Eyetracking – the basics of how it works

I am often asked how the eyetrackers work (second only to why the name Bunnyfoot?) – so here it is – in essence it is really simple – a digital camera videos your pupils (the holes that let light into your eye) and a computer works out where you are looking based on the video images.

Well there is a little bit more to it than that (not much though):
Tobii eyetrackers contain infra-red emitting diodes and a high resolution digital camera
The infrared diodes shine light on the person in front of the eyetracker (it’s 14 times less strong than that emitted from a TV remote – so doesn’t burn their eyes out).

A high resolution digital camera tuned to the infrared spectrum picks up the reflections from their eyes (and some other things too, but lets not worry about that)!
the infra red lights reflect off the cornea (the shiny front of the eye) and they are just below the dark spot - the pupil
Because the angle of the diodes in relation to the camera is known, the computer looks for the dark spot near to the reflections from the infrared and identifies this as the pupil.

A clever algorithm then works out where the pupils are pointing – and so where the person is looking – it does this between 50 and 120 times per second – depending on the model of eyetracker being used.

The computer then stores the horizontal and vertical coordinates of where each eye was looking and gives it a time stamp. It also stores a picture of whatever was on screen at the time.

Simple – in fact so simple you can find instructions about how to build your own eyetracker on the net (I must admit I didn’t really understand these though)!

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