Pupilometer is the name for two different devices used in ophthalmology — one that measures the distance between pupils, and one that measures the pupil’s response to visual stimuli.
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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.
There are a variety of ways of measuring one's pupil distance. Historically a ruler is used by optometrists. State of the art are video centration devices which provide higher accuracy and repeatability compared to manual measurements. The accuracy is especially a concern for progressive lenses where small deviations severely reduce the performance of the lens. In addition to having it measured when one visits an optometrist, there are a variety of mobile phone apps and web apps that a person can use. The apps are available for the iPhone, iPod touch, and iPad. The web app is used by a variety of online sellers of eyeglasses. In order to use the phone app or web app, a camera and a credit card with a magnetic strip are needed to assist in the measuring processes.
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 (i.e. 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. The pupil response has also been found to reflect long-term memory processes both at encoding, predicting the success of memory formation, and at retrieval reflecting the operation of different recognition outcomes.