Team 6: Digital Exophthalmometer

UCSD 2013 Spring MAE 156B DESIGN PROJECT

Team 6 Digital Exophthalmometer

Sponsored by Shiley Eye Center

Two Parallel Approach of Design: 

(1) Infrared Sensor & (2) Ultrasonic Sensor

Background: Overview of Project

        By comparing and contrasting the measurement of the excessive projection of eye, the progress of several diseases can be analyzed. The typical exophthalmometer that is widely being used throughout ophthalmology measures the clinical relationship of the globe to the orbit using prisms and mirrors. Every doctor uses the device slightly differently, and every patient has a dissimilar facial shape, forcing this device to a greater degree of possible error. The objective of this design project is to design and build a portable instrument capable of accurately measuring the distance between the cornea and the orbital rim.

The Objective

The main objective of this design project is to establish a prototype with the following functional requirements:

• Accurately measure the distance between the orbital bone and the very tip of cornea 

• Accuracy of 500 microns (0.5 mm)

• Final prototype of a lightweight and simple design with straightforward operation

• Standalone device that will provide a digital readout without a computer connection for extra data processing

Review of Current Device

The current device that is widely used consists of a set of prisms and mirrors that present a ruler-like scale on top of the profile view of a patient's eye. This particular non-digital device holds a great amount of systematic error due to the following variances:

• Application and positioning of the device on the oribtal bone

• Reading of the scale varying for every doctor based on angular changes 

Statement of Requirements

The main performance requirements that are expected with this device include the following:

• Perfectly safe for the eye for any patient

• Portable and lightweight to carry around and to be applied without discomfort

• Any doctor should be able to easily use the device and be able to gain the digital readouts quickly when needed.

Key Components of Final Design Solution

Design 1. Infrared Approach

• Sensing mechanism: infrared emitter and receiver placed with an angle

• Linear potentiometer: voltage outputs converted into distance readings on horizontal sliders

• Ambient light filtering circuit system: detected amount of ambient light is subtracted from that of ambient + emitter to solely provide the intensity of emitter light for sensing purposes

Design 2. Ultrasonic Approach

• Sensing mechanism: based on a commercial product, Baumer UNAM ultrasonic sensor

• Collimator attached to this allows to only measure required region of eye surface (wave output beam width of 3 mm)

• Rack and pinion sliders: provides smooth slide with delrin blocks and wave washers

• Rotary encoder: attached to the rack and pinion system of horizontal sliders

Hardware Performance

1. Infrared Approach

        The infrared prototype was tested on a Teflon semi-sphere, and the average of ten trials demonstrated an overall error of 0.39 mm. The possible sources of error include the painting of mount, the cut hole on acrylic cover of emitter, the accuracy of 3D printer, and the alignment of rays which were concluded to mainly be systematic rather than a concern of the theoretical design.

2. Ultrasonic Approach

The prototype of ultrasonic design was tested on human eye to confirm its repeatability as well as on an acrylic sample eye to demonstrate the accuracy. A set of ten measurements all fell within a range of 1 mm, and the second controlled experiment reflected an overall error value of 0.4 mm which satisfies the initial functional requirements of repeatability and accuracy.

Impact on Society

Considering that many individuals suffer from minor bulging of eyes to serious proptosis that require immediate attention, the automation of exophthalmometry will assist and improve the process of diagnosis and treatment. Using the current system, the diagnosis and test of patients’ progress of such conditions withheld many variables due to the systematic error caused by each doctor who applies and reads the measurements from a manual device. With the digital version of exophthalmometer, an accurate measurement of the distance between one’s orbital bone and the tip of cornea is delivered on a digital display which eliminates possible errors resulting from various users.

Conclusion

The design process was completed with two sensing mechanisms. The first prototype was established using a commercial ultrasonic sensor that was chosen and purchased to fit the functional requirements presented from the success and errors of the current device. The infrared sensor is still under development with possible further research and improvement; however, it holds an optimistic future considering its independence of refraction as well as the low fractional overall component expenses in comparison to the previous approach with the commercial ultrasonic sensor.