Comparing Bright and Dark Pupil Tracking Techniques

There is extensive research being done around the world on eye tracking technology. Eye tracking, regardless of the method used, is essentially detecting the rotation of the eye using the pupil as a reference point to determine where a person is looking. There is a variety of techniques being developed and tested, and so far there hasn’t been one declared superior. However, there are two approaches that are the most commonly used for eye tracking, bright pupil and dark pupil tracking.

For the most part, modern eye trackers use video cameras and an infrared lighting source to create a corneal reflection to determine the location of the pupil. Bright or dark pupils occur as a result of the position of the source of illumination. The bright pupil effect occurs when the lighting source is in line with the optical axis, causing the light to reflect off the retina. This is similar to what occurs in flash photography to create red-eye effect. Dark pupils occur in a photograph or video image when the lighting source is offset from the optical axis, and the reflection is projected away from the camera, making the pupil appear black.

Each method has benefits and pitfalls, so the optimal method depends on the conditions under which the technique is being used and the physical characteristics of the person whose eye movement is being tracked. The bright pupil technique results in a dramatic contrast between the pupil and the iris, making the pupil easily distinguishable and therefore easier to track. There is little interference from eyelashes and shadows because the image-processing algorithm recognizes a white elliptical region as the pupil. Creating a pronounced bright pupil effect, however, is highly dependent on pupil size, which is affected by several external factors like age, emotional response to stimuli, and lighting sources. This method tends to work better in a dark environment and on children and people with blue or light eyes.

The dark pupil method, which is the method Eye-Com Corporation uses in its eye tracking technology, detects the dark ellipse of the pupil within the iris. This methods works well in bright environments and outside in natural lighting conditions, but there are issues with eyelashes and shadows causing false positives during pupil detection. Dark colored eyes work best because the IR light reflection off the iris makes the dark color of the iris appear light in the digital image, thus making the pupil more easily discernible.

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