The eye is designed by evolution to be biologically sensitive to natural light energy; indeed, this is the basis of vision. Therefore, the various structures and tissues of the eye are designed to collect and focus light on the retina. These structures and tissues are affected in various ways by light, and at intense levels, this effect may be adverse. Because of this, any light source intended for human use must be carefully tested and conform to safety regulations.
Natural Defence Mechanisms and Photobiological Effects
The eye is well adapted to protecting itself against overly intense broad-band optical radiation from the natural environment (i.e., ultraviolet, visible, and infrared radiant energy), and mankind has learned to use protective measures, such as hats and eye-protectors, to shield against the harmful effects on the eye from very intense ultraviolet radiation (UVR) and the blue light present in sunlight over snow or sand. The eye is also protected against bright light by the natural aversion response to viewing bright visible light sources. The aversion response includes blinking and/or head movements to avoid strongly perceived light. It normally protects the eye against injury from viewing bright light sources such as the sun, arc lamps, and welding arcs, since this aversion limits the duration of exposure to a fraction of a second (about 0.25 s).
The infrared LEDs employed in most infrared LED eye trackers do not, however, produce a strong aversion response, as they are barely visible to the human eye, and the spectral emission is limited to the near-infrared (IR-A, 780–1400 nm) spectral band. If a conventional incandescent lamp or discharge lamp that has been filtered to block most visible light and transmit IR-A is employed, some emissions of note are possible outside the IR-A range and must be evaluated separately.