LEDs have many features that turn them into very versatile sources of light, they possess high energy efficiency and with them a new world of lighting creativity is opening its doors.
However human eyes have evolved to deal with the wide output spectrum of sunlight, how does that compare with the type of light produced by an LED fixture? This can be important. We need to know at least some information to ensure we protect our sight from any hazard. Actually there are different levels for safety if we look into any type of lighting.
We need to know a bit about how our eyes perceive light. Light enters the eye, it goes through the cornea and gets to the retina, where light can be actually detected. The retina has two types of cells, rods and cones. Rods allow us to see in low light, and cones let us see colors and detail. Also there is a third type of light sensor in the eye that does not perceive colors, called intrinsically photosensitive retinal ganglion cells these have control over the pupil, and help synchronizing the circadian rhythms with daylight. These ganglion cells can keep the pupil small, while rods and cones can only shrink the pupil briefly (10 seconds), so this inner retinal photoreceptive system, mainly photo-ganglions generate sustained an enhanced responses under continuous high levels of illumination, producing continual pupillary constriction under bright light conditions (researchers Yanli Zhu and Daniel Tu, 2007). This is considered a short-term adaptation, and there are long-term adaptations. For example UVA from sunlight can be harmful for young children but adults accumulate filtering chemicals in the eyes that protect them from such harm. For damage to occur the light has to be bright and its spectrum becomes important. Shorter wavelength light has more energy, blue photons can do more damage than red photons (longer light wavelengths)
Ultra-violet photons would be the most damaging. Fortunately LEDs do not produce UV. If UV from sunlight reaches the eyes it is absorbed by the filtering chemicals in the front of the eye. At higher levels this could cause ‘arc eye’ in the cornea – a reversible UV damage. Long-term UV absorption in the lens from sunlight appears to contribute to cataract formation.
Leaving apart UV, blue light radiation is left as the most harmful within visible spectrum, it can affect the retina through photo-chemical action. Blue light damage at high intensity is well studied and is cumulative. The ICNIRP (International Commission on Non-Ionizing Radiation Protection) through expert review produces best-practice guidelines which are the basis for global standards in light safety, they provide advice on most precautions to avoid harmful effects of sunlight UV. Long term exposure to high levels of blue light may cause cumulative damage over years and it has been correlated with age-related macular degeneration or AMD, however there is not strong evidence yet on blue light and AMD. Researchers state that sunlight effect is far greater than LEDs’ “I would be very surprised if blue LEDs increased AMD. Blue light exposure from the sun is far more than this” (O’Hagan). Only blue light above a certain level could be accumulated, potential effects would be small and this is still being studied.