To track the eyes, we need a camera to look back at them. To make this affordable, we have been working with low cost (less than 6€ per unit) endoscopic cameras such as this one...
First, you need to get the camera out of the casing. In these more modern (water proof) designs, you can simply "wobble" the cable at the end of casing to break the glue joint holding the casing onto the cable.
You'll then be able to carefully slide out the camera and the attached PCB.
The endoscopic cameras come with a tiny ring of LEDs. These will be replaced later by an Infra Red (IR) light source. Simply clip off the LED ring.
Next, we need to unscrew the lens on the camera so that we can remove the IR filter. Why? IR light isn't seen by the human eye, which makes it great for illuminating the pupil without distracting the user. However, these cameras (almost all cameras) have an inbuilt IR filter which blocks infra-red light, so it needs to go!
To remove the filter, we use the handy 3D printed aide you can get here
Now comes the hard part. You need to smash out the filter. Just poke it until it breaks and take out the pieces. More elegant suggestions are welcome ;-)
Make sure you clean the lens before continuing...
Now we can use the power supply to the LED array to power an IR LED. Make sure you calculate the right resistor value for your choice of IR LED. Infra-red can be harmful for the eye! This is a good source of information for your calculations.
You can reuse the usb cable. The larger "illumination dial" at the main USB connector is superfluous. The chip controlling image processing and USB transfer is the one right next to the camera!
Solder this up to the nano usb hub of your choice. In this case the NanoHub from Berkel en Rodenrijs!
You'll do this again for the two cables which connect to the cameras. They need to connect to the camera PCB and also the nano hub.
Now you should be able to just plug the camera guts into your laptop/pc/android phone and see both working!