The pupilometer is actually two different opthalmic equipment devices that share the same name. One of the instruments measures the distance between each pupil of the eyes and the other is one that measures the response of the pupil to any kind of visual stimuli. The normal usage of pupilometer is pertaining to the ophthalmic equipment used by optometrists or ophthalmologists to measure inter pupillary distance (PD). This is done in order to fit a person for eyeglasses so that the lenses are focused correctly in the frame. This will ensure that the glasses are positioned directly in front of the visual axis. Measuring for PD is done twice, one with the eyes focusing on a target that is far away from the patient and another one with the patient looking at a closer object. Measurements from the latter are helpful in making reading glasses. While measuring for PD can be done with a ruler using millimetres, the results may not be as accurate as it were done properly with an ophthalmic equipment called the pupilometer.

The other ophthalmic instrument with the same name is used to observe pupil response. This pupilometer is a monocular instrument that measures the amount of pupil dilation in the eye as triggered by a stimulus, which is in this case, light. Pupillary response is different from the response to focus. Usually, the pupils of the eyes constrict when subjected to bright light to allow less light in, while dim lighting makes the pupils dilate. This reflex to light allows the pupils to regulate the amount of illumination entering the eye. Studying the pupil’s reaction to light can help doctors in diagnosing lesions or damage to the optic nerve. This is by looking at the eye’s direct response (the response of the eye directly receiving the light) and consensual response (the constriction or dilation of the eye that follows the reaction of the stimulated eye). Lack of response in one eye may indicate damage in the nerves that connect both visual organs.

Examining the pupil reflex to light may also help doctors in diagnosing several diseases like tertiary syphilis, which is characterized by the eyes’ lack of response to light. A person with Argyll Robertson pupil will have pupils that constrict when focusing on an object that is close but do not react when the eye is exposed to bright light. Ophthalmic equipment used for observing the dilation of the human eye can be useful in several applications.  The dilation and constriction of the pupil of the eye happen to be indicative of a person’s psychological state. Dilated pupils may be a sign of interest or attentiveness, and also an increase in the rate of mental process. This kind of study is helpful in marketing researches on the effectiveness of TV commercials in catching the attention of people.

On another hand, a study showed that people who could allocate their attention fully to stimuli that are relevant to their task can perform their work better than those who get distracted by task-irrelevant stimuli. In this same way, people suffering from schizophrenia have more dilation on task-irrelevant stimuli and less dilation on the relevant ones. Fear, anxiety and even sexual arousal are also factors in pupil dilation. Pupils also constrict under the influence of some opiates and other drug depressants. While stimulants and hallucinogenic drugsmake the pupils dilate. In this way the ophthalmic instrument called the pupilometer can also be used in detecting drug abuse.