Eye Tracking Terms Part 1: The Basics

Eye Tracking Terms Part 1:The Basics
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If you have recently begun your journey into the field of eye tracking, you may feel a bit overwhelmed by all of the terms and measurements that can be associated with it. I’m going to illustrate and explain some of these oculometrics to help you out.

 

Let’s start with Pupil-related terms:

 

• The size of the pupil, the black part in the center of the eye that allows light to enter the retina is called Pupil area. This is usually measured in pixels.
• The size of the pupil in relation to the size of the iris, the part around the pupil which controls its size is the Pupil area to iris area ratio.
• The length of the pupil from side to side is the Pupil diameter. Pupil diameter will vary as the pupil constricts and expands. A normal pupil diameter will range from 2 to 8mm.
• The rate at which the pupil gets larger or smaller to absorb more or less light Pupil constriction rate.

 

Now let’s move on to eye movement terms

• When a user’s gaze maintains its position focused on a single location, this is a Fixation.
• There are fast, simultaneous movements of both eyes in the same direction from one fixation point to another called Saccades.
• A measurement of the rate and direction of change in position of the eyes is Saccade velocity.

 

Finally, here are a few terms related to blink behavior:

 

• The number of times an individual blinks in a given time period is the Blink rate . The average blink rate of an adult is usually around 10 blinks per minute.
• Length of a blink from beginning to end is Blink duration. Blink duration is often used in conjunction with the next metric, PERCLOS, as an indicator of drowsiness.
• PERCLOS is a measurement regarding the percentage of closure of the eyelid. PERCLOS measures slow eyelid closures, rather than blinks, and is the measurement of choice for drowsiness detection.

 

Stay tuned for part 2, where we will go deeper into some of the more complex eye tracking terms you will find in research studies and eye tracking results.