Do not stare at high brightness LEDs, because doing so may cause permanent damage to your eyes.
The optics of our eyes focus visible light onto the retina, which is the light sensitive part of our eyes, similar to the way a magnifying glass can focus an image of the sun onto a piece of paper. Just as the piece of paper might smolder and catch fire, a light that is brightly focused on your retina can cook and thus permanently damage your eye. Exposure to bright white and blue LEDs can also cause permanent damage to your eyes through photochemical action.
Here is a link to an article about the particular hazards of blue LEDs: http://www.mdsupport.org/library/hazard.html#blue
The key factors that determine whether or not a particular exposure to light from an LED causes permanent damage are the wavelength and intensity of the light, the length of the exposure occurs, and the distance from the light source to the eye.
This danger is most pronounced with the use of the newer high output blue and white LEDs. These are intended to illuminate large areas, and for those designed for direct view applications, to be viewed at a significant distance. These LEDs can be identified by packaging or mountings that are designed to carry heat away from the junction. LEDs without these special thermal management features can also pose a danger when operated beyond their specified maximum current.
Infrared and ultraviolet LEDs can also be hazardous, and when using those, please consult the manufacturer’s data sheets or IEC 60825-1:2001 for guidance with respect to safety. You may find IEC 60825-1:2001 summarized on the web. The standard can be purchased from IEC’s web store http://webstore.iec.ch/.
The IEC specification was written to apply to primarily to lasers , though it also applies to LEDs. A new standard, IEC TR 60825, is in development that promises that includes a section that is intended to address LEDs directly. You might want to check the web for drafts or summaries of this standard in the future.
The standards mentioned above rely on measurements that can only be made with very expensive specialized equipment, though some manufacturers state the safety classification of their LEDs, under specified operating conditions, in their data sheets. Take note of these comments on the data sheets for LEDs you may use. In the mean time, play it safe by not staring at any bright LEDs.
My thanks to Geoff Davies of Lucid Optical Services Ltd. (http://www.lucidos.co.uk), for raising the issue of permanent eye damage, and providing the information contained in this section.