Eye tracking devices - different approaches to eye tracking technology

Eye tracking systems measure the eye position, movement, and pupil size. This way eye trackers detect the areas in which the user has an interest. And eye tracking technology has very much advanced in recent years! It is now used in various fields for research, source of real-time data interaction and much more. Eye tracking is no longer limited to vision and attention measurement studies. As a result, there are different types of eye tracking devices designed to be used for specific purposes. Four major types of eye tracking devices will be discussed here, along with the basic examples of their applications.

Eye Tracking Systems

There are differences between eye tracking devices in case of form and functionality. Here are some ways that eye tracking hardware can differ:

• Human Interface: The way eye tracking devices interact with users and the environment can differ. Some systems require head-stabilization using a chinrest or bite-bar. Others are built into a headband or glasses and worn by the participant of the study. The most common type doesn't require physical contact and measures the eye gaze from a distance.
• Tracking Area: Most eye tracking devices use a computer screen as the stimulus area and don't track eye movements elsewhere. Some systems can track eye movement in more complex environments (like a cockpit or multiple-screen area), and a few are designed for real-world tracking of almost anything the participant looks at.
• Specifications: Measures such as spatial resolution, sample rate, and accuracy are important for research and can impact other areas as well. There are tradeoffs in terms of performance, human interface, and tracking area.

Types of eye-tracking devices

Most currently used eye tracking systems fall into one of four categories: head-stabilized, remote, mobile (head-mounted), and embedded (integrated).

1. Head-Stabilized Eye Tracking

These eye tracking gear keeps the participant's head still, usually with a bite-bar or chinrest. They are mainly used in research experiments for neurophysiology or vision experiments, where accuracy and precision are more important than participant comfort. Sometimes, head stabilization is used alongside other technologies that already keep the head still, like fMRI or MEG.
There are usually three reasons for using head stabilization:
A. For more accuracy and precision: Eye tracking systems need to take head movements into account, providing some kind of compensation. By stabilizing the head, these systems can reduce artifacts and noise caused by head movements in the eye tracking data. 
‍B. To control visual experience: Head-stabilized systems control the visual experience for participants. Providing consistent visual experience for all of them is beneficial for those eye tracking studies that aim to understand perception and the visual system.
‍C. It is used with other technologies that already require head stabilization: Eye tracking systems used in fMRI, MEG, and other research areas need to be head-fixed because the other research tool already requires it. In fMRI experiments, the head is stabilized to control the quality of the scanner data, but this also immobilizes the eyes for the eye tracking device.
Head-stabilized eye tracking systems can achieve a higher level of precision. This is partly because a high-resolution camera can capture a close-up image of the eye without adjusting for head movements. These systems also often have a higher sample rate, allowing for faster analysis of eye movements and they can be used with one eye or both eyes.

‍Limitations:The first and main limitation of head-stabilized tracking devices is the comfort and natural interaction of the participant. So understandably these systems are used only in controlled lab settings.

2. Remote Eye Tracking

Remote eye tracking systems do not require contact with the participant at all. The camera is placed at a distance to capture the participant's eye positions and eye movement, and the systems can adjust the camera's view to account for head movements. These systems track eye position and head orientation using the center of the pupil and the reflection on the cornea.
These systems typically consist of a camera and an infrared (IR) source positioned below the stimulus area, usually a computer screen. It is also possible for the camera to be positioned above the display, which is useful for touch screens. However, because of the shape of the eye and eyelids, the pupil is more visible and less likely to be blocked when the camera is below the display. The remote device for measuring eye positions can be placed in front of the screen, attached to it, or embedded in a laptop, monitor, or kiosk. 
These systems have a designated working area called a "head box" and can usually only track eye movements on a defined "calibration plane," typically the computer screen. If the participant moves outside of the head box or looks away from the calibration plane, the tracking will be temporarily interrupted, as data is only collected for the computer screen.Remote systems are most commonly used for screen-based interaction or experiments. They are also useful for gaze-contingent interfaces, such as assistive technology devices, or gaming laptops.This type of eye tracking has some significant advantages:
‍A. Natural interactions: Ideally, the participant can use a computer naturally while the eye tracking system is working. This approach is great for usability testing, psychology experiments, market research, etc., where a more intrusive interface would potentially influence the participant's behavior.
‍B. No physical contact: Remote systems are often the only option for studying infants, people with disabilities or participants with neurological conditions who may not tolerate anything touching their head. It is also the basis for assistive communication devices. For example, a person with quadriplegia or locked-in syndrome can use a remote eye tracking device to communicate using eye movements.
‍C. Compatibility with other technologies: Since they measure fixation and visual attention without touching the participant and the electronics are at a distance, remote systems work well with other research technologies such as EEG, NIRs, biosignals, etc. Remote eye tracking systems are almost always binocular. 

‍Limitations:
A. Working area: It can be hard to track gaze interaction when the object being observed, such as a mobile device or document, is not fixed relative to the camera. 
‍B. Head-Movement: Moving significantly closer or further from the screen requires the system to adjust the calibration and account for changes in focus, which can lead to increased errors.
‍C. Be careful not to have more than one participant visible to the camera. 
‍D. Sunlight: Remote systems can be sensitive to infrared sources like sunlight, that can affect eye tracking process, especially if the sun reflects into the participant's eyes (e.g., facing a sunny window). 

3. Mobile Eye Tracking

Mobile eye tracking systems, also known as "head-mounted", involves wearing a device that tracks eye movement. This can be eye tracking glasses or a headband. The system uses a camera or mirror positioned in front of one or both eyes to record what is seen, as mobile systems usually track both eyes. Using a head-mounted system allows for real-world experiments in various areas such as sports, driving, navigation, social communication, hand-eye coordination, and testing products on store shelves. Modern mobile systems are wireless, allowing for experiments in realistic contexts like simulators, vehicles, sports training, and navigation.
Head-mounted systems built into glasses are comfortable, less invasive, and can be used together with other technologies like EEG. With mobile eye tracking, studies that used to be simulated on screens can now be conducted in naturalistic and realistic settings.

‍Limitations:
A. Sunlight: Mobile eye tracking devices can have trouble tracking eye movements in sunlight. 
‍B. Eccentric Eye Movements: As people tend to look at the horizon and lower, tracking eye movements towards the edges of vision can be difficult and less accurate, especially when the cameras are positioned below. 
‍C. Relative Coordinate System: When using a mobile eye tracking device, there is no absolute coordinate system. Instead, the system records gaze data in a coordinate system defined by the scene camera. Think of this as an imaginary screen that moves with the participant's head. For example, let's say you're using a remote system and showing a moving target on a computer screen. If you know the target's position relative to the screen, it would be easy to determine whether each participant's gaze position (also in screen pixels) coincided with the target. However, with a mobile eye tracking system, the target may be a real-world object captured by the scene camera, like a kicked soccer ball. The position of the ball in the scene camera depends on the participant's head position and can change as both the ball and the participant move. This can vary significantly between each participant.

4. Webcam Eye Tracking

Mobile eye tracking systems, also known as "head-mounted", involves wearing a device that tracks eye movement. This can be eye tracking glasses or a headband. The system uses a camera or mirror positioned in front of one or both eyes to record what is seen, as mobile systems usually track both eyes. Using a head-mounted system allows for real-world experiments in various areas such as sports, driving, navigation, social communication, hand-eye coordination, and testing products on store shelves. Modern mobile systems are wireless, allowing for experiments in realistic contexts like simulators, vehicles, sports training, and navigation.
Head-mounted systems built into glasses are comfortable, less invasive, and can be used together with other technologies like EEG. With mobile eye tracking, studies that used to be simulated on screens can now be conducted in naturalistic and realistic settings.

‍Limitations:
A. Sunlight: Mobile eye tracking devices can have trouble tracking eye movements in sunlight. 
‍B. Eccentric Eye Movements: As people tend to look at the horizon and lower, tracking eye movements towards the edges of vision can be difficult and less accurate, especially when the cameras are positioned below. 
Webcam eye-tracking is an innovative method that brings together remote and mobile eye tracking. And it gains interest on the market, as it is one of the low cost eye tracking technologies, yet effective one. It is one of the methods suited for the digital era, asit uses the computing power of a regular PC/laptop to run AI (Deep neural network) to analyze images coming from a webcam. Any webcam: the one in participants' laptop or a smartphone, which means a major bow towards human factors and ergonomics. The AI detects a panelist's face, pupils and predicts a gaze point. And all this is performed entirely in a web browser in real-time. 
Webcam eye-tracking works perfectly for creating digital assets like heat maps, but this method of eye tracking is also used for package design testing. This way webcam eye-tracking truly revolutionizes eye tracking research, as it doesn’t require any additional gear to conduct a study, making most efficient time wise and money wise as well. 

5. Integrated or Embedded Systems

This category includes eye tracking devices integrated into various technologies. For example: 
- Aiming devices in eye surgery systems and other medical products.
- Canon cameras with an autofocus system based on gaze position.
- Integration of eye tracking devices into vehicle dashboards.More recently, eye tracking systems have been embedded in virtual or augmented reality devices. This integration serves purposes like:
A. Research applications: Eye tracking in immersive screens allows for remote monitoring and control of gaze.
B. Control schemes: Users can interact with content using eye movements, providing an intuitive control method in AR and VR environments where a mouse or keyboard is not available.
One interesting application of eye tracking in VR is foveated rendering. The human visual system has high acuity at the fovea (the point of gaze), while peripheral vision has lower acuity. Foveated rendering optimizes graphical processing power by rendering higher quality graphics at the point of gaze and lower quality in the periphery. To achieve this, embedded eye tracking is necessary, with a fast sample rate and real-time data transmission to react to fast eye movements.

Eye tracking may be over 100 years old, but it has come a long way. Only in the past 15 to 20 years, it has transitioned from clean lab settings to almost everyday use in natural conditions. This is how eye tracking technologies have advanced! And as the market grows, with neuromarketing being one of the big trends for the upcoming years, technology will surely follow. So, whether you are happy or not, staying updated and doing your lessons is the best way to actually use technology that suits your needs best. 

Bibliography: 
‍https://www.bitbrain.com/blog/eye-tracking-devices 
‍https://www.bitbrain.com/blog/eye-tracking-technology 
‍https://www.sciencedirect.com/science/article/abs/pii/S0957417420308071 
‍https://www.questionpro.com/blog/eye-tracking/

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