STUDIO NEXT: Source / Design / Build: Eye Tracker

SAFETY OF INFRARED LIGHT

Gareth Roberts

How does eye-tracking work?

Eye tracking is the act of following the direction of the pupil of a subjects eye in order to gain data about where they are looking and the duration of their gaze. It is conducted by directing a source of infrared light at the eye of the subject, which is then reflected back and detected by an infrared sensitive camera. When the incoming infrared light source is directly in line with the subjects eye and the camera is within approximately one foot, the infrared camera will likely pick up an image of the eye with the pupil brightly shining, whereas an indirect infrared source or a camera positioned further away will show the pupil as a ‘black hole’.

The phenomonen of the bright pupil is much like that of red-eye in traditional photography, and occurs when the camera is close to the subject and picks up the infrared reflection off the retina of the eye. In either case, the pupil is clearly distinguishable from the rest of the eye, and can be used with computation to estimate the gaze direction of the subject.

Is it safe to expose your eyes to the infrared light emitted by the camera?

The human eye is extremely well equipped to deal with incident light to prevent damage, and we have many safeguards pre-programmed to prevent damage to our eyes. Our blinking reflex ensures our eyes are properly lubricated, and we have a natural aversion reflex when we are exposed to strong light intensity in the visible spectrum. Unfortunately, the eye cannot detect Infrared light, and thus severe damage can be caused by exposure to the infrared spectrum without us knowing it.

Infrared light, like any form of light in the spectrum has the potential to damage the eye. Infrared Light-Emiting Diodes (LED’s) typically emit near-infrared light at a wavelengths between 820nm abd 950nm, and any given Inrfared (IR) camera can include up to 40 individual light sources depending on the quality of the device.

When exposed to excessive amounts of IR, two types of eye damage can be incurred; thermal injury to the retina (Burning) can occur with light wavelengths between 400nm and 1400nm, and burning of the lens of the eye with light wavelengths between 800nm and 3000nm.

In the past two decades, scientists around the world have developed a keen interest in this subject, and both North America agencies and their counterparts in Europe have developed a series of standards regarding exposure to IR light. With these standards in mind, and given normal viewing conditions, the infrared light sources used in the IR Cameras pose no threat for the health of eyes, even when viewed continuously for hours.

Saying this, the international standards developed to govern the use of IR light were created at a time when there was no use for continuous exposure to artificial IR light sources, and no comprehensive studies have been completed on the effects of long-term exposure to IR light (For example, over a decade of use). Naturally, as the length of exposure increases, so does the chance for damage to the eyes, so it is best to limit the amount of exposure to IR light where possible.

Other eye related health concerns with eye-tracking

Aside from exposure to Infrared light, there are several other health concerns with eye-tracking systems. Firstly, eye-tracking usually involves ensuring a user is seated in front of an interface for hours on end, and results in intense fixations on the screen or other medium. While a person is fixated on an interface, their natural blink reaction is slowed significantly, and over a short amout of time results in the eye becoming under-lubricated. When we blink, tears that contain important antibodies are used to lubricate the eye, and when our eyes are dry they are more suseptable to infection.

This is especially important to consider when eye-tracking is used to augment actions for disabled or incapacitated subject. People who are paralized could rely heavily on an eye-tracking interface, and that intense fixation on a screen could make them even more suseptable to eye damage from under-lubricated eyes.

Another important consideration is that incapacitated or disabled users of these devices are often taking many different medications, and some drugs can have side effects that heighten sensitivity to exposure to light, or restrict the ability for the eye to lubricate itself. These patients will be especially at risk to infection or eye damage, so care must be taken when designing interfaces for their use.