To understand how and eye tracker works it is first important to know how our eyes and the humans vision system works.
An eye-tracking system like the Tobii T60 / T120 / X120 measures how the eyes typically move over a digital image that is shown to a respondent from the target group. The human eyes are constantly moving until they stop and focus on a point. There are over ten different types of eye movements, of which the most important ones are saccades, fixations, and smooth pursuit. When the eyes stop to focus it is called a fixation and the movements between these fixations are called saccades. See image below:
The length of the stops, when the eye fixates, varies from about 100 to 600 milliseconds and during this stop the brain starts to process the visual information received from the eyes. Saccades are extremely fast jumps from one fixation to another. The average length of a saccade is about 20-40 ms. During this period the eyes do not send visual information to the brain.
The human eye has a visual field of about 200º but the highest number of light sensitive cells on the retina are located in the part called fovea, which is the only point in our eyes where we are able to see a sharp and colorful image of the world around us. This area is fairly small and covers only about 1 – 2 degrees of our vision (which is about the size of a thumbnail at an arm length’s distance). It is only from these cells our brain can receive detailed visual information. See image below:
Our perception is slightly larger than the area of foveal vision. When, for example, reading a text, we can read about 12-15 letters to the right and 3-4 letters to the left, which would indicate there is a perceptual span of around 18 characters centered asymmetrically around the fixation point. Using our peripheral vision outside the area of the foveal vision we can see an unclear image, but our peripheral vision is generally very poor and is only good at picking up movements and contrasts. See image below:
It is possible to move our covert attention (the attention of our mind) around the entire visual field when our eyes are at rest. Thus it is possible to move our attention around without eye movements. However, our foveal vision is usually a valid measure for determining the target of our covert attention as our brain can process very little information from complex stimuli from the area outside the fovea (such as packages on a shelf). In such cases it is more efficient for our brain to focus attention on the fovea rather than on our peripheral vision, as the brain needs to process blurry visual information requiring more effort to interpret than the visual information from the fovea. This is why it is possible to tell something about human behavior by just following eye movements, especially the fixations, as we know that we can only see something clearly when we fixate on an object or are very close to it. If a person does not fixate on, or is not very close to the package on a shelf, he or she would not be able to see the product and read the text on it. The eye tracker records these movements and the location of the foveal vision when the eyes fixate.
By analyzing our eye movements it is possible to tell something about our behavior. The length of a fixation is usually an indication of information processing or cognitive activities as this is when the brain interprets the visual information from the eyes. For example, in reading studies it has been proven that for common words the fixation lengths are shorter than for less common words where the fixation lengths are longer. The amount of fixations can indicate how easy it is to find, for example, a specific package on a shelf. The fewer fixations used the more efficient the search process.
Tobii Technology Whitepaper: Tobii Eye Tracking. An introduction to eye tracking and Tobii Eye Trackers. (2010) [pdf]
Rayner, K.: Eye Movements in Reading and Information Processing: 20 Years of Research. In: Psychological Bulletin, vol. 124, pp. 372–422 (1998) [pdf]
How does an eye tracker work?
Read more in the Tobii Technology Whitepaper: Tobii Eye Tracking. An introduction to eye tracking and Tobii Eye Trackers. (2010) [pdf]