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Eye Pupil Control for the Shiley Eye Institute

Lihua Wei, Jessica Kim, Robert Haeuser, Wanfang Wu

Sponsored by Drs. Ben Xu and Robert Weinreb at the Shiley Eye Institute and Hamilton Glaucoma Center 

Objective:

The goal of this project is to develop a device that uses visible light to modulate the size of a pupil based on images captured by the Tomey Casia Anterior Segment Optical Coherence Tomography device. Researchers at the Shiley Eye Institute and Hamilton Glaucoma Center conduct research on glaucoma, a leading cause of permanent blindness worldwide. The motivation for this project is to develop a device that helps Shiley researchers study how the iris and its biomechanics contribute to the development of certain forms of glaucoma. The design parameters are to

1. Measure the pupil diameter to within 0.3mm

2. Drive pupil to desired diameter in under 6 seconds and within 0.5mm

3. Light source have small enough resolution to accommodate individuals who are very light sensitive.

4. Have large enough upper intensity limit to accommodate individuals who are light insensitive.

Design Solution:

Pupil diameter is primarily dependant on the amount of light entering the eye. The device uses four LEDs bundled together to elicit pupillary responses. The total brightness is controlled by the amount of current entering the LEDs, which is adjusted through an low pass filter and op-amp circuit attached to an Arduino. The Arduino receives PWM commands from matlab which sets the driving voltage that determines current. Additionally, the op-amp circuit prevents ripples in DC current which otherwise induces the LEDs to flicker and thus preventing a stable pupil diameter.

Matlab controls the entire process by analyzing the camera feed of Tommey, the eye-imaging machine, which sees the patient’s eye. Matlab uses Hough transforms to measure pupil diameter and then uses integral control to determine step changes in brightness.

Besides the closed-loop system, the project included a screen cover that would cover the screen of the computer that operates the eye imaging machine to reduce ambient light to minimum level. It also included a case that contains most part of the hardware (Arduino board and the circuit excluding LEDs and AC to DC adapter with works as power supply) to ensure that the device is easily movable.

The final design would drive the subject’s pupil diameter to desired value in the range of 3mm to 6mm under 10 seconds.

Results:

Detection is accurate up to 0.05mm:

Controller able to drive pupil diameter to desired diameter with oscillation amplitude less than 0.3 mm. Below, desired diameter is 3.5 mm. Blue dots are diameter measurements, red dots are diameter measurements when Tomey OCT scanner takes pictures, green line is current output.