Eye tracking is the process of measuring eye movements in order to determine where a person is looking, what they are looking at, and for how long their gaze is in a particular spot. The technology is capable of converting eye movements into a data stream that contains information such as pupil position, the gaze vector for each eye, and gaze point. The insights developed from the translation of eye movements can further be used in a range of applications or as an input modality.
Often, eye tracking systems use invisible, near-infrared light and high definition cameras to project light onto the eye and record the direction it is reflected off the cornea. Algorithms are then used to calculate the position of the eye and where it is focused. This can make it possible to measure and study visual behavior and fine eye movements.
The brain processes images through light-sensing cells in the retina. These cells, rod cells, and cone cells detect light through the pupil and send visual data to the brain. While there are fewer cone cells than rod cells; the former permits a person to see in high resolution. Cone cells are in the fovea, the center of the visual field, and is the region with the highest visual acuity in the eye, but it is small. Outside the area of the foveal vision is the peripheral vision, where clarity decreases. This is why the eye has to move in order to process detailed visual information of interest. These movements are what is tracked by eye tracking technology, among other observations.
The eye executes a number of movements, including vergence or torsion, but the most pertinent ones measured through eye tracking are fixations, saccades, and smooth pursuit.
- Fixations occur when the eye stops to collect visual data. Although the duration of one is highly variable, the longer a fixation is, the more visual information is processed.
- Saccades are fast jumps the eye performs between fixations in a static environment. The eye moves from one object of interest to another with the goal of acquiring new visual data in high resolution.
- Smooth pursuit is the eye movement that takes place when looking at an object in motion and following it. As visual intake is possible during smooth pursuit, the movement is relevant for eye tracking.
Video-based gaze tracking also allows measurement and analysis of pupil size. To obtain accurate pupil tracking, however, the environment needs to be rigorously controlled. In adequate test conditions, pupil dilation can be observed and monitored as a result of the following:
- Strong emotional stimuli
- Acute attention
- Working memory load
Perhaps of most importance, eye tracking facilitates the study of visual attention. Due to the limited nature of image processing sources, the brain selectively chooses relevant visual information based on:
- interest—the conscious or unconscious decision to look at an object, and
- environment—elements detected through peripheral vision.
In academia, the examination of visual attention leads to understanding of attention mechanisms in general. It sheds light on the cognitive processing that takes place during the execution of a given task. This same measurement makes it of importance and of value to the realm of marketing and advertising.
While the general principles of of eye tracking devices tend to be the same, there are several types of tracking devices. Which device is used depends on the nature of the research being conducted, with some being better suited than others to a specific research objective.
Types of eye tracking device
These are stand-alone, remote devices that either come as an individual unit or a smaller panel, which can be attached to a laptop or monitor.
These devices are usually used in research where participants interacts with or is exposed to stimuli on a screen. These remote eye trackers offer broad sampling rates, and those with a high frequency can provide a large amount of data and a very high level of detail relating to the movement of the eye. Screen-based eye trackers with a large tolerance for head movement are commonly used when studying people with certain medical conditions and infants who are unable to control their movements.
These include tracking glasses and virtual reality headsets with integrated eye tracking.
These devices are considered ideal for studying behavior in real-world situations, such as browsing the aisles of a supermarket, playing sports, navigating the subway system, consuming media in the home, human interaction, or working in a factory. They offer an unobtrusive system capable of evaluating a natural movement and attention tracking. Often, these systems include built-in scene camera and microphone to record the environment.
Webcam eye trackers do not have sensors or specialized cameras, they compromise a webcam either attached or built-in to a computer.
These tend to use built-in or external webcams attached to a laptop or monitor to collect information on where the person is looking. This method does not use infrared light beams or specialized cameras, and instead relies on the image generated from the webcam. An algorithm is then often used to calculate the position of the head and eyes. While the depth and accuracy of the information of this method is somewhat limited, webcam eye tracking enables large-scale studies and a fast turnaround which can be ideal for quantitative research. The method is commonly used early in a design process, such as A/B testing of a website or product design.
The category of embedded or integrated eye tracking devices is a growing category with increased use cases, some of which tend to draw the most interest from users. This has included devices aimed in surgery systems or related medical devices, autofocus camera systems based on gaze position within a camera's viewfinder, devices embedded into vehicle dashboards, and those integrated into virtual and augmented reality devices.
For research purposes, systems can be integrated into virtual reality and augmented reality devices to provide a control scheme and reduce variables into a study. This technology can also provide intuitive control method to menus within VR and AR technologies to eliminate the need of controllers or a mouse or keyboard.
Another possible use of eye tracking in virtual reality is foveated rendering. The human visual system is, as noted above, capable of high acuity at the fovea, and the peripheral vision has lower acuity and is incapable of seeing the same level of detail. This means graphical rendering in VR could focus on saving processing power by only rendering high quality graphics at the point of gaze, and lower quality in the periphery. However, this also requires embedded tracking systems fast enough to sample the movements and fast enough real-time data transmission to react to fast eye movements.
The human eye is constantly used for different tasks, and almost anything that involves a visual component can become a subject of eye tracking and the data collected by eye tracking devices can be leveraged to gain insights and understand human behavior. The following are use cases of the technology in this field.
Use cases of eye tracking technology
Eye tracking makes it possible for users with physical difficulties in performing mouse navigation, and can help users with disabilities move the cursor as efficiently as anyone.
Advertising and market research
Knowing where customers and users look, and where they do not, can be invaluable for online, TV, and print advertising. Eye trackers on monitors and kiosks can glean insights into how many users see key messages and components of ads, while mobile gear can be used to weigh customer reaction to print materials, posters, and product packages. Eye tracking devices can also help store owners research customer behavior and navigation patterns in order to better understand how customers look at products on shelves, which sections of a store get the most attention, and how a store can make better use of their space.
With distracted driving and drowsiness, two prominent causes of road incidents, eye tracking has been proposed as a chance to track the driver's attention and state of awareness and issue warnings. Combined with other innovative technologies such as smart sensors and image analysis software, eye tracking can help direct driver's attention to where it matters most and prevent incidents from happening.
One explored use of eye tracking is improving gaming experience. There are a variety of areas in which eye tracking can make it easier for users to interact with the user interface of games, and could be used to replace traditional input methods. They can also be used to analyze eye interaction with the interface and help players improve their gaming by giving them insights into what details are being ignored. This could further be used to improve the gaming environment, such as having game characters react when a user looks at them.
As mentioned, eye tracking has significant contributions to qualitative and quantitative research. In an academic context, the technology can be used as a tool for visual behavior study, attentional mechanisms evaluation, visual stimuli response measurement, learning patterns insight, and group behavior comparison.
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