P16590: Low-Cost Fundus Camera 2
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Preliminary Detailed Design

Table of Contents

Moveable Base

A mounting base was provided which allows for fine control of the position of the camera to get it properly focused on the fundus.

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In order to attach the camera to this base a mounting plate needs to be 3D printed based on measurements.

Optics

An old camera was taken apart to examine the lens setup. This was used as the basis for our lens setup. A 20 diopter lens was provided to use in the lens setup and magnify the fundus. Initially there was difficulty getting an image.

Further research found a paper by a group of researchers working on a fundus camera. A group member reached out to them to get details on their lens setup. Their setup is below and the bill of materials for their pieces shown after that.

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The paper this material came from is found here.

Source: http://iovs.arvojournals.org/article.aspx?articleid=2127233

Small modifications will need to be made in order to account for differences in lenses used.

Illumination

Many options were considered on how to properly illuminate the eye, due to the need to illuminate through the pupil while avoiding the reflection off the cornea and iris. Three light sources are needed: an exposure source to cause a flash for image capture, an high frequency infrared source to allow the proper focus without constricting the pupil, and a set of red fixation lights in order to get the gaze of eye in the proper orientation.

Light Path

Liquid Light Guide

One thought was to use a liquid light guide in order to indirectly illuminate the eye. This would require a shutter to block off the light and then allow a brief flash to cause exposure for image capture. The infrared and fixation lights would need to be in a different position so they are not blocked by the shutter. This may cause an issue with focusing as the light used to focus would follow a different path than the light being used to

Beam Splitter - Coaxial Light

Another thought was to use a beam splitter and shine the light so it would be coaxial to the eye. This would make it easier to place the infrared and fixation lights in a separate place from the shuttered light source, while still allowing them to be used for focusing. Some concern over the potential cost of a beam splitter. More feasibility needs to be done to check if a beam splitter can transmit IR light.

Flash Options

Mechanical Shutter

A mechanical shutter was acquired from a previous camera. This can be used to create a flash in order to illuminate the image briefly for capture. Some concerns were raised about the complexity of using a mechanical shutter. These include: vibrations caused by the movement which may disrupt the camera focus, the complexity of coding the software to move the shutter at the same time as capturing the image, the energy required to initiate the mechanical motion, and ensuring a consistent exposure time by the shutter each time.

LED Flash

Another option which was considered was to use high power LEDs as the illumination source. This could then be quickly discharged to cause a flash. Some concern with this process included the cost of LEDs that would be able to discharge that rapidly, the time it would take to get to full brightness, and the power required to illuminate the LEDs.

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Ultimately it was decided the LED flash would be a better choice. This allowed for an easier integration of the software controlling everything, and also allows all light sources to be placed in the same location, as it is a matter of turning them on/off instead of a mechanical barrier blocking them.

Focusing Light

One other concern is getting the light to focus directly on the retina and not reflect off other areas. To accomplish this a donut like aperture needs to be acquired to focus the beam of light down. To accomplish this the team looked for a premade device and also considered constructing our own. Acrylic was found to be a low cost solution as it could easily be machined down into the desired shape, and coated as necessary to prevent external light entering causing interference.

Software

The software is being developed to interact with all the hardware and run on the ODROID. The software architecture is shown:

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Casing

An industrial design student joined our team and helped develop a casing based on the dimensions of the equipment.

This is a mockup of what the case could potentially look like. public/Photo Gallery/casemockup.jpg

The full presentation is included here.


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