|Optical bench showing various adjustments|
|Camera with adjustable focus lens and LED board ahead of it|
|Board containing prototype bi-color LEDs|
|Board containing conventional LEDs|
LEDs -- I built two versions of the board containing the LEDs. One contained two of bi-color prototype LEDs (white and infrared) that I mentioned last time, and one contained two each of conventional through-hole white and infrared LEDs. Through experimentation we found that a single LED of each type was able to provide sufficient light. The reason we decided to have two, at right angles to each other, is an attempt to overcome specular reflection from the various optical components. The highlights of these LEDs potentially hide important information in the image so we felt it was important to deal with it. More about that later. There is a ribbon cable that runs from the LED board to a breadboard that contains the circuitry that drives the LEDs based on control signals from the Pi. There are also two potentiometers that can be used to control the brightness of the LEDs.
|Infrared and White LED brightness controls|
Condensing Lens -- This lens provides the primary magnification for the image of the retina. Other focal length lenses are sometimes used but this seems to be the most common for this type of screening test.
|Chin and head rest as well a condensing lens|
Raspberry Pi --
Hardware -- We used a Pi 3 Model B Version 2. Using GPIO pins it is connected to the circuit board that drives the LEDs mentioned above. It is also connected via USB cable to an Arduino. We wanted the ability to control a number of Pi Camera parameters on the fly. (See Software below) We did this by using four potentiometers as voltage dividers, read the voltage via the analog to digital converters on the Arduino, and then pass that information to the Pi via the USB.
|Brightness, Contrast, Sharpness and ISO camera controls|
Here is an example of a picture we took using our device. The optic nerve is the yellow circular area in the upper left of the picture. This picture is not perfect, of course. You can see evidence of the specular reflection I mentioned earlier. We are, however, pleased with the resolution and the field of view. Also we still haven't been able to achieve sharp focus using the IR LEDs so it should be noted that this photo is taken with a chemically dilated pupil.
Next time I'll talk about what we're doing about that.
Oh, and one other thing. There is a group that has made excellent progress on this type of device. If you're interested in the topic be sure to look here.