in response to a visual stimulus.

In ophthalmology, a pupillary response to light is differentiated from a pupillary response to focus (ie. pupils may constrict on near focus, as with the Argyll Robertson pupil) in the diagnosis of tertiary syphilis. Although a pupillometer can be used, the diagnosis is often made with a penlight & near-point card.

The extent of dilation of the pupil in the eye could be an indicator of interest and attention. Methods of reliable measurement of cognitive load, such as the dilation or constriction of the pupils, are used in marketing research to assess the attractiveness of TV commercials. Dilation of the pupils reflects an increase in mental processes, whether it be attentiveness, or psychomotor responsiveness.