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The response of the pupil to a flash of light, the pupillary light reflex (PLR), is an important measure in optometry and in other fields of medicine that is typically evaluated by qualitative observation. Here we describe a simple, portable, iPhone-based pupillometer that quantifies the PLR in real time.The purposes of this study were to describe a novel application that records the PLR and to compare its technical capabilities with a laboratory-based infrared (IR) camera system.Pupil sizes were measured from 15 visually normal subjects (age, 19 to 65 years) using an IR camera system and the Sensitometer test. This test elicits pupillary constriction using the iPhone flash, records pupil size using the camera, and provides measurements in real time. Simultaneous recordings were obtained with the Sensitometer test and IR camera, and two measures were calculated: (1) dark-adapted steady-state pupil size and (2) minimum pupil size after the flash. The PLR was defined as the difference between these two measures. Pupil size was also recorded during the redilation phase after the flash. Bland-Altman analysis was used to assess the limits of agreement between the two methods.Statistically significant correlations between the IR and Sensitometer test measures were found for the PLR (r = 0.91, P < .001) and redilation size (r = 0.65, P = .03). Bland-Altman analysis indicated a mean PLR difference of 6% between these two methods. The PLR limit of agreement was 14%, indicating that 95% of subjects are expected to have IR and Sensitometer test measurements that differ by 14% or less. Bland-Altman analysis indicated a mean redilation size difference of 1% between the two methods; the limit of agreement was 5%.There is excellent agreement between pupil responses recorded using the Sensitometer test and IR camera. The Sensitometer test provides a highly promising approach for simple, portable, inexpensive pupillary measurements.