Dynamic Pupillometer

by Adrian Glasser

This is a blog about a dynamic infrared pupillometer for real-time tracking and measurement of pupil diameter.

Often, clinically, pupil diameter is measured by a clinician by holding a ruler in front of the pupil of the eye of a patient to measure the pupil diameter to the nearest millimeter. This may be perfectly suitable for many clinical applications that require nothing more. However, today, with the advent of inexpensive USB video cameras, many with excellent infrared sensitivity, it is possible to do far more. One possible application, where more accurate and more detailed pupil diameter information might be required, is during near-vision testing. The ability of an eye to see near objects clearly, normally relies on an optical change in refracting power of the eye called accommodation. However, with aging and the onset of presbyopia, accommodation is progressively and ultimately completely lost by about age 55 years. However, the pupil of the eye continues to constrict when the eye makes an effort to focus at near even in the fully presbyopic eye. This near pupil constriction effectively increases the depth of focus of the eye which can aid distance corrected near vision in the presbyopic eye.

In this application, an infrared sensitive video camera with associated infrared LED is mounted on a spectacle frame to view the pupil of the eye at high magnification. The video camera and LED are controlled from software run on a laptop computer. The software allows for control of one or two LEDs. The LEDs can be turned on and off and the brightness adjusted. The live video image allows for the pupil to be tracked in real-time and the pupil diameter to be recorded. The data is accumulated and can be saved to a file for later analysis. In this way, the pupil diameter can be measured to a fraction of a millimeter and can be measured multiple times per second and can be recorded for long durations.

The video is best viewed by clicking on the full-screen icon at lower right on the video.

The software records the measurement number, the measurement time and the pupil diameter. At the end of a recording the recorded data can be saved to a file, such as in the image below, for later analysis. The analysis on this data can include the dynamics of the pupil response, such as to a flash of light, for example. With this approach, the pupil diameter could also be recorded as patients are performing an acuity task so that the pupil diameter can be known at the same time that the acuity is being measured.

Example data file.
CAD diagram of stretcher

An approach such as this could readily be implemented clinically to get more accurate and more useful information on the pupil diameter.